QO Groupmeeting

Klein Tunneling of a Quasirelativistic Bose-Einstein Condensate in an Optical Lattice

2012-02-23T16:35:00.002-05:00

Tobias Salger, Christopher Grossert, Sebastian Kling, and Martin Weitz

A proof-of-principle experiment simulating effects predicted by relativistic wave equations with ultracold atoms in a bichromatic optical lattice that allows for a tailoring of the dispersion relation is reported. We observe the analog of Klein tunneling, the penetration of relativistic particles through a potential barrier without the exponential damping that is characteristic for nonrelativistic quantum tunneling. Both linear (relativistic) and quadratic (nonrelativistic) dispersion relations are investigated, and significant barrier transmission is observed only for the relativistic case.

**Groupmeeting by Shreyas Potnis, Jan 25th, 2011**

Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators

2012-01-19T11:41:00.002-05:00

M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauß, C. Gross, L. Mazza, M. C. Bañuls, L. Pollet, I. Bloch, S. Kuhr

Quantum phases of matter are characterized by the underlying correlations of the many-body system. Although this is typically captured by a local order parameter, it has been shown that a broad class of many-body systems possesses a hidden nonlocal order. In the case of bosonic Mott insulators, the ground state properties are governed by quantum fluctuations in the form of correlated particle-hole pairs that lead to the emergence of a nonlocal string order in one dimension. By using high-resolution imaging of low-dimensional quantum gases in an optical lattice, we directly detect these pairs with single-site and single-particle sensitivity and observe string order in the one-dimensional case.

**Groupmeeting by Carolyn Kierans, Jan 18th, 2011**

Spin–orbit-coupled Bose–Einstein condensates

2011-12-02T11:04:00.002-05:00

Y.-J. Lin, K. Jiménez-García & I. B. Spielman

Spin–orbit (SO) coupling—the interaction between a quantum particle’s spin and its momentum—is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators; it contributes to the electronic properties of materials such as GaAs, and is important for spintronic devices. Quantum many-body systems of ultracold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom’s intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose–Einstein condensate by dressing two atomic spin states with a pair of lasers. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin-mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling—equally applicable for bosons and fermions—sets the stage for the realization of topological insulators in fermionic neutral atom systems.

**Groupmeeting by Graham Edge, Nov 30th, 2011**

The quantum state cannot be interpreted statistically

2011-11-25T11:51:00.013-05:00

Matthew F. Pusey, Jonathan Barrett, Terry Rudolph

Quantum states are the key mathematical objects in quantum theory. It is therefore surprising that physicists have been unable to agree on what a quantum state represents. There are at least two opposing schools of thought, each almost as old as quantum theory itself. One is that a pure state is a physical property of system, much like position and momentum in classical mechanics. Another is that even a pure state has only a statistical significance, akin to a probability distribution in statistical mechanics. Here we show that, given only very mild assumptions, the statistical interpretation of the quantum state is inconsistent with the predictions of quantum theory. This result holds even in the presence of small amounts of experimental noise, and is therefore amenable to experimental test using present or near-future technology. If the predictions of quantum theory are confirmed, such a test would show that distinct quantum states must correspond to physically distinct states of reality.

**Groupmeeting by Lee Rozema, Nov 23rd, 2011**

Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction

2011-11-25T11:51:00.012-05:00

Nanfang Yu, Patrice Genevet, Mikhail A. Kats, Francesco Aieta, Jean-Philippe Tetienne, Federico Capasso, Zeno Gaburro

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

**Groupmeeting by Rockson Chang, Nov 16th, 2011**

Exploring Symmetry Breaking at the Dicke Quantum Phase Transition

2011-11-25T11:51:00.011-05:00

K. Baumann, R. Mottl, F. Brennecke, and T. Esslinger

We study symmetry breaking at the Dicke quantum phase transition by coupling a motional degree of freedom of a Bose-Einstein condensate to the field of an optical cavity. Using an optical heterodyne detection scheme, we observe symmetry breaking in real time and distinguish the two superradiant phases. We explore the process of symmetry breaking in the presence of a small symmetry-breaking field and study its dependence on the rate at which the critical point is crossed. Coherent switching between the two ordered phases is demonstrated.

**Groupmeeting by Nathan Cheng, Nov 9th, 2011**

Unconditional room-temperature quantum memory

2011-11-03T16:14:00.001-04:00

M. Hosseini, G. Campbell, B. M. Sparkes, P. K. Lam & B. C. Buchler

Just as classical information systems require buffers and memory, the same is true for quantum information systems. The potential that optical quantum information processing holds for revolutionizing computation and communication is therefore driving significant research into developing optical quantum memory. A practical optical quantum memory must be able to store and recall quantum states on demand with high efficiency and low noise. Ideally, the platform for the memory would also be simple and inexpensive. Here, we present a complete tomographic reconstruction of quantum states that have been stored in the ground states of rubidium in a vapour cell operating at around 80 °C. Without conditional measurements, we show recall fidelity up to 98% for coherent pulses containing around one photon. To unambiguously verify that our memory beats the quantum no-cloning limit we employ state-independent verification using conditional variance and signal-transfer coefficients.

**Groupmeeting by Amir Feizpour, Nov 2nd, 2011**

Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond

2011-10-24T11:14:00.002-04:00

Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin-ichi Karimoto, Hayato Nakano, William J. Munro, Yasuhiro Tokura, Mark S. Everitt, Kae Nemoto, Makoto Kasu, Norikazu Mizuochi & Kouichi Semba

During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention since the first demonstration of macroscopic quantum coherence in Josephson junction circuits. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen–vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 × 10^7 such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.

**Groupmeeting by Xingxing Xing, Oct 19th, 2011**

Cavity-Enhanced Frequency Comb Spectroscopy

2011-10-13T09:38:00.002-04:00

A. Foltynowicz, T. Ban, P. Maslowski, F. Adler, J. Ye

What happens when you combine a frequency comb with a cavity

How Jun Ye knows if you're a smoker

**Groupmeeting by Dylan Jervis, Oct 12th, 2011**

Non-Hermitian Quantum Mechanics

2011-10-12T18:38:00.005-04:00

Y. Choi, S. Kang, S. Lim, W. Kim, J-R. Kim, J-H. Lee, K. An

We report the first direct observation of an exceptional point (EP) in an open quantum composite of a single atom and a high-Q cavity mode. The atom-cavity coupling constant was made a continuous variable by utilizing the multisublevel nature of a single rubidium atom when it is optimally coupled to the cavity mode. The spectroscopic properties of quasieigenstates of the atom-cavity composite were experimentally investigated near the EP. Branch-point singularity of quasieigenenergies was observed and its 4π symmetry was demonstrated. Consequently, the cavity transmission at the quasieigenstate was observed to exhibit a critical behavior at the EP.

**Groupmeeting by Chao Zhuang, Oct 5th, 2011**

Interaction-induced orbital excitation blockade of ultracold atoms in an optical lattice

2011-10-12T18:38:00.002-04:00

W. S. Bakr, P. M. Preiss, M. E. Tai, R. Ma, J. Simon, M. Greiner

Interaction blockade occurs when strong interactions in a confined few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow the detection and manipulation of the constituent particles, whether they are electrons, spins, atoms, or photons. The diverse applications range from single-electron transistors based on electronic Coulomb blockade to quantum logic gates in Rydberg atoms. We have observed a new kind of interaction blockade in transferring ultracold atoms between orbitals in an optical lattice. In this system, atoms on the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth and observe a staircase-type excitation behavior as we cross the interaction-split resonances by tuning the modulation frequency. As an application of orbital excitation blockade (OEB), we demonstrate a novel algorithmic route for cooling quantum gases. Our realization of algorithmic cooling utilizes a sequence of reversible OEB-based quantum operations that isolate the entropy in one part of the system, followed by an irreversible step that removes the entropy from the gas. This work opens the door to cooling quantum gases down to ultralow entropies, with implications for developing a microscopic understanding of strongly correlated electron systems that can be simulated in optical lattices. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a roadmap for the implementation of two-qubit gates in a quantum computing architecture with natural scalability.

**Groupmeeting by Alma Bardon, Sept 28, 2011**

Universal Digital Quantum Simulation with Trapped Ions

2011-10-05T15:00:00.002-04:00

B. P. Lanyon, C. Hempel, D. Nigg, M. Müller, R. Gerritsma, F. Zähringer, P. Schindler, J. T. Barreiro, M. Rambach, G. Kirchmair, M. Hennrich, P. Zoller, R. Blatt, C. F. Roos

A digital quantum simulator is an envisioned quantum device that can be programmed to efficiently simulate any other local system. We demonstrate and investigate the digital approach to quantum simulation in a system of trapped ions. Using sequences of up to 100 gates and 6 qubits, the full time dynamics of a range of spin systems are digitally simulated. Interactions beyond those naturally present in our simulator are accurately reproduced and quantitative bounds are provided for the overall simulation quality. Our results demonstrate the key principles of digital quantum simulation and provide evidence that the level of control required for a full-scale device is within reach.

**Groupmeeting by Dylan Mahler, Sept 21, 2011**

Atoms in Optical Fibers

2011-09-26T12:56:00.004-04:00

Summary of several papers, including:

Kasturi Saha, Vivek Vankataraman, Pablo Londero, and Alexander L. Gaeta

We show that two-photon absorption (TPA) in rubidium atoms can be greatly enhanced by the use of a hollow-core photonic-band-gap fiber. We investigate off-resonant, degenerate Doppler-free TPA on the 5S_1/2→5D_5/2 transition and observe 1% absorption of a pump beam with a total power of only 1 mW in the fiber. These results are verified by measuring the amount of emitted blue fluorescence and are consistent with the theoretical predictions which indicate that transit-time effects play an important role in determining the two-photon absorption cross section in a confined geometry.

**Groupmeeting by Greg Dmochowski, Sept 14, 2011**

Quantum Simulation of Frustrated Classical Magnetism in Triangular Optical Lattices

2011-09-07T12:23:00.002-04:00

J. Struck, C. Ölschläger, R. Le Targat, P. Soltan-Panahi, A. Eckardt, M. Lewenstein, P. Windpassinger, K. Sengstock

Magnetism plays a key role in modern technology and stimulates research in several branches of condensed matter physics. Although the theory of classical magnetism is well developed, the demonstration of a widely tunable experimental system has remained an elusive goal. Here, we present the realization of a large-scale simulator for classical magnetism on a triangular lattice by exploiting the particular properties of a quantum system. We use the motional degrees of freedom of atoms trapped in an optical lattice to simulate a large variety of magnetic phases: ferromagnetic, antiferromagnetic, and even frustrated spin configurations. A rich phase diagram is revealed with different types of phase transitions. Our results provide a route to study highly debated phases like spin-liquids as well as the dynamics of quantum phase transitions.

**Groupmeeting by Dan Fine, Aug 31, 2011**

Realization of an optomechanical interface between ultracold atoms and a membrane

2011-09-05T20:15:00.003-04:00

Stephan Camerer, Maria Korppi, Andreas Jockel, David Hunger, Theodor W. Hansch, Philipp Treutlein

We have realized a hybrid optomechanical system by coupling ultracold atoms to a micromechanical membrane. The atoms are trapped in an optical lattice, which is formed by retro-reflection of a laser beam from the membrane surface. In this setup, the lattice laser light mediates an optomechanical coupling between membrane vibrations and atomic center-of-mass motion. We observe both the effect of the membrane vibrations onto the atoms as well as the backaction of the atomic motion onto the membrane. By coupling the membrane to laser-cooled atoms, we engineer the dissipation rate of the membrane. Our observations agree quantitatively with a simple model.

**Groupmeeting by Matin Hallaji, Aug 24, 2011**

Rydberg Excitations in Bose-Einstein Condensates in Quasi-One-Dimensional Potentials and Optical Lattices

2011-08-18T15:57:00.004-04:00

M Viteau, M.G. Bason, J. Radogostrowicz, N. Malossi, D. Ciampini, O. Morsch, and E. Arimondo

We experimentally realize Rydberg excitations in Bose-Einstein condensates of rubidium atoms loaded into quasi-one-dimensional traps and in optical lattices. Our results for condensates expanded to different sizes in the one-dimensional trap agree well with the intuitive picture of a chain of Rydberg excitations. We also find that the Rydberg excitations in the optical lattice do not destroy the phase coherence of the condensate, and our results in that system agree with the picture of localized collective Rydberg excitations including nearest-neighbor blockade.

**Groupmeeting by Shreyas Potnis, Aug 17, 2011**

Vacuum Induced Transparency

2011-08-08T16:16:00.003-04:00

Haruka Tanji-Suzuki,
Wenlan Chen,
Renate Landig,
Jonathan Simon,
Vladan Vuletić

Photons are excellent information carriers but normally pass through each other without consequence. Engineered interactions between photons would enable applications from quantum information processing to simulation of condensed matter systems. Using an ensemble of cold atoms strongly coupled to an optical cavity, we demonstrate experimentally that the transmission of light through a medium may be controlled with few photons and even by the electromagnetic vacuum field. The vacuum induces a group delay of 25 ns on the input optical pulse, corresponding to a light velocity of 1600 m/s, and a transparency of 40% that increases to 80% when the resonator is filled with 10 photons. This strongly nonlinear effect provides prospects for advanced quantum devices such as photon-number-state filters.

**Groupmeeting by Graham Edge, Aug 3, 2011**

Experimental high-dimensional two-photon entanglement and violations of generalized Bell inequalities

2011-08-02T11:27:00.004-04:00

Adetunmise C. Dada, Jonathan Leach, Gerald S. Buller, Miles J. Padgett & Erika Andersson

Quantum entanglement^{1, 2} plays a vital role in many quantum-information and communication tasks³. Entangled states of higher-dimensional systems are of great interest owing to the extended possibilities they provide. For example, they enable the realization of new types of quantum information scheme that can offer higher-information-density coding and greater resilience to errors than can be achieved with entangled two-dimensional systems (see ref. 4 and references therein). Closing the detection loophole in Bell test experiments is also more experimentally feasible when higher-dimensional entangled systems are used⁵. We have measured previously untested correlations between two photons to experimentally demonstrate high-dimensional entangled states. We obtain violations of Bell-type inequalities generalized to d-dimensional systems⁶ up to d=12. Furthermore, the violations are strong enough to indicate genuine 11-dimensional entanglement. Our experiments use photons entangled in orbital angular momentum⁷, generated through spontaneous parametric down-conversion^{8, 9}, and manipulated using computer-controlled holograms.

**Groupmeeting by Lee Rozema, July 27, 2011**

Measurement of the internal state of a single atom without energy exchange

2011-07-21T13:32:00.003-04:00

Jürgen Volz, Roger Gehr, Guilhem Dubois, Jérôme Estève & Jakob Reichel

A measurement necessarily changes the quantum state being measured, a phenomenon known as back-action. Real measurements, however, almost always cause a much stronger back-action than is required by the laws of quantum mechanics. Quantum non-demolition measurements have been devised^{1, 2, 3, 4, 5, 6} that keep the additional back-action entirely within observables other than the one being measured. However, this back-action on other observables often imposes its own constraints. In particular, free-space optical detection methods for single atoms and ions (such as the shelving technique⁷, a sensitive and well-developed method) inevitably require spontaneous scattering, even in the dispersive regime⁸. This causes irreversible energy exchange (heating), which is a limitation in atom-based quantum information processing, where it obviates straightforward reuse of the qubit. No such energy exchange is required by quantum mechanics⁹. Here we experimentally demonstrate optical detection of an atomic qubit with significantly less than one spontaneous scattering event. We measure the transmission and reflection of an optical cavity^{10,11, 12, 13} containing the atom. In addition to the qubit detection itself, we quantitatively measure how much spontaneous scattering has occurred. This allows us to relate the information gained to the amount of spontaneous emission, and we obtain a detection error below 10 per cent while scattering less than 0.2 photons on average. Furthermore, we perform a quantum Zeno-type experiment to quantify the measurement back-action, and find that every incident photon leads to an almost complete state collapse. Together, these results constitute a full experimental characterization of a quantum measurement in the ‘energy exchange-free’ regime below a single spontaneous emission event. Besides its fundamental interest, this approach could significantly simplify proposed neutral-atom quantum computation schemes¹⁴, and may enable sensitive detection of molecules and atoms lacking closed transitions.

**Groupmeeting by Rockson Chang, July 20, 2011**

Optical Precursor of a Single Photon

2011-07-21T11:55:00.002-04:00

Shanchao Zhang, J.F. Chen, Chang Liu, M.M.T Loy, G.K.L. Wong, Shengwang Du

We report the direct observation of optical precursors of heralded single photons with step- and square-modulated wave packets passing through cold atoms. Using electromagnetically induced transparency and the slow-light effect, we separate the single-photon precursor, which always travels at the speed of light in vacuum, from its delayed main wave packet. In the two-level superluminal medium, our result suggests that the causality holds for a single photon.

**Groupmeeting by Amir Feizpour, July 13, 2011**

Quantum annealing with manufactured spins

2011-05-26T20:41:00.001-04:00

M. W. Johnson,M. H. S. Amin,S. Gildert,T. Lanting,F. Hamze,N. Dickson,R. Harris,A. J. Berkley,J. Johansson,P. Bunyk,E. M. Chapple,C. Enderud,J. P. Hilton,K. Karimi,E. Ladizinsky,N. Ladizinsky,T. Oh,I. Perminov,C. Rich,M. C. Thom,E. Tolkacheva,C. J. S. Truncik,S. Uchaikin,J. Wang,B. Wilson& G. Rose

Many interesting but practically intractable problems can be reduced to that of finding the ground state of a system of interacting spins; however, finding such a ground state remains computationally difficult¹. It is believed that the ground state of some naturally occurring spin systems can be effectively attained through a process called quantum annealing^{2, 3}. If it could be harnessed, quantum annealing might improve on known methods for solving certain types of problem^{4, 5}. However, physical investigation of quantum annealing has been largely confined to microscopic spins in condensed-matter systems^{6, 7, 8, 9, 10, 11, 12}. Here we use quantum annealing to find the ground state of an artificial Ising spin system comprising an array of eight superconducting flux quantum bits with programmable spin–spin couplings. We observe a clear signature of quantum annealing, distinguishable from classical thermal annealing through the temperature dependence of the time at which the system dynamics freezes. Our implementation can be configured in situ to realize a wide variety of different spin networks, each of which can be monitored as it moves towards a low-energy configuration^{13, 14}. This programmable artificial spin network bridges the gap between the theoretical study of ideal isolated spin networks and the experimental investigation of bulk magnetic samples. Moreover, with an increased number of spins, such a system may provide a practical physical means to implement a quantum algorithm, possibly allowing more-effective approaches to solving certain classes of hard combinatorial optimization problems.

**Groupmeeting by XingXing Xing, May 25th, 2011**

Single-ion quantum lock-in amplifier

2011-05-24T09:16:00.000-04:00

Shlomi Kotler, Nitzan Akerman, Yinnon Glickman, Anna Keselman & Roee Ozeri

Quantum metrology1 uses tools from quantum information science to improve measurement signal-to-noise ratios. The challenge is to increase sensitivity while reducing susceptibility to noise, tasks that are often in conflict. Lock-in measurement is a detection scheme designed to overcome this difficulty by spectrally separating signal from noise. Here we report on the implementation of a quantum analogue to the classical lock-in amplifier. All the lock-in operations—modulation, detection and mixing—are performed through the application of non-commuting quantum operators to the electronic spin state of a single, trapped Sr+ ion. We significantly increase its sensitivity to external fields while extending phase coherence by three orders of magnitude, to more than one second. Using this technique, we measure frequency shifts with a sensitivity of 0.42 Hz Hz−1/2 (corresponding to a magnetic field measurement sensitivity of 15 pT Hz−1/2), obtaining an uncertainty of less than 10 mHz (350 fT) after 3,720 seconds of averaging. These sensitivities are limited by quantum projection noise and improve on other single-spin probe technologies2, 3 by two orders of magnitude. Our reported sensitivity is sufficient for the measurement of parity non-conservation4, as well as the detection of the magnetic field of a single electronic spin one micrometre from an ion detector with nanometre resolution. As a first application, we perform light shift spectroscopy of a narrow optical quadrupole transition. Finally, we emphasize that the quantum lock-in technique is generic and can potentially enhance the sensitivity of any quantum sensor.

**Groupmeeting by Dylan Jervis, May 18th, 2011**

Shortcut to adiabaticity for an interacting Bose-Einstein condensate

2011-05-12T10:23:00.000-04:00

Jean-François Schaff, Xiao-Li Song, Pablo Capuzzi, Patrizia Vignolo, Guillaume Labeyrie

We present an investigation of the fast decompression of a three-dimensional (3D) Bose-Einstein condensate (BEC) at finite temperature using an engineered trajectory for the harmonic trapping potential. Taking advantage of the scaling invariance properties of the time-dependent Gross-Pitaevskii equation, we exhibit a solution yielding a final state identical to that obtained through a perfectly adiabatic transformation, in a much shorter time. Experimentally, we perform a large trap decompression and displacement within a time comparable to the final radial trapping period. By simultaneously monitoring the BEC and the non-condensed fraction, we demonstrate that our specific trap trajectory is valid both for a quantum interacting many-body system and a classical ensemble of non-interacting particles.

**Groupmeeting by Zhenfu Zhang, May 11th, 2011**

Self contained quantum heat engines

2011-05-12T10:20:00.000-04:00

Noah Linden, Sandu Popescu, Paul Skrzypczyk

We construct the smallest possible self contained heat engines; one composed of only two qubits, the other of only a single qutrit. The engines are self-contained as they do not require external sources of work and/or control. They are able to produce work which is used to continuously lift a weight. Despite the dimension of the engine being small, it is still able to operate at the Carnot efficiency.

**Groupmeeting by Chao Zhuang, May 4th, 2011**

Towards Quantum Chemistry On a Quantum Computer

2011-04-28T11:55:00.003-04:00

B. P. Lanyon, J. D. Whitfield, G. G. Gillett, M. E. Goggin, M. P. Almeida, I. Kassal, J. D. Biamonte, M. Mohseni, B. J. Powell, M. Barbieri, A. Aspuru-Guzik & A. G. White

Exact first-principles calculations of molecular properties are currently intractable because their computational cost grows exponentially with both the number of atoms and basis set size. A solution is to move to a radically different model of computing by building a quantum computer, which is a device that uses quantum systems themselves to store and process data. Here we report the application of the latest photonic quantum computer technology to calculate properties of the smallest molecular system: the hydrogen molecule in a minimal basis. We calculate the complete energy spectrum to 20 bits of precision and discuss how the technique can be expanded to solve large-scale chemical problems that lie beyond the reach of modern supercomputers. These results represent an early practical step toward a powerful tool with a broad range of quantum-chemical applications.

**Groupmeeting by Dylan Mahler, Apr 27th, 2011**

Spin Drag in a Perfect Fluid

2011-04-21T16:40:00.001-04:00

Ariel Sommer, Mark Ku, Giacomo Roati, & Martin W. Zwierlein

Transport of fermions, particles with half-integer spin, is central to many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin is being explored as a new carrier of information¹. Neutrino transport energizes supernova explosions following the collapse of a dying star², and hydrodynamic transport of the quark–gluon plasma governed the expansion of the early Universe³. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics⁴. Even above the superfluid transition, such gases flow as an almost perfect fluid with very low viscosity when interactions are tuned to a scattering resonance^{3, 5, 6, 7, 8}. In this hydrodynamic regime, collective density excitations are weakly damped^{6, 7}. Here we experimentally investigate spin excitations in a Fermi gas of ⁶Li atoms, finding that, in contrast, they are maximally damped. A spin current is induced by spatially separating two spin components and observing their evolution in an external trapping potential. We demonstrate that interactions can be strong enough to reverse spin currents, with components of opposite spin reflecting off each other. Near equilibrium, we obtain the spin drag coefficient, the spin diffusivity and the spin susceptibility as a function of temperature on resonance and show that they obey universal laws at high temperatures. In the degenerate regime, the spin diffusivity approaches a value set by /m, the quantum limit of diffusion, where /m is Planck’s constant divided by 2π and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state^{9, 10, 11}.

**Groupmeeting by Alma Bardon, Apr 20th, 2011**

A Single-Atom Quantum Memory

2011-04-17T15:52:00.001-04:00

Holger P. Specht, Christian Nölleke, Andreas Reiserer, Manuel Uphoff, Eden Figueroa, Stephan Ritter, Gerhard Rempe

The faithful storage of a quantum bit of light is essential for long-distance quantum communication, quantum networking and distributed quantum computing. The required optical quantum memory must, first, be able to receive and recreate the photonic qubit and, second, store an unknown quantum state of light better than any classical device. These two requirements have so far been met only by ensembles of material particles storing the information in collective excitations. Recent developments, however, have paved the way for a new approach in which the information exchange happens between single quanta of light and matter. This single-particle approach allows one to address the material qubit and thus has fundamental advantages for realistic implementations: First, to combat inevitable losses and finite efficiencies, it enables a heralding mechanism that signals the successful storage of a photon by means of state detection. Second, it allows for individual qubit manipulations, opening up avenues for in situ processing of the stored quantum information. Here we demonstrate the most fundamental implementation of such a quantum memory by mapping arbitrary polarization states of light into and out of a single atom trapped inside an optical cavity. The memory performance is analyzed through full quantum process tomography. The average fidelity is measured to be 93% and low decoherence rates result in storage times exceeding 180\mu s. This makes our system a versatile quantum node with excellent perspectives for optical quantum gates and quantum repeaters.

**Groupmeeting by Greg Dmochowski, Apr 6th, 2011**

Quantum ground state and single-phonon control of a mechanical resonator

2011-03-24T15:40:00.003-04:00

A. D. O’Connell, M. Hofheinz, M. Ansmann, Radoslaw C. Bialczak, M. Lenander, Erik Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, John M. Martinis & A. N. Cleland

Quantum mechanics provides a highly accurate description of a wide variety of physical systems. However, a demonstration that quantum mechanics applies equally to macroscopic mechanical systems has been a long-standing challenge, hindered by the difficulty of cooling a mechanical mode to its quantum ground state. The temperatures required are typically far below those attainable with standard cryogenic methods, so significant effort has been devoted to developing alternative cooling techniques. Once in the ground state, quantum-limited measurements must then be demonstrated. Here, using conventional cryogenic refrigeration, we show that we can cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator—a ‘quantum drum’—coupled to a quantum bit, which is used to measure the quantum state of the resonator. We further show that we can controllably create single quantum excitations (phonons) in the resonator, thus taking the first steps to complete quantum control of a mechanical system.

**Groupmeeting by Dave McKay, Mar 23rd, 2011**

Efficient Measurement of Quantum Dynamics via Compressive Sensing

2011-03-21T13:20:00.002-04:00

A. Shabani, R. L. Kosut, M. Mohseni, H. Rabitz, M. A. Broome, M. P. Almeida, A. Fedrizzi, and A. G. White

The resources required to characterize the dynamics of engineered quantum systems—such as quantum computers and quantum sensors—grow exponentially with system size. Here we adapt techniques from compressive sensing to exponentially reduce the experimental configurations required for quantum process tomography. Our method is applicable to processes that are nearly sparse in a certain basis and can be implemented using only single-body preparations and measurements. We perform efficient, high-fidelity estimation of process matrices of a photonic two-qubit logic gate. The database is obtained under various decoherence strengths. Our technique is both accurate and noise robust, thus removing a key roadblock to the development and scaling of quantum technologies.

**Groupmeeting by Ardavan Darabi, Mar 16th, 2011**

Making optical atomic clocks more stable with 10^−16-level laser stabilization

2011-03-11T13:50:00.001-05:00

Y. Y. Jiang, A. D. Ludlow, N. D. Lemke, R. W. Fox, J. A. Sherman, L.-S. Ma & C. W. Oates

The superb precision of an atomic clock is derived from its stability. Atomic clocks based on optical (rather than microwave) frequencies are attractive because of their potential for high stability, which scales with operational frequency. Nevertheless, optical clocks have not yet realized this vast potential, due in large part to limitations of the laser used to excite the atomic resonance. To address this problem, we demonstrate a cavity-stabilized laser system with a reduced thermal noise floor, exhibiting a fractional frequency instability of 2 × 10−16. We use this laser as a stable optical source in a ytterbium optical lattice clock to resolve an ultranarrow 1 Hz linewidth for the 518 THz clock transition. With the stable laser source and the signal-to-noise ratio afforded by the ytterbium optical clock, we dramatically reduce key stability limitations of the clock, and make measurements consistent with a clock instability of 5 × 10−16 .

**Groupmeeting by Matin Hallaji, Mar 9th, 2011**

Itinerant Ferromagnetism in Ultracold Fermions

2011-03-04T11:48:00.002-05:00

Gyu-Boong Jo, Ye-Ryoung Lee, Jae-Hoon Choi, Caleb A. Christensen, Tony H. Kim, Joseph H. Thywissen, David E. Pritchard and Wolfgang Ketterle

Can a gas of spin-up and spin-down fermions become ferromagnetic because of repulsive interactions? We addressed this question, for which there is not yet a definitive theoretical answer, in an experiment with an ultracold two-component Fermi gas. The observation of nonmonotonic behavior of lifetime, kinetic energy, and size for increasing repulsive interactions provides strong evidence for a phase transition to a ferromagnetic state. Our observations imply that itinerant ferromagnetism of delocalized fermions is possible without lattice and band structure, and our data validate the most basic model for ferromagnetism introduced by Stoner.

**Groupmeeting by Boris Braverman, Mar 2nd, 2011**

Broadband waveguide quantum memory for entangled photons

2011-02-23T18:19:00.002-05:00

Erhan Saglamyurek, Neil Sinclair, Jeongwan Jin, Joshua A. Slater, Daniel Oblak, Félix Bussières, Mathew George, Raimund Ricken, Wolfgang Sohler & Wolfgang Tittel

The reversible transfer of quantum states of light into and out of matter constitutes an important building block for future applications of quantum communication: it will allow the synchronization of quantum information¹, and the construction of quantum repeaters² and quantum networks³. Much effort has been devoted to the development of such quantum memories¹, the key property of which is the preservation of entanglement during storage. Here we report the reversible transfer of photon–photon entanglement into entanglement between a photon and a collective atomic excitation in a solid-state device. Towards this end, we employ a thulium-doped lithium niobate waveguide in conjunction with a photon-echo quantum memory protocol⁴, and increase the spectral acceptance from the current maximum⁵ of 100 megahertz to 5 gigahertz. We assess the entanglement-preserving nature of our storage device through Bell inequality violations⁶ and by comparing the amount of entanglement contained in the detected photon pairs before and after the reversible transfer. These measurements show, within statistical error, a perfect mapping process. Our broadband quantum memory complements the family of robust, integrated lithium niobate devices⁷. It simplifies frequency-matching of light with matter interfaces in advanced applications of quantum communication, bringing fully quantum-enabled networks a step closer.

**Groupmeeting by Shreyas Potnis, Feb 23rd, 2011**

Evidence for orbital superfluidity in the P-band of a bipartite optical square lattice

2011-02-15T17:21:00.002-05:00

Georg Wirth, Matthias Ölschläger & Andreas Hemmerich

The successful emulation of the Hubbard model in optical lattices has stimulated extensive efforts to extend their scope to also capture more complex, incompletely understood scenarios of many-body physics. A promising approach is to consider higher bands, where the orbital degree of freedom gives rise to a structural diversity that is directly relevant, for example, for the physics of strongly correlated electronic matter. Here we report evidence for the formation of a superfluid in the P-band of a bipartite optical square lattice with S-orbits and P-orbits arranged in a chequerboard pattern. The observed momentum spectra feature cross-dimensional coherence with a lifetime of nearly 20 ms. Depending on the value of a small adjustable anisotropy of the lattice, our findings are explained either by real-valued striped superfluid order parameters with different orientations P_x±P_y, or by a complex-valued P_x±iP_y order parameter, which breaks time-reversal symmetry.

**Groupmeeting by Dan Fine, Feb 9th, 2011**

The uncertainty principle in the presence of quantum memory

2011-02-03T20:32:00.002-05:00

Mario Berta, Matthias Christandl,Roger Colbeck,Joseph M. Renes & Renato Renner

The uncertainty principle, originally formulated by Heisenberg1, clearly illustrates the difference between classical and quantum mechanics. The principle bounds the uncertainties about the outcomes of two incompatible measurements, such as position and momentum, on a particle. It implies that one cannot predict the outcomes for both possible choices of measurement to arbitrary precision, even if information about the preparation of the particle is available in a classical memory. However, if the particle is prepared entangled with a quantum memory, a device that might be available in the not-too-distant future2, it is possible to predict the outcomes for both measurement choices precisely. Here, we extend the uncertainty principle to incorporate this case, providing a lower bound on the uncertainties, which depends on the amount of entanglement between the particle and the quantum memory. We detail the application of our result to witnessing entanglement and to quantum key distribution.

**Groupmeeting by Lee Rozema, Feb 2nd, 2011**

Near-deterministic preparation of a single atom in an optical microtrap

2011-01-27T12:46:00.003-05:00

*T. Grünzweig,*A. Hilliard,*M. McGovern*& M. F. Andersen

Neutral atoms stored in optical traps are strong candidates for a physical realization of a quantum logic device1, 2. Far off-resonance optical traps provide conservative potentials and excellent isolation from the environment, and they may be arranged to produce arbitrary arrays of traps, where each trap is occupied by a single atom that can be individually addressed3, 4, 5, 6. At present, significant effort is being expended on developing two-qubit gates based on coupling individual Rydberg atoms in adjacent optical microtraps7, 8, 9. A major challenge associated with this approach is the reliable generation of single-atom occupancy in each trap, as the loading efficiency in the past experiments has been limited to 50% (refs 4, 7, 8, 10, 11, 12). Here we report a loading efficiency of 82.7% in an optical microtrap. We achieve this by manipulating the collisions between pairs of trapped atoms through tailored optical fields and directly observing the resulting single atoms in the trap.

**Groupmeeting by Graham Edge, Jan 26th, 2011**

Optomechanically Induced Transparency

2011-01-24T18:15:00.001-05:00

Stefan Weis, Rémi Rivière, Samuel Deléglise1, Emanuel Gavartin1, Olivier Arcizet3, Albert Schliesser and Tobias J. Kippenberg

Electromagnetically induced transparency is a quantum interference effect observed in atoms and molecules, in which the optical response of an atomic medium is controlled by an electromagnetic field. We demonstrated a form of induced transparency enabled by radiation-pressure coupling of an optical and a mechanical mode. A control optical beam tuned to a sideband transition of a micro-optomechanical system leads to destructive interference for the excitation of an intracavity probe field, inducing a tunable transparency window for the probe beam. Optomechanically induced transparency may be used for slowing and on-chip storage of light pulses via microfabricated optomechanical arrays.

**Groupmeeting by Amir Feizpour, Jan 19th, 2011**

Spin Hall Effect Transistor

2011-01-24T18:12:00.001-05:00

Jörg Wunderlich, Byong-Guk Park, Andrew C. Irvine, Liviu P. Zârbo, Eva Rozkotová, Petr Nemec, Vít Novák, Jairo Sinova and Tomás Jungwirth

The field of semiconductor spintronics explores spin-related quantum relativistic phenomena in solid-state systems. Spin transistors and spin Hall effects have been two separate leading directions of research in this field. We have combined the two directions by realizing an all-semiconductor spin Hall effect transistor. The device uses diffusive transport and operates without electrical current in the active part of the transistor. We demonstrate a spin AND logic function in a semiconductor channel with two gates. Our study shows the utility of the spin Hall effect in a microelectronic device geometry, realizes the spin transistor with electrical detection directly along the gated semiconductor channel, and provides an experimental tool for exploring spin Hall and spin precession phenomena in an electrically tunable semiconductor layer.

**Groupmeeting by Nathan Cheng, Jan 12th, 2011**

A time-symmetric formulation of quantum mechanics

2010-11-25T20:24:00.002-05:00

Yakir Aharonov, Sandu Popescu, and Jeff Tollaksen

Quantum mechanics allows one to independently select both the initial and final states of a single system. Such pre- and postselection reveals novel effects that challenge our ideas about what time is and how it flows.

**Groupmeeting by Greg Dmochowski, Nov 24th, 2010**

Generation of three-qubit entangled states using superconducting phase qubits

2010-11-18T18:49:00.001-05:00

Matthew Neeley, Radoslaw C. Bialczak, M. Lenander, E. Lucero, Matteo Mariantoni, A. D. O’Connell, D. Sank, H. Wang, M. Weides, J. Wenner, Y. Yin, T. Yamamoto, A. N. Cleland & John M. Martinis

Entanglement is one of the key resources required for quantum computation¹, so the experimental creation and measurement of entangled states is of crucial importance for various physical implementations of quantum computers². In superconducting devices³, two-qubit entangled states have been demonstrated and used to show violations of Bell’s inequality⁴ and to implement simple quantum algorithms⁵. Unlike the two-qubit case, where all maximally entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways⁶. These are typified by the states |GHZ = (|000 + |111)/ and |W = (|001 + |010 + |100)/ . Here we demonstrate the operation of three coupled superconducting phase qubits⁷ and use them to create and measure |GHZ and |W states. The states are fully characterized using quantum state tomography⁸ and are shown to satisfy entanglement witnesses⁹, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.

**Groupmeeting by Dylan Jervis, Nov 18th, 2010**

Dynamics of a tunable superfluid junction

2010-09-24T20:56:00.001-04:00

L. J. LeBlanc, A. B. Bardon, J. McKeever, M. H. T. Extavour, D. Jervis, J. H. Thywissen, F. Piazza, A. Smerzi

We study the population dynamics of a Bose-Einstein condensate in a double-well potential with tunable barrier height. In the regime of weak inter-well coupling, we observe Josephson plasma oscillations as expected. However, in the strong-coupling regime, a second frequency enters the dynamics. We explain the amplitude, frequency, and nature of these two modes by with Gross-Pitaevskii calculations throughout the weak- to strong-coupling crossover. Our results interpolate between two standard paradigms of superfluidity: hydrodynamics and Josephson dynamics.

**Groupmeeting by Lindsay LeBlanc, September 22nd, 2010**

Simple approach to the relation between laser frequency noise and laser line shape

2010-09-24T20:49:00.002-04:00

Gianni Di Domenico, Stéphane Schilt, and Pierre Thomann

Frequency fluctuations of lasers cause a broadening of their line shapes. Although the relation between the frequency noise spectrum and the laser line shape has been studied extensively, no simple expression exists to evaluate the laser linewidth for frequency noise spectra that does not follow a power law. We present a simple approach to this relation with an approximate formula for evaluation of the laser linewidth that can be applied to arbitrary noise spectral densities.

**Groupmeeting by Amir Feizpour, September 15th, 2010**

Thermometry with spin-dependent lattices

2010-09-24T20:47:00.001-04:00

D McKay and B DeMarco

We propose a method for measuring the temperature of strongly correlated phases of ultracold atom gases confined in spin-dependent optical lattices. In this technique, a small number of 'impurity' atoms—trapped in a state that does not experience the lattice potential—are in thermal contact with atoms bound to the lattice. The impurity serves as a thermometer for the system because its temperature can be straightforwardly measured using time-of-flight expansion velocity. This technique may be useful for resolving many open questions regarding thermalization in these isolated systems. We discuss the theory behind this method and demonstrate proof-of-principle experiments, including the first realization of a three-dimensional (3D) spin-dependent lattice in the strongly correlated regime.

**Groupmeeting by Dave McKay, September 8th, 2010**

Photons: Still Bosons

2010-09-01T18:44:00.002-04:00

D. English, V. V. Yashchuk, D. Budker

Using Bose-Einstein-statistics-forbidden two-photon excitation in atomic barium, we have limited the rate of statistics-violating transitions, as a fraction $\nu$ of an equivalent statistics-allowed transition rate, to $\nu<4.0\times10^{-11}$ at the 90% confidence level. This is an improvement of more than three orders of magnitude over the best previous result. Additionally, hyperfine-interaction enabling of the forbidden transition has been observed, to our knowledge, for the first time.

**Groupmeeting by Rockson Chang, September 1st, 2010**

ECDL with frequency modulation saturation spectroscopy laser lock

2010-08-27T15:36:00.001-04:00

We have built an "ECDL laser" in the lab and are locking it succesfully to the potassium D2-line using "frequency modulation spectroscopy".

**Groupmeeting by Felix Stubenrauch, August 25th, 2010**

An entangled-light-emitting diode

2010-08-12T11:30:00.001-04:00

C. L. Salter, R. M. Stevenson, I. Farrer, C. A. Nicoll, D. A. Ritchie & A. J. Shields

An optical quantum computer, powerful enough to solve problems so far intractable using conventional digital logic, requires a large number of entangled photons^{1, 2}. At present, entangled-light sources are optically driven with lasers^{3, 4, 5, 6, 7}, which are impractical for quantum computing owing to the bulk and complexity of the optics required for large-scale applications. Parametric down-conversion is the most widely used source of entangled light, and has been used to implement non-destructive quantum logic gates^{8, 9}. However, these sources are Poissonian^{4, 5} and probabilistically emit zero or multiple entangled photon pairs in most cycles, fundamentally limiting the success probability of quantum computational operations. These complications can be overcome by using an electrically driven on-demand source of entangled photon pairs¹⁰, but so far such a source has not been produced. Here we report the realization of an electrically driven source of entangled photon pairs, consisting of a quantum dot embedded in a semiconductor light-emitting diode (LED) structure. We show that the device emits entangled photon pairs under d.c. and a.c. injection, the latter achieving an entanglement fidelity of up to 0.82. Entangled light with such high fidelity is sufficient for application in quantum relays¹¹, in core components of quantum computing such as teleportation^{12, 13, 14}, and in entanglement swapping^{15, 16}. The a.c. operation of the entangled-light-emitting diode (ELED) indicates its potential function as an on-demand source without the need for a complicated laser driving system; consequently, the ELED is at present the best source on which to base future scalable quantum information applications¹⁷.

**Groupmeeting by Yasaman Soudagar, August 11th, 2010**

Thermalization of photons

2010-08-09T11:47:00.001-04:00

Jan Klaers1, Frank Vewinger1 & Martin Weitz1

Bose–Einstein condensation1, the macroscopic accumulation of bosonic particles in the energetic ground state below a critical temperature, has been demonstrated in several physical systems2, 3, 4, 5, 6, 7, 8. The perhaps best known example of a bosonic gas, blackbody radiation9, however exhibits no Bose–Einstein condensation at low temperatures10. Instead of collectively occupying the lowest energy mode, the photons disappear in the cavity walls when the temperature is lowered—corresponding to a vanishing chemical potential. Here we report on evidence for a thermalized two-dimensional photon gas with a freely adjustable chemical potential. Our experiment is based on a dye-filled optical microresonator, acting as a ‘white wall’ box for photons. Thermalization is achieved in a photon-number-conserving way by photon scattering off the dye molecules, and the cavity mirrors provide both an effective photon mass and a confining potential—key prerequisites for the Bose–Einstein condensation of photons. As a striking example of the unusual system properties, we demonstrate a yet unobserved light concentration effect into the centre of the confining potential, an effect with prospects for increasing the efficiency of diffuse solar light collection11.

**Groupmeeting by Xingxing Xing, August 4th, 2010**

Deterministic entanglement of two neutral atoms via Rydberg blockade

2010-08-09T11:43:00.002-04:00

X. L. Zhang, L. Isenhower, A. T. Gill, T. G. Walker, M. Saffman

We demonstrate the first deterministic entanglement of two individually addressed neutral atoms using a Rydberg blockade mediated controlled-NOT gate. Parity oscillation measurements reveal an entanglement fidelity of $F=0.58\pm0.04$, which is above the entanglement threshold of $F=0.5$, without any correction for atom loss, and $F=0.71\pm0.05$ after correcting for background collisional losses. The fidelity results are shown to be in good agreement with a detailed error model.

**Groupmeeting by Greg Dmochowski, July 28th, 2010**

Selective and Efficient Process Tomography

2010-07-27T17:10:00.003-04:00

Cecilia C. López, Ariel Bendersky, Juan Pablo Paz, David G. Cory

We present in a unified manner the existing methods for scalable partial quantum process tomography. We focus on two main approaches: the one presented in Bendersky et al. [Phys. Rev. Lett. 100, 190403 (2008)], and the ones described, respectively, in Emerson et al. [Science 317, 1893 (2007)] and L\'{o}pez et al. [Phys. Rev. A 79, 042328 (2009)], which can be combined together. The methods share an essential feature: They are based on the idea that the tomography of a quantum map can be efficiently performed by studying certain properties of a twirling of such a map. From this perspective, in this paper we present extensions, improvements and comparative analyses of the scalable methods for partial quantum process tomography. We also clarify the significance of the extracted information, and we introduce interesting and useful properties of the $\chi$-matrix representation of quantum maps that can be used to establish a clearer path toward achieving full tomography of quantum processes in a scalable way.

**Groupmeeting by Dylan Mahler, July 21st, 2010**

Noise-Powered Probabilistic Concentration of Phase Information

2010-07-18T16:57:00.002-04:00

Mario A. Usuga, Christian R. Mueller, Christoffer Wittmann, Petr Marek, Radim Filip, Christoph Marquardt, Gerd Leuchs, Ulrik L. Andersen

Phase insensitive optical amplification of an unknown quantum state is known to be a fundamentally noisy operation that inevitably adds noise to the amplified state [1 - 5]. However, this fundamental noise penalty in amplification can be circumvented by resorting to a probabilistic scheme as recently proposed and demonstrated in refs [6 - 8]. These amplifiers are based on highly non-classical resources in a complex interferometer. Here we demonstrate a probabilistic quantum amplifier beating the fundamental quantum limit utilizing a thermal noise source and a photon number subtraction scheme [9]. The experiment shows, surprisingly, that the addition of incoherent noise leads to a noiselessly amplified output state with a phase uncertainty below the uncertainty of the state prior to amplification. This amplifier might become a valuable quantum tool in future quantum metrological schemes and quantum communication protocols.

**Groupmeeting by Ardavan Darabi, July 14th, 2010**

Adding control to arbitrary quantum operations

2010-07-07T21:41:00.000-04:00

Xiao-Qi Zhou, Timothy C. Ralph, Pruet Kalasuwan, Mian Zhang, Alberto Peruzzo, Benjamin P. Lanyon, Jeremy L. O'Brien

Quantum computers promise exponential power for particular tasks, however, the complexity of quantum algorithms remains a major technological challenge. We have developed and demonstrated an architecture independent technique for adding control qubits to arbitrary quantum operations (unitary or otherwise) - a key requirement in many quantum algorithms. The technique is independent of how the operation is done and does not even require knowledge of what the operation is. In this way the technical problems of how to implement a quantum operation and how to add a control are separated. The number of computational resources required is independent of the depth of the operation and increases only linearly with the number of qubits on which it acts. Our approach will significantly reduce the complexity of quantum computations such as Shor's factoring algorithm and the near-term prospect of quantum simulations. We use this new approach to implement a number of two-qubit photonic quantum gates in which the operation of the control circuit is completed independent of the choice of quantum operation.

**Groupmeeting by Lee Rozema, July 7th, 2010**

Probing general relativity using atom interferometry

2010-07-07T10:01:00.002-04:00

T. van Zoest, N. Gaaloul, Y. Singh, H. Ahlers, W. Herr, S. T. Seidel, W. Ertmer, E. Rasel, M. Eckart, E. Kajari, S. Arnold, G. Nandi, W. P. Schleich, R. Walser, A. Vogel, K. Sengstock, K. Bongs, W. Lewoczko-Adamczyk, M. Schiemangk, T. Schuldt, A. Peters, T. Könemann, H. Müntinga, C. Lämmerzahl, H. Dittus, T. Steinmetz, T. W. Hänsch, J. Reichel

Albert Einstein’s insight that it is impossible to distinguisha local experiment in a "freely falling elevator" from one infree space led to the development of the theory of general relativity.The wave nature of matter manifests itself in a striking wayin Bose-Einstein condensates, where millions of atoms lose theiridentity and can be described by a single macroscopic wave function.We combine these two topics and report the preparation and observationof a Bose-Einstein condensate during free fall in a 146-meter-tallevacuated drop tower. During the expansion over 1 second, theatoms form a giant coherent matter wave that is delocalizedon a millimeter scale, which represents a promising source formatter-wave interferometry to test the universality of freefall with quantum matter.

**Groupmeeting by Timur Rvachov, June 30th, 2010**

Towards high-speed optical quantum memories

2010-06-23T09:12:00.002-04:00

K. F. Reim, J. Nunn, V. O. Lorenz, B. J. Sussman, K. C. Lee, N. K. Langford, D. Jaksc & I. A. Walmsley

Quantum memories, capable of controllably storing and releasing a photon, are a crucial component for quantum computers¹ and quantum communications². To date, quantum memories^3,^4,^5,⁶ have operated with bandwidths that limit data rates to megahertz. Here we report the coherent storage and retrieval of sub-nanosecond low-intensity light pulses with spectral bandwidths exceeding 1 GHz in caesium vapour. The novel memory interaction takes place through a far off-resonant two-photon transition in which the memory bandwidth is dynamically generated by a strong control field^7,⁸. This should allow data rates more than 100 times greater than those of existing quantum memories. The memory works with a total efficiency of 15%, and its coherence is demonstrated through direct interference of the stored and retrieved pulses. Coherence times in hot atomic vapours are on the order of microseconds⁹, the expected storage time limit for this memory.

**Groupmeeting by Amir Feizpour, June 16th, 2010**

Ground State Laser Cooling with Electromagnetically Induced Transparency

2010-06-23T09:08:00.004-04:00

C. F. Roos*, D. Leibfried, A. Mundt, F. Schmidt-Kaler, J. Eschner, and R. Blatt

Ground state laser cooling of a single trapped Ca+ ion is achieved with a technique which tailors the absorption profile for the cooling laser by exploiting electromagnetically induced transparency. Using the Zeeman structure of the S1/2 to P1/2 dipole transition we achieve up to 90% ground state probability. The new method is robust, easy to implement, and proves particularly useful for cooling several motional degrees of freedom simultaneously, which is of great practical importance for the implementation of quantum logic schemes with trapped ions.

**Groupmeeting by Chao Zhuang June 9th, 2010**

Suppression of Density Fluctuations in a Quantum Degenerate Fermi Gas

2010-06-23T09:05:00.003-04:00

Christian Sanner, Edward J. Su, Aviv Keshet, Ralf Gommers, Yong-il Shin, Wujie Huang, Wolfgang Ketterle

We study density profiles of an ideal Fermi gas and observe Pauli suppression of density fluctuations (atom shot noise) for cold clouds deep in the quantum degenerate regime. Strong suppression is observed for probe volumes containing more than 10,000 atoms. Measuring the level of suppression provides sensitive thermometry at low temperatures. After this method of sensitive noise measurements has been validated with an ideal Fermi gas, it can now be applied to characterize phase transitions in strongly correlated many-body systems.

**Groupmeeting by Dylan Jervis, June 2nd, 2010**

Non-Ground State BEC

2010-05-13T18:38:00.000-04:00

**Groupmeeting by Joon Cho**

Quantum Theory from 5 reasonable Axioms

2010-04-28T18:41:00.005-04:00

Lucien Hardy

The usual formulation of quantum theory is based on rather obscure axioms (employing complex Hilbert spaces, Hermitean operators, and the trace rule for calculating probabilities). In this paper it is shown that quantum theory can be derived from five very reasonable axioms. The first four of these are obviously consistent with both quantum theory and classical probability theory. Axiom 5 (which requires that there exists continuous reversible transformations between pure states) rules out classical probability theory. If Axiom 5 (or even just the word "continuous" from Axiom 5) is dropped then we obtain classical probability theory instead. This work provides some insight into the reasons quantum theory is the way it is. For example, it explains the need for complex numbers and where the trace formula comes from. We also gain insight into the relationship between quantum theory and classical probability theory.

**Groupmeeting by Omar Gamel**

Collective spin squeezing with a cavity

2010-04-28T18:37:00.002-04:00

Monika H. Schleier-Smith, Ian D. Leroux, Vladan Vuletić

We generate entangled states of an ensemble of 5*10^4 rubidium-87 atoms by optical quantum nondemolition measurement. The resonator-enhanced measurement leaves the atomic ensemble, prepared in a superposition of hyperfine clock levels, in a squeezed spin state. By comparing the resulting reduction of quantum projection noise (up to 8.8(8) dB) with the concomitant reduction of coherence, we demonstrate a clock input state with spectroscopic sensitivity 3.0(8) dB beyond the standard quantum limit.

**Groupmeeting by Lindsay LeBlanc**

Delocalization of a disordered bosonic system by repulsive interactions

2010-04-19T14:34:00.002-04:00

B. Deissler, M. Zaccanti, G. Roati, C. D?Errico, M. Fattori, M. Modugno, G. Modugno & M. Inguscio

In bosonic many-body systems, disorder tends to localize particles, whereas weak
repulsive interactions between the particles have a delocalizing effect. The crossover
between these regimes has now been studied experimentally, using an optical lattice to
control disorder and interactions independently.

**Groupmeeting by Rockson Chang**

Non-dispersive optics using storage of light

2010-04-19T14:29:00.003-04:00

Leon Karpa, Martin Weitz

We demonstrate the non-dispersive deflection of an optical beam in a Stern-Gerlach magnetic field. An optical pulse is initially stored as a spin-wave coherence in thermal rubidium vapour. An inhomogeneous magnetic field imprints a phase gradient onto the spin wave, which upon reacceleration of the optical pulse leads to an angular deflection of the retrieved beam. We show that the obtained beam deflection is non-dispersive, i.e. its magnitude is independent of the incident optical frequency. Compared to a Stern-Gerlach experiment carried out with propagating light under the conditions of electromagnetically induced transparency, the estimated suppression of the chromatic aberration reaches 10 orders of magnitude.

**Groupmeeting by Greg Dmochowski**

Quantum non-demolition measurements of a qubit coupled to a harmonic oscillator

2010-03-18T15:48:00.000-04:00

Luca Chirolli, Guido Burkard

We theoretically describe the weak measurement of a two-level system (qubit) and quantify the degree to which such a qubit measurement has a quantum non-demolition (QND) character. The qubit is coupled to a harmonic oscillator which undergoes a projective measurement. Information on the qubit state is extracted from the oscillator measurement outcomes, and the QND character of the measurement is inferred by the result of subsequent measurements of the oscillator. We use the positive operator valued measure (POVM) formalism to describe the qubit measurement. Two mechanisms lead to deviations from a perfect QND measurement: (i) the quantum fluctuations of the oscillator, and (ii) quantum tunneling between the qubit states $|0>$ and $|1>$ during measurements. Our theory can be applied to QND measurements performed on superconducting qubits coupled to a circuit oscillator.

**Groupmeeting by Xingxing Xing**

The Quantum Random Walk

2010-03-11T22:19:00.002-05:00

M. A. Broome, A. Fedrizzi, B. P. Lanyon, I. Kassal, A. Aspuru-Guzik, A. G. White

Quantum walks have a host of applications, ranging from quantum computing to the simulation of biological systems. We present an intrinsically stable, deterministic implementation of discrete quantum walks with single photons in space. The number of optical elements required scales linearly with the number of steps. We measure walks with up to 6 steps and explore the quantum-to-classical transition by introducing tunable decoherence. Finally, we also investigate the effect of absorbing boundaries and show that decoherence significantly affects the probability of absorption.

**Groupmeeting by Dylan Mahler**

An Invisible Quantum Tripwire

2010-03-03T08:05:00.002-05:00

Petr M. Anisimov, Daniel J. Lum, S. Blane McCracken, Jonathan P. Dowling.

We present here a quantum tripwire, which is a quantum optical interrogation technique capable of detecting an intrusion with very low probability of the tripwire being revealed to the intruder. Our scheme combines interaction-free measurement with the quantum Zeno effect in order to interrogate the presence of the intruder without interaction. The tripwire exploits a curious nonlinear behavior of the quantum Zeno effect we discovered, which occurs in a lossy system. We also employ a statistical hypothesis testing protocol, allowing us to calculate a confidence level of interaction-free measurement after a given number of trials. As a result, our quantum intruder alert system is robust against photon loss and dephasing under realistic atmospheric conditions and its design minimizes the probabilities of false positives and false negatives as well as the probability of becoming visible to the intruder.

**Groupmeeting by Ardavan Darabi**

Observation of a kilogram-scale oscillator near its quantum ground state

2010-02-26T15:04:00.002-05:00

LIGO Scientific Collaboration – B Abbott et a. lot.

We introduce a novel cooling technique capable of approaching the quantum ground state of a kilogram-scale system—an interferometric gravitational wave detector. The detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) operate within a factor of 10 of the standard quantum limit (SQL), providing a displacement sensitivity of 10−18 m in a 100 Hz band centered on 150 Hz. With a new feedback strategy, we dynamically shift the resonant frequency of a 2.7 kg pendulum mode to lie within this optimal band, where its effective temperature falls as low as 1.4 μK, and its occupation number reaches about 200 quanta. This work shows how the exquisite sensitivity necessary to detect gravitational waves can be made available to probe the validity of quantum mechanics on an enormous mass scale.

**Groupmeeting by Michael Sprague**

Violation of the Leggett-Garg inequality with weak measurements of photons

2010-02-26T14:57:00.002-05:00

M. E. Goggin, M. P. Almeida, M. Barbieri, B. P. Lanyon, J. L. O'Brien, A. G. White, G. J. Pryde

By weakly measuring the polarization of a photon between two strong polarization measurements, we experimentally investigate the correlation between the appearance of anomalous values in quantum weak measurements, and the violation of realism and non-intrusiveness of measurements. A quantitative formulation of the latter concept is expressed in terms of a Leggett-Garg inequality for the outcomes of subsequent measurements of an individual quantum system. We experimentally violate the Leggett-Garg inequality for several measurement strengths. Furthermore, we experimentally demonstrate that there is a one-to-one correlation between achieving strange weak values and violating the Leggett-Garg inequality.

**Groupmeeting by Lee Rozema**

Coherently wired light-harvesting in photosynthetic marine algae at ambient temperature

2010-02-11T12:42:00.003-05:00

Elisabetta Collini, Cathy Y. Wong, Krystyna E. Wilk, Paul M. G. Curmi, Paul Brumer & Gregory D. Scholes

Photosynthesis makes use of sunlight to convert carbon dioxide into useful biomass and is vital for life on Earth. Crucial components for the photosynthetic process are antenna proteins, which absorb light and transmit the resultant excitation energy between molecules to a reaction centre. The efficiency of these electronic energy transfers has inspired much work on antenna proteins isolated from photosynthetic organisms to uncover the basic mechanisms at play^1,^2,^3,^4,⁵. Intriguingly, recent work has documented^6,^7,⁸ that light-absorbing molecules in some photosynthetic proteins capture and transfer energy according to quantum-mechanical probability laws instead of classical laws⁹ at temperatures up to 180 K. This contrasts with the long-held view that long-range quantum coherence between molecules cannot be sustained in complex biological systems, even at low temperatures. Here we present two-dimensional photon echo spectroscopy^10,^11,^12,¹³ measurements on two evolutionarily related light-harvesting proteins isolated from marine cryptophyte algae, which reveal exceptionally long-lasting excitation oscillations with distinct correlations and anti-correlations even at ambient temperature. These observations provide compelling evidence for quantum-coherent sharing of electronic excitation across the 5-nm-wide proteins under biologically relevant conditions, suggesting that distant molecules within the photosynthetic proteins are ‘wired’ together by quantum coherence for more efficient light-harvesting in cryptophyte marine algae.

**Groupmeeting by Dylan Jervis**

Coherent control in the classical limit: Symmetry breaking in an optical lattice

2010-02-03T18:48:00.003-05:00

By Michael Spanner, Ignacio Franco, and Paul Brumer

The quantum-to-classical transition of a symmetry-breaking coherent control scenario is computationally demonstrated in an optical lattice arrangement. Control is shown to survive in the classical limit and, for small effective ℏ, to be comparable in magnitude to quantum control. Moderate decoherence is seen to eliminate structure from the momentum space distribution, but not to cause loss of control. The proposed scenario is designed so as to be demonstrable experimentally in a moving or shaken one-dimensional optical lattice.

**Groupmeeting by Chao Zhuang**

Implementation of a non-deterministic optical noiseless amplifier

2010-01-21T11:54:00.002-05:00

By Franck Ferreyrol, Marco Barbieri, Remi Blandino, Simon Fossier, Rosa Tualle-Brouri, Philippe Grangier

Quantum mechanics imposes that any amplifier that works independently on the phase of the input signal has to introduce some excess noise. The impossibility of such a noiseless amplifier is rooted into unitarity and linearity of quantum evolution. A possible way to circumvent this limitation is to interrupt such evolution via a measurement, providing a random outcome able to herald a successful - and noiseless - amplification event. Here we show a successful realisation of such an approach; we perform a full characterization of an amplified coherent state using quantum homodyne tomography, and observe a strong heralded amplification, with about 6dB gain and a noise level significantly smaller than the minimal allowed for any ordinary phase-independent device.

**Groupmeeting by Amir Feizpour**

Entropy Exchange in a Mixture of Ultracold Atoms

2009-10-19T14:30:00.002-04:00

By J. Catani, G. Barontini, G. Lamporesi, F. Rabatti, G. Thalhammer, F. Minardi, S. Stringari, and M. Inguscio

We investigate experimentally the entropy transfer between two distinguishable atomic quantum gases at ultralow temperatures. Exploiting a species-selective trapping potential, we are able to control the entropy of one target gas in presence of a second auxiliary gas. With this method, we drive the target gas into the degenerate regime in conditions of controlled temperature by transferring entropy to the auxiliary gas. We envision that our method could be useful both to achieve the low entropies required to realize new quantum phases and to measure the temperature of atoms in deep optical lattices. We verified the thermalization of the two species in a 1D lattice.

**Groupmeeting by Lindsay LeBlanc**

A phonon laser

2009-09-30T13:44:00.000-04:00

By K. Vahala, ..., & T. Hansch and Th. Udem

Red-detuned laser pumping of an atomic resonance will cool the motion of an ion or atom. The complementary regime of blue-detuned pumping is investigated in this work using a single, trapped Mg⁺ ion interacting with two laser beams, tuned above and below resonance. Widely thought of as a regime of heating, theory and experiment instead show that stimulated emission of centre-of-mass phonons occurs, providing saturable amplification of the motion. A threshold for transition from thermal to coherent oscillating motion has been observed, thus establishing this system as a mechanical analogue to an optical laser—a phonon laser. Such a system has been sought in many different physical contexts.

**Groupmeeting by Xing-xing Xing**

Phase shaping of single-photon wave packets

2009-09-23T13:47:00.000-04:00

By H.P. Specht, ..., & D. Rempe

Although the phase of a coherent light field can be precisely known, this is not true for the phase of the individual photons that create the field, considered individually¹. Phase changes within single-photon wave packets, however, have observable effects. In fact, actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols^2,³. Here we demonstrate arbitrary phase control of a single photon. The phase modulation is applied without affecting the photon's amplitude profile and is verified by means of a two-photon quantum interference measurement^4,⁵, demonstrating fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon's amplitude^6,^7,^8,^9,¹⁰, frequency¹¹, and polarization¹², the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets.

**Groupmeeting by Amir Feizpour**

Laser cooling by collisional redistribution of radiation

2009-09-16T13:49:00.001-04:00

By Ulrich Vogl & Martin Weitz

The general idea that optical radiation may cool matter was put forward 80 years ago1. Doppler cooling of dilute atomic gases is an extremely successful application of this concept2,3.More recently, anti-Stokes cooling in multilevel systems has been explored4,5, culminating in the optical refrigeration of solids6–9. Collisional redistribution of radiation has been proposed10 as a different cooling mechanism for atomic two-level systems, although experimental investigations using moderate-density gases have not reached the cooling regime11. Here we experimentally demonstrate laser cooling of an atomic gas based on collisional redistribution of radiation, using rubidiumatoms in argon buffer gas at a pressure of 230 bar. The frequent collisions in the ultradense gas transiently shift a highly red-detuned laser beam(that is, one detuned to amuch lower frequency) into resonance,whereas spontaneous decay occurs close to the unperturbed atomic resonance frequency. During each excitation cycle, kinetic energy of order kBT—that is, the thermal energy (kB, Boltzmann’s constant; T, temperature)—is extracted from the dense atomic sample. In a proof-of-principle experiment with a thermally non-isolated sample, we demonstrate relative cooling by 66 K. The cooled gas has a density more than ten orders of magnitude greater than the typical values used in Doppler-cooling experiments, and the cooling power reaches 87mW. Future applications of the technique may include supercooling beyond the homogeneous nucleation temperature12,13 and optical chillers9.

**Groupmeeting by Dylan Jervis**

Optical entanglement of co-propagating modes

2009-09-09T12:53:00.000-04:00

By J. Janousek, .., & H.A. Bachor

Optical entanglement is a key requirement for many quantum communication protocols1. Conventionally, entanglement is formed between two distinct beams, with the quantum corre- lation measurements being performed at separate locations. Such setups can be complicated, requiring the repeated combi- nation of complex resources, a task that becomes increasingly difficult as the number of entangled information channels, or modes, increases. We pave the way towards the realization of optical multimode quantum information systems by showing continuous variable entanglement between two spatial modes within one beam. Our technique is a major advance towards practical systems with minimum complexity. We demonstrate three major experimental achievements. First, only one source is required to produce squeezed light in two orthogonal spatial modes. Second, entanglement is formed through lenses and beam rotation, without the need for a beamsplitter. Finally, quantum correlations are measured directly and simul- taneously using a multipixel quadrant detector.

**Group Meeting By Zachari Medendorp**

Simple pulses for elimination of leakage in weakly nonlinear qubits

2009-09-02T13:37:00.000-04:00

By F. Motzoi, ..., F. K. Wilhelm

In realizations of quantum computing, a two-level system (qubit) is often singled out of the many levels of an anharmonic oscillator. In these cases, simple qubit control fails on short time scales because of coupling to leakage levels. We provide an easy to implement analytic formula that inhibits this leakage from any single-control analog or pixelated pulse. It is based on adding a second control that is proportional to the time-derivative of the first. For realistic parameters of superconducting qubits, this strategy reduces the error by an order of magnitude relative to the state of the art, all based on smooth and feasible pulse shapes. These results show that even weak anharmonicity is sufficient and in general not a limiting factor for implementing quantum gates.

**Groupmeeting by Chao Zhuang**

A High Phase-Space-Density Gas of Polar Molecules

2009-08-19T13:31:00.001-04:00

By K.-K. Ni, ..., & D. Jin and J. Ye

A quantum gas of ultracold polar molecules, with long-rangeand anisotropic interactions, not only would enable explorationsof a large class of many-body physics phenomena but also couldbe used for quantum information processing. We report on thecreation of an ultracold dense gas of potassium-rubidium (⁴⁰K⁸⁷Rb)polar molecules. Using a single step of STIRAP (stimulated Ramanadiabatic passage) with two-frequency laser irradiation, wecoherently transfer extremely weakly bound KRb molecules tothe rovibrational ground state of either the triplet or thesinglet electronic ground molecular potential. The polar moleculargas has a peak density of 10¹² per cubic centimeter and an expansion-determinedtranslational temperature of 350 nanokelvin. The polar moleculeshave a permanent electric dipole moment, which we measure withStark spectroscopy to be 0.052(2) Debye (1 Debye = 3.336 x 10^–30coulomb-meters) for the triplet rovibrational ground state and0.566(17) Debye for the singlet rovibrational ground state.

**Groupmeeting by Karl Pilch**

Measure for the Non-Markovianity of Quantum Processes

2009-08-05T13:14:00.000-04:00

By Heinz-Peter Breuer, et. al.

We construct a general measure for the degree of non-Markovian behavior in open quantum systems. This measure is based on the trace distance which quantifies the distinguishability of quantum states. It represents a functional of the dynamical map describing the time evolution of physical states, and can be interpreted in terms of the information flow between the open system and its environment. The measure takes on nonzero values whenever there is a flow of information from the environment back to the open system, which is the key feature of non-Markovian dynamics.

**Groupmeeting by Asma Al-Qasimi**

Collective Oscillations of an Imbalanced Fermi Gas: Axial Compression Modes and Polaron Effective Mass

2009-07-29T13:19:00.001-04:00

By S. Nascimbene, ..., & C. Salomon

We investigate the low-lying compression modes of a unitary Fermi gas with imbalanced spin populations. For low polarization, the strong coupling between the two spin components leads to a hydrodynamic behavior of the cloud. For large population imbalance we observe a decoupling of the oscillations of the two spin components, giving access to the effective mass of the Fermi polaron, a quasi-particle composed of an impurity dressed by particle-hole pair excitations in a surrounding Fermi sea. We find $m^*/m=1.17(10)$, in agreement with the most recent theoretical predictions.

**Groupmeeting by Jason McKeever**

Entangled Mechanical Oscillators

2009-07-22T13:27:00.001-04:00

By J.D. Jost, ..., & D. Wineland

Superposition and entanglement are hallmarks of quantum mechanics. One system ubiquitous to nature where entanglement has not previously been shown is distinct mechanical oscillators, such as springs or pendula. Here, deterministic entanglement of separated mechanical oscillators—consisting of the vibrational states of two pairs of atomic ions held in different locations—is demonstrated.

**Groupmeeting by Chris Paul**

Quantum Walk in Position Space with Single Optically Trapped Atoms

2009-07-15T13:11:00.000-04:00

By Michal Karski, ..., & Dieter Meschede

The quantum walk is the quantum analog of the well-known randomwalk, which forms the basis for models and applications in manyrealms of science. Its properties are markedly different fromthe classical counterpart and might lead to extensive applicationsin quantum information science. In our experiment, we implementeda quantum walk on the line with single neutral atoms by deterministicallydelocalizing them over the sites of a one-dimensional spin-dependentoptical lattice. With the use of site-resolved fluorescenceimaging, the final wave function is characterized by local quantumstate tomography, and its spatial coherence is demonstrated.Our system allows the observation of the quantum-to-classicaltransition and paves the way for applications, such as quantumcellular automata.

**Groupmeeting by Alma Bardon**

Attosecond Ionization and Tunneling Delay Time Measurements in Helium

2009-07-08T13:04:00.000-04:00

By P. Eckle, ... , & U. Keller

It is well established that electrons can escape from atomsthrough tunneling under the influence of strong laser fields,but the timing of the process has been controversial and fartoo rapid to probe in detail. We used attosecond angular streakingto place an upper limit of 34 attoseconds and an intensity-averagedupper limit of 12 attoseconds on the tunneling delay time instrong field ionization of a helium atom. The ionization fieldderives from 5.5-femtosecond-long near-infrared laser pulseswith peak intensities ranging from 2.3 x 10¹⁴ to 3.5 x 10¹⁴watts per square centimeter (corresponding to a Keldysh parametervariation from 1.45 to 1.17, associated with the onset of efficienttunneling). The technique relies on establishing an absolutereference point in the laboratory frame by elliptical polarizationof the laser pulse, from which field-induced momentum shiftsof the emergent electron can be assigned to a temporal delayon the basis of the known oscillation of the field vector..

**Groupmeeting by Rockson Chang**

Control of a magnetic Feshbach resonance with laser light

2009-06-29T11:09:00.005-04:00

By Dominik M. Bauer, Matthias Lettner, Christoph Vo, Gerhard Rempe & Stephan Dürr

The capability to tune the strength of the elastic interparticle interaction is crucial for many experiments with ultracold gases. Magnetic Feshbach resonances [1, 2] are widely harnessed for this purpose, but future experiments [3, 4, 5, 6, 7, 8] would benefit from extra flexibility, in particular from the capability to spatially modulate the interaction strength on short length scales. Optical Feshbach resonances [9, 10, 11, 12, 13, 14, 15] do offer this possibility in principle, but in alkali atoms they induce rapid loss of particles due to light-induced inelastic collisions. Here, we report experiments that demonstrate that light near-resonant with a molecular bound-to-bound transition in 87Rb can be used to shift the magnetic field at which a magnetic Feshbach resonance occurs. This enables us to tune the interaction strength with laser light, but with considerably less loss than using an optical Feshbach resonance.

**Groupmeeting by Adam Weir**

Bio-Imaging and Super-resolution

2009-06-29T10:54:00.002-04:00

Imaging Intracellular Fluorescent Proteins at Nanometer Resolution

Eric Betzig,¹^,2^* George H. Patterson,³ Rachid Sougrat,³ O. Wolf Lindwasser,³ Scott Olenych,⁴Juan S. Bonifacino,³ Michael W. Davidson,⁴ Jennifer Lippincott-Schwartz,³ Harald F. Hess⁵^*

We introduce a method for optically imaging intracellular proteinsat nanometer spatial resolution. Numerous sparse subsets ofphotoactivatable fluorescent protein molecules were activated,localized (to 2 to 25 nanometers), and then bleached. The aggregateposition information from all subsets was then assembled intoa superresolution image. We used this method—termed photoactivatedlocalization microscopy—to image specific target proteinsin thin sections of lysosomes and mitochondria; in fixed wholecells, we imaged vinculin at focal adhesions, actin within alamellipodium, and the distribution of the retroviral proteinGag at the plasma membrane.

Three-Dimensional Super-Resolution Imaging by Stochastic Optical Reconstruction Microscopy

Bo Huang,¹^,2 Wenqin Wang,³ Mark Bates,⁴ Xiaowei Zhuang¹^,2^,3^*

^{Recent advances in far-field fluorescence microscopy have ledto substantial improvements in image resolution, achieving anear-molecular resolution of 20 to 30 nanometers in the twolateral dimensions. Three-dimensional (3D) nanoscale-resolutionimaging, however, remains a challenge. We demonstrated 3D stochasticoptical reconstruction microscopy (STORM) by using optical astigmatismto determine both axial and lateral positions of individualfluorophores with nanometer accuracy. Iterative, stochasticactivation of photoswitchable probes enables high-precision3D localization of each probe, and thus the construction ofa 3D image, without scanning the sample. Using this approach,we achieved an image resolution of 20 to 30 nanometers in thelateral dimensions and 50 to 60 nanometers in the axial dimension.This development allowed us to resolve the 3D morphology ofnanoscopic cellular structures.}

Interferometric fluorescent super-resolution microscopy resolves 3D cellular ultrastructure

Gleb Shtengel^a,
James A. Galbraith^b,
Catherine G. Galbraith^c,
Jennifer Lippincott-Schwartz^d,¹,
Jennifer M. Gillette^d,
Suliana Manley^d,
Rachid Sougrat^d,
Clare M. Waterman^e,
Pakorn Kanchanawong^e,
Michael W. Davidson^f,
Richard D. Fetter^a and
Harald F. Hess^a,¹

Understanding molecular-scale architecture of cells requires determination of 3D locations of specific proteins with accuracy matching their nanometer-length scale. Existing electron and light microscopy techniques are limited either in molecular specificity or resolution. Here, we introduce interferometric photoactivated localization microscopy (iPALM), the combination of photoactivated localization microscopy with single-photon, simultaneous multiphase interferometry that provides sub-20-nm 3D protein localization with optimal molecular specificity. We demonstrate measurement of the 25-nm microtubule diameter, resolve the dorsal and ventral plasma membranes, and visualize the arrangement of integrin receptors within endoplasmic reticulum and adhesion complexes, 3D protein organization previously resolved only by electron microscopy. iPALM thus closes the gap between electron tomography and ight microscopy, enabling both molecular specification and resolution of cellular nanoarchitecture.

**Groupmeeting by Krister Shalm**

Q-bits from Nitrogen Vacancy Centers in Diamond

2009-06-11T12:31:00.009-04:00

The first half of the talk is a background on nitrogen-vacancy defects. Some references on this are:

Optical Properties of Solids by Mark Fox,
Spin-flip and spin-conserving optical transitions of the nitrogen-vacancy centre in diamond, NJP 10, 045004 (2008)
Ab initio supercell calculations on nitrogen-vacancy center in diamond: Electronic structure and hyperfine tensors, PRB 79, 075203 (2009)
Quantum Mechanics by Landau and Lifshitz (good reference for symmetry groups)

The second half of the talk was based on the paper Coherent Dynamics of Coupled Electron and Nuclear Spins in Diamond. L. Childress et al., Science 314, 281 (2006)

Abstract: Understanding and controlling the complex environment of solid-state quantum bits is a central challenge in spintronics and quantum information science. Coherent manipulation of an individual electron spin associated with a nitrogen-vacancy center in diamond was used to gain insight into its local environment. We show that this environment is effectively separated into a set of individual proximal 13C nuclear spins, which are coupled coherently to the electron spin, and the remainder of the 13C nuclear spins, which cause the loss of coherence. The proximal nuclear spins can be addressed and coupled individually because of quantum back-action from the electron, which modifies their energy levels and magnetic moments, effectively distinguishing them from the rest of the nuclei. These results open the door to coherent manipulation of individual isolated nuclear spins in a solid-state environment even at room temperature.

**Groupmeeting by David McKay**

Theoretical On-Demand Adiabatic Transfer of Light Between Adjacent Optical Cavities

2009-06-08T11:58:00.003-04:00

By Nick Chisholm, Ian Linington, Duncan O'Dell

Cavity quantum electrodynamics is one of the most promising systems for realizing quantum computing and communication. One of the most important problems facing researchers today is ﬁnding a way to coherently transfer light between two optical cavities connected by an optical ﬁbre. In this work, we provide a method for coherently transferring light between two adjacent optical cavities that share a slightly transmissive common mirror. By using the mode structure of our model, we expect a Landau-Zener adiabatic approach to the time dependence of the shared mirror’s position will allow for this coherent transfer. We believe this work is the ﬁrst step towards resolving the problem involving two optical cavities connected by an optical ﬁbre.

**Groupmeeting by Nick Chisholm**

Efficient all-optical switching using slow light within a hollow-core fiber

2009-05-29T11:49:00.005-04:00

By M. Bajcsy, ... , V. Vuletic & M. Lukin

Wedemonstrate a fiber-optical switch that is activated at tiny energiescorresponding to a few hundred optical photons per pulse. Thisis achieved by simultaneously confining both photons and a smalllaser-cooled ensemble of atoms inside the microscopic hollow core ofa single-mode photonic-crystal fiber and using quantum optical techniques forgenerating slow light propagation and large nonlinear interaction between lightbeams.

**Groupmeeting by Luciano Cruz**

Complete path entanglement of two photons

2009-05-14T11:33:00.004-04:00

By A. Rossi, ... , F. De Martini & P Mattaloni

We present a novel optical device based on an integrated system of microlenses and single mode optical fibers. It allows to send in many directions two photons generated by spontaneous parametric down conversion. By this device multiqubit entangled states and/or multilevel qu-dit states of two photons, encoded in the longitudinal momentum degree of freedom, are created. The
multipath photon entanglement realized by this device is expected to find important applications in modern quantum information technology.

**Groupmeeting by Yasaman Soudagar**

Experimental Quantum Process Discrimination

2009-05-11T15:35:00.005-04:00

By A. Laing, T. Rudolph & J. O'Brien

Discriminationbetween unknown processes chosen from a finite set is experimentallyshown to be possible even in the case of nonorthogonalprocesses. We demonstrate unambiguous deterministic quantum process discrimination of nonorthogonalprocesses using properties of entanglement, additional known unitaries, or classicalcommunication. Single qubit measurement and unitary processes and multipartite unitaries(where the unitary acts nonseparably across two distant locations) actingon photons are discriminated with a confidence of >97% inall cases.

**Groupmeeting by Lee Rozema**

Probing Interactions Between Ultracold Fermions

2009-04-30T19:37:00.006-04:00

By G. K. Campbell, ... , Jun Ye, A. D. Ludlow

At ultracold temperatures, the Pauli exclusion principle suppresses collisions between identical fermions. This has motivated the development of atomic clocks with fermionic isotopes. However,by probing an optical clock transition with thousands of lattice-confined, ultracold fermionic strontium atoms, we observed density-dependent collisional frequency shifts. These collision effects were measured systematically and are supported by a theoretical description attributing them to inhomogeneities in the probe excitation process that render the atoms distinguishable. This work also yields insights for zeroing the clock density shift.

**Groupmeeting by Mr. T**

2009-04-23T12:16:00.011-04:00

Repulsive Fermi Gas in a harmonic trap: Ferromagnetism and spin textures
by L. LeBlanc, ...., A. Paramekanti

We study ferromagnetism in a repulsively interacting two-component Fermi gas in a harmonic trap at zero net magnetization. Within a local density approximation, the two components phase-separate beyond a critical interaction strength, with one species having a higher density at the trap center. The mean field release energy depends on the interaction strength and contains a sharp signature of this transition. To go beyond the local density approximation, we derive an energy functional which includes a term that depends on the local magnetization gradient and acts as a `surface tension'. Using this energy functional, we numerically study the energetics of some candidate spin textures which may be stabilized in a harmonic trapping potential. We find that a hedgehog state has a lower energy than an `in-out' domain wall state in an isotropic trap. Upon inclusion of trap anisotropy we find that the hedgehog magnetization profile gets distorted due to the surface tension term, this distortion being more apparent for small atom numbers. We estimate that the magnetic dipole interaction does not play a significant role in this system. We consider implications for experiments on trapped two-component Fermi systems such as Li-6 and K-40 gases.

**Groupmeeting by Lindsay LeBlanc**

2009-04-21T12:04:00.006-04:00

Observation of Rydberg blockade between two atoms
by E. Urban, ...., T. Walker and M. Saffman

Blockade interactions whereby a single particle prevents the flow or excitation of other particles provide a mechanism for control of quantum states, including entanglement of two or more particles. Blockade has been observed for electrons^1,^2,³, photons⁴ and cold atoms⁵. Furthermore, dipolar interactions between highly excited atoms have been proposed as a mechanism for 'Rydberg blockade'^6,⁷, which might provide a novel approach to a number of quantum protocols^8,^9,^10,¹¹. Dipolar interactions between Rydberg atoms were observed several decades ago¹²^13,^14,^15,^16,^17,¹⁸. However, to harness Rydberg blockade for controlled quantum dynamics, it is necessary to achieve strong interactions between single pairs of atoms. Here, we demonstrate that a single Rydberg-excited rubidium atom blocks excitation of a second atom located more than 10 m away. The observed probability of double excitation is less than 20%, consistent with a theoretical model of the Rydberg interaction augmented by Monte Carlo simulations that account for experimental imperfections. and have been studied recently in a many-body regime using cold atoms

**Groupmeeting by Xingxing Xing**

2009-04-08T19:33:00.002-04:00

Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms
by Andre Schirotzek, ...., and Martin Zwierlein

We have observed Fermi polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid coexisting with a Fermi sea.

**Groupmeeting by Dylan Jervis**

2009-04-01T22:47:00.004-04:00

Violation of the principles of Complementarity, and its implications
by Shahriar Afshar

Bohr's principle of complementarity predicts that in a welcher weg ("which-way") experiment, obtaining fully visible interference pattern should lead to the destruction of the path knowledge. Here I report a failure for this prediction in an optical interferometry experiment. Coherent laser light is passed through a dual pinhole and allowed to go through a converging lens, which forms well-resolved images of the respective pinholes, providing complete path knowledge. A series of thin wires are then placed at previously measured positions corresponding to the dark fringes of the interference pattern upstream of the lens. No reduction in the resolution and total radiant flux of either image is found in direct disagreement with the predictions of the principle of complementarity. In this paper, a critique of the current measurement theory is offered, and a novel nonperturbative technique for ensemble properties is introduced. Also, another version of this experiment without an imaging lens is suggested, and some of the implications of the violation of complementarity for another suggested experiment to investigate the nature of the photon and its "empty wave" is briefly discussed.

**Groupmeeting by Zach Medendorp**

2009-03-25T18:45:00.002-04:00

Entanglement Test on a Microscopic-Macroscopic System
by F. De Martini, et. al.

Amacrostate consisting of N~3.5×10⁴ photons in a quantum superposition andentangled with a far apart single-photon state (microstate) is generated.Precisely, an entangled photon pair is created by a nonlinearoptical process; then one photon of the pair is injectedinto an optical parametric amplifier operating for any input polarizationstate, i.e., into a phase-covariant cloning machine. Such transformation establishesa connection between the single photon and the multiparticle fields.We then demonstrate the nonseparability of the bipartite system byadopting a local filtering technique within a positive operator valuedmeasurement.

**Groupmeeting by Amir Feizpour**

2009-03-11T15:23:00.000-04:00

The Wigner Distribution Function and its Optical Production
by H.O. Bartelt, et. al.

An optical signal (image etc.) can be described by its complex amplitude u (x,y), or by its spatial frequency spectrum. Both descriptions are complete and also equivalent, because one can be derived from the other by a Fourier transformation. Neither the complex amplitude nor the spatial frequency spectrum is suitable for answering a question like "what is the spatial frequency in a certain part of the image?". Here the term "local spectrum" is adequate. A rigorous definition of the "local spectrum" can be based on the Wigner distribution function. We developed optical methods for producing this "local spectrum" and we applied these methods to the investigation of sound patterns.

**Groupmeeting by Asma Al-Qasimi**

2009-03-05T18:40:00.000-05:00

Unambiguous modification of non-orthogonal qubit states
by F.A. Torres-Ruiz et. al.

A probabilistic method for the unambiguous modification of non-orthogonal quantum states is proposed. This is based on conclusive modifications of the inner product between two pure quantum states in a discrimination-like process. We experimentally implemented this protocol by using two-photon polarization states generated in the process of spontaneous parametric down conversion. In the experiment, for codifying initial quantum states, we consider single photon states and heralded detection. As application, we show that when our protocol is applied to partially entangled states it allows a fine control of the amount of entanglement of the modified states.

**Groupmeeting by Fabian Torres-Ruiz**

2009-02-26T15:26:00.000-05:00

Tripartite entanglement versus tripartite nonlocality in 3 qubit GHZ-class states
by Shohini Ghose et. al.

We analyze the relationship between tripartite entanglement and genuine tripartite nonlocality for 3-qubit pure states in the GHZ class. We consider a family of states known as the generalized GHZ states and derive an analytical expression relating the 3-tangle, which quantifies tripartite entanglement, to the Svetlichny inequality, which is a Bell-type inequality that is violated only when all three qubits are nonlocally correlated. We show that states with 3-tangle less than 1/2 do not violate the Svetlichny inequality. On the other hand, a set of states known as the maximal slice states always violate the Svetlichny inequality, and exactly analogous to the two-qubit case, the amount of violation is directly related to the degree of tripartite entanglement. We discuss further interesting properties of the generalized GHZ and maximal slice states.

**Groupmeeting by Rene Stock**

2009-02-18T18:52:00.000-05:00

Coherent excitation of a strongly coupled quantum dot - cavity system
by Dirk Englund, ....., Jelena Vuckovic

Photonic nanocavities coupled to semiconductor quantum dots are becoming well developed systems for studying cavity quantum electrodynamics and constructing the basic architecture for quantum information science. One of the key challenges is to coherently control the state of the quantum dot/cavity system for quantum memory and gates that exploit the non-linearity of such a system. Recently, coherent control of quantum dots has been studied in bulk semiconductor. Here we investigate the coherent excitation of a strongly coupled InAs quantum dot - photonic crystal cavity system. When the quantum dot and cavity are on resonance, we observe time-domain Rabi oscillation in the transmission of a laser pulse. This coherent excitation promises to enable an all-optical method to observe and manipulate the state a single quantum dot in a cavity. When the detuned dot is resonantly excited, we show that the resonantly driven quantum dot efficiently emits through the cavity mode, an effect that is explained in part by an incoherent dephasing mechanism similar to recent theoretical models. When the detuned quantum dot is resonantly excited, the cavity signal represents a spectrally separated read-out channel for high resolution single quantum dot spectroscopy. In this case, we observe antibunching of the cavity mode. Such a single photon source could allow photon indistinguishability that approaches unity as could lift the limi-itation due to dephasing and timing jitter. The single photon emission is controlled by the cavity resonance, which relaxes the demands for spectrally matching quantum dots for two-photon interference and may therefore be of use in linear optics quantum computation and quantum communication.

**Groupmeeting by Jason McKeever**

2009-01-29T12:11:00.000-05:00

Direct Observation of Anderson Localization of Matter Waves in a Controlled Disorder
by Juliette Billy, ....., Philippe Bouyer & Alain Aspect

In 1958, Anderson predicted the localization¹ of electronic wavefunctions in disordered crystals and the resulting absence of diffusion. It is now recognized that Anderson localization is ubiquitous in wave physics² because it originates from the interference between multiple scattering paths. Experimentally, localization has been reported for light waves^3,^4,^5,^6,⁷, microwaves^8,⁹, sound waves¹⁰ and electron gases¹¹. However, there has been no direct observation of exponential spatial localization of matter waves of any type. Here we observe exponential localization of a Bose–Einstein condensate released into a one-dimensional waveguide in the presence of a controlled disorder created by laser speckle¹². We operate in a regime of pure Anderson localization, that is, with weak disorder—such that localization results from many quantum reflections of low amplitude—and an atomic density low enough to render interactions negligible. We directly image the atomic density profiles as a function of time, and find that weak disorder can stop the expansion and lead to the formation of a stationary, exponentially localized wavefunction—a direct signature of Anderson localization. We extract the localization length by fitting the exponential wings of the profiles, and compare it to theoretical calculations. The power spectrum of the one-dimensional speckle potentials has a high spatial frequency cutoff, causing exponential localization to occur only when the de Broglie wavelengths of the atoms in the expanding condensate are greater than an effective mobility edge corresponding to that cutoff. In the opposite case, we find that the density profiles decay algebraically, as predicted in ref. 13. The method presented here can be extended to localization of atomic quantum gases in higher dimensions, and with controlled interactions.

**Groupmeeting by Rockson Chang**

2009-01-21T19:03:00.000-05:00

Experimental Joint Weak Measurement on a Photon Pair as a Probe of Hardy's Paradox

by Jeff Lundeen and Aephraim Steinberg

It has been proposed that the ability to perform joint weak measurements on postselected systems would allow us to study quantum paradoxes. These measurements can investigate the history of those particles that contribute to the paradoxical outcome. Here we experimentally perform weak measurements of joint (i.e., nonlocal) observables. In an implementation of Hardy’s paradox, we weakly measure the locations of two photons, the subject of the conflicting statements behind the paradox. Remarkably, the resulting weak probabilities verify all of these statements but, at the same time, resolve the paradox.

**Groupmeeting by Chris Paul**

2009-01-14T19:18:00.000-05:00

Cavity Optomechanics Using a Bose-Einstein Condensate

by Ferdinand Brennecker, Stephan Ritter, Tobias Donner, Tilman Esslinger

Cavity optomechanics studies the coupling between a mechanical oscillator and the electromagnetic field in a cavity. We report on a cavity optomechanical system in which a collective density excitation of a Bose-Einstein condensate serves as the mechanical oscillator coupled to the cavity field. A few photons inside the ultrahigh-finesse cavity trigger strongly driven back-action dynamics, in quantitative agreement with a cavity optomechanical model. We approach the strong coupling regime of cavity optomechanics, where a single excitation of the mechanical oscillator substantially influences the cavity field. The results open up new directions for investigating mechanical oscillators in the quantum regime and the border between classical and quantum physics.

**Groupmeeting by Alma Bardon**

2009-01-07T19:18:00.000-05:00

Simplifying Quantum Logic using Higher Dimensional Hilbert Spaces

by Benjamin Lanyon,......., and Andrew White

Quantum Computation promises to solve fundamental, yet otherwise intractable, problems across a range of active fields of research. Recently, universal quantum logic gate sets - the elemntal blocks for a quantum computer - have been demonstrated in several physical architectures. A serious obstacle to a full-scale implementation is the large number of these gates required to uild even small quantum circuits. Here, we present and demonstrate a general technique that harnesses multi-level information carriers to significantly reduce this number, enabling the construction of key quantum circuits with existing technology. we present implementations of two key quantum circuits: the three-qubit Toffoli gate and the general two-qubit controlled-unitary gate. Although our experiment is carried out in a phtonic architecture, the technique is independent of the particular physical encoding of quantum information, and has the potential for wider application.

**Groupmeeting by Hoda Hossein-Nejad**