Monday, October 19, 2009

Entropy Exchange in a Mixture of Ultracold Atoms

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.

Wednesday, September 30, 2009

A phonon laser

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.

Wednesday, September 23, 2009

Phase shaping of single-photon wave packets

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 protocols2, 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 measurement4, 5, demonstrating fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon's amplitude6, 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**

Wednesday, September 16, 2009

Laser cooling by collisional redistribution of radiation

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.

Wednesday, September 9, 2009

Optical entanglement of co-propagating modes

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**

Wednesday, September 2, 2009

Simple pulses for elimination of leakage in weakly nonlinear qubits

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**

Wednesday, August 19, 2009

A High Phase-Space-Density Gas of Polar Molecules

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

A quantum gas of ultracold polar molecules, with long-range and anisotropic interactions, not only would enable explorations of a large class of many-body physics phenomena but also could be used for quantum information processing. We report on the creation of an ultracold dense gas of potassium-rubidium (40K87Rb) polar molecules. Using a single step of STIRAP (stimulated Raman adiabatic passage) with two-frequency laser irradiation, we coherently transfer extremely weakly bound KRb molecules to the rovibrational ground state of either the triplet or the singlet electronic ground molecular potential. The polar molecular gas has a peak density of 1012 per cubic centimeter and an expansion-determined translational temperature of 350 nanokelvin. The polar molecules have a permanent electric dipole moment, which we measure with Stark spectroscopy to be 0.052(2) Debye (1 Debye = 3.336 x 10–30 coulomb-meters) for the triplet rovibrational ground state and 0.566(17) Debye for the singlet rovibrational ground state.

**Groupmeeting by Karl Pilch**

Wednesday, August 5, 2009

Measure for the Non-Markovianity of Quantum Processes

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**

Wednesday, July 29, 2009

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

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.

Wednesday, July 22, 2009

Entangled Mechanical Oscillators

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.

Wednesday, July 15, 2009

Quantum Walk in Position Space with Single Optically Trapped Atoms

By Michal Karski, ..., & Dieter Meschede

The quantum walk is the quantum analog of the well-known random walk, which forms the basis for models and applications in many realms of science. Its properties are markedly different from the classical counterpart and might lead to extensive applications in quantum information science. In our experiment, we implemented a quantum walk on the line with single neutral atoms by deterministically delocalizing them over the sites of a one-dimensional spin-dependent optical lattice. With the use of site-resolved fluorescence imaging, the final wave function is characterized by local quantum state tomography, and its spatial coherence is demonstrated. Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata.

**Groupmeeting by Alma Bardon**

Wednesday, July 8, 2009

Attosecond Ionization and Tunneling Delay Time Measurements in Helium

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

It is well established that electrons can escape from atoms through tunneling under the influence of strong laser fields, but the timing of the process has been controversial and far too rapid to probe in detail. We used attosecond angular streaking to place an upper limit of 34 attoseconds and an intensity-averaged upper limit of 12 attoseconds on the tunneling delay time in strong field ionization of a helium atom. The ionization field derives from 5.5-femtosecond-long near-infrared laser pulses with peak intensities ranging from 2.3 x 1014 to 3.5 x 1014 watts per square centimeter (corresponding to a Keldysh parameter variation from 1.45 to 1.17, associated with the onset of efficient tunneling). The technique relies on establishing an absolute reference point in the laboratory frame by elliptical polarization of the laser pulse, from which field-induced momentum shifts of the emergent electron can be assigned to a temporal delay on the basis of the known oscillation of the field vector..

Monday, June 29, 2009

Control of a magnetic Feshbach resonance with laser light

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

Imaging Intracellular Fluorescent Proteins at Nanometer Resolution

Eric Betzig,1,2*{dagger} 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 proteins at nanometer spatial resolution. Numerous sparse subsets of photoactivatable fluorescent protein molecules were activated, localized (to ~2 to 25 nanometers), and then bleached. The aggregate position information from all subsets was then assembled into a superresolution image. We used this method—termed photoactivatedlocalization microscopy—to image specific target proteins in thin sections of lysosomes and mitochondria; in fixed whole cells, we imaged vinculin at focal adhesions, actin within a lamellipodium, and the distribution of the retroviral protein Gag 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 led to substantial improvements in image resolution, achieving a near-molecular resolution of 20 to 30 nanometers in the two lateral dimensions. Three-dimensional (3D) nanoscale-resolution imaging, however, remains a challenge. We demonstrated 3D stochastic optical reconstruction microscopy (STORM) by using optical astigmatism to determine both axial and lateral positions of individual fluorophores with nanometer accuracy. Iterative, stochasticactivation of photoswitchable probes enables high-precision 3D localization of each probe, and thus the construction of a 3D image, without scanning the sample. Using this approach, we achieved an image resolution of 20 to 30 nanometers in the lateral dimensions and 50 to 60 nanometers in the axial dimension. This development allowed us to resolve the 3D morphology of nanoscopic cellular structures.

Interferometric fluorescent super-resolution microscopy resolves 3D cellular ultrastructure
  • Gleb Shtengela
  • James A. Galbraithb
  • Catherine G. Galbraithc
  • Jennifer Lippincott-Schwartzd,1,
  • Jennifer M. Gilletted
  • Suliana Manleyd
  • Rachid Sougratd
  • Clare M. Watermane,
  • Pakorn Kanchanawonge
  • Michael W. Davidsonf
  • Richard D. Fettera and 
  • Harald F. Hessa,1
    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.

    Thursday, June 11, 2009

    Q-bits from Nitrogen Vacancy Centers in Diamond

    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.

    Monday, June 8, 2009

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

    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.

    Friday, May 29, 2009

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

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

    We demonstrate a fiber-optical switch that is activated at tiny energies corresponding to a few hundred optical photons per pulse. This is achieved by simultaneously confining both photons and a small laser-cooled ensemble of atoms inside the microscopic hollow core of a single-mode photonic-crystal fiber and using quantum optical techniques for generating slow light propagation and large nonlinear interaction between light beams.

    Thursday, May 14, 2009

    Complete path entanglement of two photons

    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**