Atoms in Optical Fibers

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

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

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

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

• 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

• 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

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**