Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session B7: Invited Session: Quantum Control for Quantum Information Processing |
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Chair: Ivan Deutsch, University of New Mexico Room: Terrace |
Tuesday, June 5, 2012 10:30AM - 11:00AM |
B7.00001: Quantum feedback experiments with atoms and cavities Invited Speaker: Jean-Michel Raimond Quantum feedback transposes the usual feedback loop concept into the quantum world. A measurement performed on the system by the sensor is used by a controller to infer the system's state and to steer it towards the target by the action of the actuator. This scheme has to face a fundamental difficulty, since the measurement changes the system's state. This back-action makes quantum feedback algorithms more complex than their classical counterparts. We report the first successful operation of a repeated quantum feedback loop [1]. It prepares photon number states (from 0 to 4 photons) on-demand in a superconducting microwave cavity and subsequently reverses the effect of decoherence-induced quantum jumps. The quantum sensors are circular Rydberg atoms, performing a Quantum Non Demolition (QND) measurement of the cavity field. Information they provide is used by the controller (real-time computer) to estimate the field state. The controller determines the amplitude of a coherent displacement leading the cavity closer to the target This displacement is performed by a microwave source acting as the actuator. Iterations of this loop rapidly drive the cavity towards the prescribed target. When it is reached, the actuator idles. It resumes operation when atomic detections indicate that a photon has been lost, or that a thermal photon has appeared. The feedback compensates for these quantum jumps and rapidly restores the field in the target state. In a variant of the experiment, we use quantum actuators, resonant atoms that feed photons back in the cavity when they get lost. This more efficient scheme allows us to stabilize higher photon numbers. These experiments are a first step towards the use of quantum feedback to protect fragile quantum resources. We also consider an alternative route towards state protection based on reservoir engineering [2]. \\[4pt] [1] C. Sayrin et al. Nature (London) 477, 73 (2011)\\[0pt] [2] A. Sarlette et al. Phys. Rev. Lett.\textbf{ 107,} 010402 (2011) [Preview Abstract] |
Tuesday, June 5, 2012 11:00AM - 11:30AM |
B7.00002: Quantum Control and Tomography in the 16-Dimensional Ground Manifold of Atomic Cesium Invited Speaker: Poul Jessen The standard paradigm for Quantum Information Science involves a collection of qubits, whereas the physical building blocks of a quantum processor or simulator often have more than two accessible levels. Taking advantage of these higher dimensional Hilbert spaces (qudits) requires the development of good laboratory tools for qudit manipulation and readout. We have successfully implemented a protocol for quantum state-to-state mapping in the 16-dimensional hyperfine ground manifold of individual Cesium atoms, using only DC, rf and microwave magnetic fields to drive the atomic evolution. Our control waveforms (rf and $\mu $w phases versus time) are found by numerical optimization, and designed to compensate for errors in the driving and background magnetic fields. Experimentally we achieve a state-to-state mapping fidelity of better than 99{\%}, averaged over a sample of randomly chosen initial and target states. Preliminary results suggest that unitary transformations can be designed and implemented in a similar manner. To perform quantum state tomography, we drive an ensemble of identically prepared atoms with phase modulated rf and $\mu $w fields while performing a continuous weak measurement of an atomic observable via polarization spectroscopy. The resulting measurement record is numerically inverted to obtain an estimate of the unknown quantum state. We have reconstructed the density matrices for a set of randomly chosen pure test states using algorithms based either on least squares fitting or compressed sensing. The latter is slightly more tolerant of experimental errors and achieves an average fidelity above 90{\%}. [Preview Abstract] |
Tuesday, June 5, 2012 11:30AM - 12:00PM |
B7.00003: Synthetic Quantum Matter under the Microscope Invited Speaker: Markus Greiner Ultracold atoms in optical lattices enable experimenters to create and study synthetic quantum matter, opening a window into the fascinating world of many-body quantum physics. With quantum gas microscopy we are now able to take the control of atoms in an optical lattice to the next and ultimate level of high fidelity addressing, manipulation and readout of single particles. I will present microscopic studies of strongly correlated quantum matter and the first realization of quantum magnetism in an optical lattice. This work opens a wide range of new possibilities and brings the realization of exotic states of matter within experimental reach. [Preview Abstract] |
Tuesday, June 5, 2012 12:00PM - 12:30PM |
B7.00004: Quantum simulations of magnetism with large numbers of atomic ion spins Invited Speaker: Rajibul Islam We report the engineering of the form and range of fully-connected Ising interactions and the observation of interesting spin orders in quantum simulations of magnetism with many trapped ion spins. The interaction between the spins is provided through state-dependent laser forces applied to individual ions in a laser-cooled Coulomb crystal. When such a laser force is applied globally, an effective spin-spin interaction emerges that is mediated through the collective motion of the ions. The sign and range of this effective magnetic interaction can be precisely controlled with the laser and any possible spin correlation function can be measured by imaging the state-dependent fluorescence from the ions. We simulate interesting spin models that possess nontrivial ground states for the investigation of quantum phase transitions, quantum frustration, and the emergence of spin liquid behavior. We speculate on the scaling of this system to more than 25 spins, where classical models are not able to calculate ground states or spin dynamics. [Preview Abstract] |
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