Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session T6: Quantum Measurement and Control |
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Chair: Mukund Vengalatorre, Cornell University Room: Delaware AB |
Friday, June 12, 2015 8:00AM - 8:12AM |
T6.00001: Measurement-induced generation of a macroscopic spin singlet Naeimeh Behbood, Ferran Martin Ciurana, Giorgio Colangelo, Mario Napolitano, Geza Toth, Robert J. Sewell, Morgan W. Mitchell We report the production of a macroscopic spin singlet (MSS) in an atomic system [1] using collective quantum nondemolition (QND) measurement as a global entanglement generator [2]. Using an unpolarized ensemble of up to $10^6$ cold atomic spins, we observe 3 dB of spin squeezing and detect entanglement with $5\sigma$ statistical significance using a generalized spin- squeezing inequality [3], indicating that at least half the atoms in the sample have formed singlets. [1] N. Behbood {\it et al.}, ``Generation of Macroscopic Singlet States in a Cold Atomic Ensemble,'' {\bf Phys. Rev. Lett.} 113, 093601 (2014). [2] R. J. Sewell {\it et al.}, ``Certified quantum non-demolition measurement of a macroscopic material system,'' {\bf Nat. Photon.} 7, 517 (2013). [3] G. T\'{o}th and M.W. Mitchell, ``Generation of macroscopic singlet states in atomic ensembles,'' {\bf New J. Phys.} 12, 053007 (2010). [Preview Abstract] |
Friday, June 12, 2015 8:12AM - 8:24AM |
T6.00002: Quantum Control by Imaging: The Zeno Effect in an Ultracold Lattice Gas Yogesh Sharad Patil, Srivatsan Chakram, Mukund Vengalattore We demonstrate the control of quantum tunneling in an ultracold lattice gas by the measurement backaction imposed by an imaging process. A \em in situ\em\ imaging technique is used to acquire repeated images of an ultracold gas confined in a shallow optical lattice. The backaction induced by these position measurements modifies the coherent quantum tunneling of atoms within the lattice. By varying the rate at which atoms are imaged, we observe the crossover from the weak measurement regime, where the measurement has a negligible effect on coherent dynamics, to the strong measurement regime, where measurement-induced localization leads to a dramatic suppression of tunneling. The latter effect is a manifestation of the Quantum Zeno effect [1]. We thereby demonstrate the paradigmatic Heisenberg microscope in a lattice gas, and shed light on the implications of quantum measurement on the coherent evolution of a mesoscopic quantum system. Our technique demonstrates a powerful tool for the control of an interacting many-body quantum system via spatially resolved measurement backaction.\\[4pt] [1] Y. S. Patil \em et al.\em\ arXiv:1411.2678 [Preview Abstract] |
Friday, June 12, 2015 8:24AM - 8:36AM |
T6.00003: Quantum control and squeezing of collective spin Daniel Hemmer, Enrique Montano, Poul Jessen, Ivan Deutsch Quantum control of many body atomic spins is often pursued in the context of an atom-light quantum interface, where a quantized light field can be used to entangle distant atoms. We are currently exploring new ways to improve the coherence and the amount of atom-light entanglement by optimizing the spatial geometry of the atomic ensemble and light fields, and through the control and optimization of the internal atomic state. Our basic setup consists of a quantized probe beam passing through an atom cloud held in a dipole trap, first generating spin-probe entanglement through the Faraday interaction, and then using backaction from a measurement of the probe polarization to squeeze the collective atomic spin. Using an optimized geometry and a 2-color probe scheme to suppress tensor light shifts we achieve 7 dB of spin noise reduction and 5 dB of metrological squeezing at the optimal measurement time. It is possible to further increase atom-light coupling by ``amplifying'' the initial projection noise per atom through a suitable internal state preparation. In principle we can use internal-state control to map this entanglement back to a basis where it corresponds to improved squeezing. [Preview Abstract] |
Friday, June 12, 2015 8:36AM - 8:48AM |
T6.00004: Enhanced Spin Squeezing in Atomic Ensembles via Control of the Internal Spin States Ezad Shojaee, Leigh Norris, Ben Baragiola, Enrique Montano, Daniel Hemmer, Poul Jessen, Ivan Deutsch Abstract: ~We study the process by which the collective spin squeezing of an ensemble of Cesium atoms is enhanced by control of the internal spin state of the atoms. By increasing the initial atomic projection noise, one can enhance the Faraday interaction that entangles the atoms with a probe. The light acts as a quantum bus for creating atom-atom entanglement via measurement backaction. ~Further control can be used to transfer this entanglement to metrologically useful squeezing. We numerically simulate this protocol by a stochastic master equation, including QND measurement and optical pumping, which accounts for decoherence and transfer of coherences between magnetic sub-levels. We study the tradeoff between the enhanced entangling interaction and increased rates of decoherence for different initial state preparations. Under realistic conditions, we find that we can achieve squeezing with a ``CAT-State'' superpostion \textbar F$=$4, Mz$=$4\textgreater $+$ \textbar F, Mz$=$-4\textgreater of $\sim$ 9.9 dB and for the spin coherent state \textbar F$=$4, Mx$=$4\textgreater of $\sim$ 7.5 dB. The increased entanglement enabled by the CAT state preparation is partially, but not completely reduced by the increased fragility to decoherence. [Preview Abstract] |
Friday, June 12, 2015 8:48AM - 9:00AM |
T6.00005: Reservoir Engineering of Two-mode Correlations in Mechanical Resonators Laura Chang, Yogesh Sharad Patil, Srivatsan Chakram, Mukund Vengalattore Nonlinear mechanical interactions in the quantum limit enable the manipulation and control of phonons in a manner akin to quantum optics in nonlinear media. We demonstrate, for the first time, strong quantum-compatible multimode nonlinearities in a low-loss mechanical resonator that is amenable to ground state optomechanical cooling, room temperature quantum control and quantum limited detection. These nonlinearities arise from substrate-mediated interactions between distinct modes of the resonator. We develop a model for this nonlinearity that accurately describes the experimental observations over three orders of magnitude in dynamic range, demonstrating the robustness and fidelity of the engineered nonlinear interactions. We use this nonlinearity to realize a mechanical nondegenerate parametric amplifier, and use it to demonstrate two-mode thermomechanical noise squeezing [1]. Our work opens new opportunities for nonlinear approaches to quantum metrology, transduction between optical and phononic fields, and the quantum manipulation of phononic degrees of freedom.\\[4pt] [1] Y. S. Patil \em et al.\em\ arXiv:1410.7109 [Preview Abstract] |
Friday, June 12, 2015 9:00AM - 9:12AM |
T6.00006: Work measurement in a quantum heat engine Francesco Bariani, Keye Zhang, Ying Dong, Pierre Meystre We consider an optomechanical quantum heat engine operating on an Otto cycle for photon-phonon polaritons, the working substance of the engine [1]. We discuss both the average value and quantum fluctuations of its work output, concentrating in particular on the effects of quantum non-adiabaticity due to the finite duration of the cycle. We also determine the quantum back-action of both absorptive and dispersive continuous measurements of the work, and quantify their impact on the Curzon-Ahlborn engine efficiency at maximum power and its fluctuations.\\[4pt] [1] Keye Zhang, F. Bariani, and P. Meystre, ``A Quantum Optomechanical Heat Engine,'' Phys. Rev. Lett. {\bf 112}, 150602 (2014). [Preview Abstract] |
Friday, June 12, 2015 9:12AM - 9:24AM |
T6.00007: 360-degree quantum tomography of a qudit Charles Baldwin, Amir Kalev, Hector Martinez, Nathan Lysne, Poul Jessen, Ivan Deutsch Quantum information processing consists of three components each with a respective tomography technique: preparation/state, evolution/process, and measurement/detector. Previous works have diagnosed a single component individually yielding an estimated density matrix, process-matrix, or POVM, which is compared to a corresponding target. However, all three types of tomography are interrelated, and accounting for only one implies that the estimator produced suffers from systematic errors. Other techniques exist to quantify the average error rates of a single part, e.g. randomized benchmarking, but fail to give information on the type of error. One goal of quantum tomography is to produce a reliable estimate to diagnose sources of errors. To study this we model a cold-atom testbed--the coupled electron-nuclear spins of the 16-dimensional ground manifold of Cs, initialized by optical pumping, controlled by magnetic and optical fields, and measured by Stern-Gerlach analysis. In a complete 360-degree cycle we can use known states as leverage to correct errors in POVMs, and in turn correct errors in processes, which allows us to improve state preparation, etc. This protocol allows us to produce reliable estimates while diagnosing sources of errors that one can work to correct. [Preview Abstract] |
Friday, June 12, 2015 9:24AM - 9:36AM |
T6.00008: Temporal Quantum-State Tomography of Narrowband Biphotons Xianxin Guo, Peng Chen, Chi Shu, M.M.T. Loy, Shengwang Du We demonstrate a technique of quantum-state tomography for measuring the complex temporal wave function of narrowband biphotons with polarization-dependent and time-resolved two-photon interference. While the amplitude function of the biphoton waveform is directly related to the second-order correlation function which is determined by the two-photon coincidence measurement, the phase function is retrieved from six sets of time-resolved two-photon interference measurements projected onto different polarization subspaces. We apply this technique to experimentally reconstruct the temporal quantum states of the narrow-band biphotons generated from the spontaneous four-wave mixing in cold atoms. As compared with the homodyne detection, our method doesn't require any external phase reference. [Preview Abstract] |
Friday, June 12, 2015 9:36AM - 9:48AM |
T6.00009: Quantum State Tomography of Cold Atom Qudits Hector Sosa Martinez, Nathan Lysne, Poul Jessen, Charles Baldwin, Amir Kalev, Ivan Deutsch Accurate and robust control over quantum systems plays a key role in quantum information science. The use of systems with large state spaces (qudits) may prove a useful resource for quantum information tasks if good laboratory tools for qudit manipulation and measurement can be developed. Over the past few years we have developed and experimentally implemented a protocol to perform high-fidelity unitary transformations in the 16 dimensional hyperfine ground manifold of Cesium-133 atoms, driving the system with phase modulated radio-frequency and microwave magnetic fields and using the tools of optimal control to find appropriate control waveforms. We have recently extended our protocol to investigate quantum state tomography based on a combination of unitary transformations and Stern-Gerlach analysis. Experimental results shown that optimal tomography based on mutually-unbiased-bases (MUBs) can be implemented, with reconstruction fidelities on the order of 99{\%} for arbitrarily chosen test states in a 16-dimensional Hilbert space. We are also interested in the characterization of our measurement detector for which we plan to perform POVM tomography. Ultimately, successful implementation of this kind of state tomography may prove very valuable, greatly reducing the required data for more complex procedures such as quantum process tomography. [Preview Abstract] |
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