Chaos, entanglement, and decoherence in the quantum kicked top

 

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Ghose, Shohini, Stock, Rene, Jessen, Poul, Lal, Roshan, Silberfarb, Andrew
 

We analyze the interplay of chaos, entanglement, and decoherence in a system of qubits whose collective behavior is that of a quantum kicked top. The dynamical entanglement between a single qubit and the rest can be calculated from the mean of the collective spin operators. This allows the possibility of efficiently measuring entanglement dynamics in an experimental setting. We consider a deeply quantum regime and show that signatures of chaos are present in the dynamical entanglement for parameters accessible in an experiment that we propose using cold atoms. The evolution of the entanglement depends on the support of the initial state on regular versus chaotic Floquet eigenstates, whose phase-space distributions are concentrated on the corresponding regular or chaotic eigenstructures. We include the effect of decoherence via a realistic model and show that the signatures of chaos in the entanglement dynamics persist in the presence of decoherence. In addition, the classical chaos affects the decoherence rate itself.
 

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Quantum Control of the Hyperfine Spin of a Cs Atom Ensemble

 

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Souma Chaudhury, Seth Merkel, Tobias Herr, Andrew Silberfarb, Ivan H. Deutsch, and Poul S. Jessen
 

We demonstrate quantum control of a large spin angular momentum associated with the F = 3 hyperfine ground state of 133Cs. Time-dependent magnetic fields and a static tensor light shift are used to implement near-optimal controls and map a fiducial state to a broad range of target states, with yields in the range 0.80.9. Squeezed states are produced also by an adiabatic scheme that is more robust against errors. Universal control facilitates the encoding and manipulation of qubits and qudits in atomic ground states and may lead to the improvement of some precision measurements.
 

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Efficient Quantum-State Estimation by Continuous Weak Measurement and Dynamical Control

 

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Greg A. Smith, Andrew Silberfarb, Ivan H. Deutsch, and Poul S. Jessen
 

We demonstrate a fast, robust, and nondestructive protocol for quantum-state estimation based on continuous weak measurement in the presence of a controlled dynamical evolution. Our experiment uses optically probed atomic spins as a test bed and successfully reconstructs a range of trial states with fidelities of ~90%. The procedure holds promise as a practical diagnostic tool for the study of complex quantum dynamics, the testing of quantum hardware, and as a starting point for new types of quantum feedback control.
 

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Lattice of double wells for manipulating pairs of cold atoms

 

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Sebby-Strabley J, Anderlini M, Jessen PS, Porto JV
 

We describe the design and implementation of a two-dimensional optical lattice of double wells suitable for isolating and manipulating an array of individual pairs of atoms in an optical lattice. Atoms in the square lattice can be placed in a double well with any of their four nearest neighbors. The properties of the double well the barrier height and relative energy offset of the paired sites can be dynamically controlled. The topology of the lattice is phase stable against phase noise imparted by vibrational noise on mirrors. We demonstrate the dynamic control of the lattice by showing the coherent splitting of atoms from single wells into double wells and observing the resulting double-slit atom diffraction pattern. This lattice can be used to test controlled neutral atom motion among lattice sites and should allow for testing controlled two-qubit gates.
 

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A Continuous Non-demolition Measurement of the Cs Clock Transition Pseudo-spin

 

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Souma Chaudhury, Greg A. Smith, Kevin Schulz and Poul S. Jessen
 

We demonstrate a weak continuous measurement of the pseudo-spin associated with the clock transition in a sample of Cs atoms. Our scheme uses an optical probe tuned near the D1 transition to measure the sample birefringence, which depends on the z-component of the collective pseudospin. At certain probe frequencies the differential light shift of the clock states vanishes and the measurement is non-perturbing. In dense samples the measurement can be used to squeeze the collective clock pseudo-spin, and has potential to improve the performance of atomic clocks and interferometers.
 

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Quantum State Reconstruction via Continuous Measurement

 

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Andrew Silberfarb, Poul S. Jessen, Ivan H. Deutsch
 

We present a new procedure for quantum state reconstruction based on weak continuous measurement of an ensemble average. By applying controlled evolution to the initial state, new information is continually mapped onto the measured observable. A Bayesian filter is then used to update the state estimate in accordance with the measurement record. This generalizes the standard paradigm for quantum tomography based on strong, destructive measurements on separate ensembles. This approach to state estimation induces minimal perturbation of the measured system, giving information about observables whose evolution cannot be described classically in real time and opening the door to new types of quantum feedback control.
 

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Continuous Weak Measurement and Nonlinear Dynamics in a Cold Spin Ensemble

 

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Greg A. Smith¹, Souma Chaudhury¹, Andrew Silberfarb², Ivan H. Deutsch², and Poul S. Jessen¹
 

1. Optical Sciences Center, University of Arizona, Tucson, AZ 85721
2. Department of Physics and Astronomy, University of New Mexico, Albuquerque, New Mexico 87131
 

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a non-perturbing measurement on the much longer timescale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.
 

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Quantum Information Processing with Trapped Neutral Atoms

 

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P. S. Jessen, I. H. Deutsch, and R. Stock
 

Quantum information can be processed using large ensembles of ultracold and trapped neutral atoms, building naturally on the techniques developed for high-precision spectroscopy and metrology. This article reviews some of the most important protocols for universal quantum logic with trapped neutrals, as well as the history and the state-of-the-art of experimental work to implement these in the laboratory. Some general observations are made concerning the different strategies for qubit encoding, transport and interaction, including trade-offs between de-coherence rates and the likelihood of two-qubit gate errors. These trade-offs must be addressed through further refinements of logic protocols and trapping technologies before one can undertake the design of a general-purpose neutral-atom quantum processor.
 

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Faraday Spectroscopy in an Optical Lattice: a continuous probe of atomic dynamics

 

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Greg A. Smith, Souma Chaudhury, and Poul S. Jessen

Optical Sciences Center, University of Arizona, Tucson, AZ 85721
 

The linear Faraday effect is used to implement a continuous measurement of the spin of a sample of laser cooled atoms trapped in an optical lattice. One of the optical lattice beams serves also as a probe beam, thereby allowing one to monitor the atomic dynamics in real time and with minimal perturbation. A simple theory is developed to predict the measurements sensitivity and associated cost in terms of decoherence caused by scattering of probe photons. Calculated signal-to-noise ratios in measurements of Larmor precession are found to agree with experimental data for a wide range of lattice intensity and detuning. Finally, quantum backaction is estimated by comparing the measurement sensitivity to spin projection noise, and shown to be insignificant in the current experiment. A continuous quantum measurement based on Faraday spectroscopy in optical lattices may open up new possibilities for the study of quantum feedback and classically chaotic quantum systems.
 

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Quantum Information Processing in Optical Lattices: Cold Atomic Qubits in a Virtual Crystal of Light

 

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Ivan H. Deutsch and Poul S. Jessen
 

No Abstract Available
 

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