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Journal of Optics B Quantum and Semiclassical Optics

Quantum feedback in a non-resonant cavity QED system

Wade P. Smith, Luis A. Orozco

Published Feb 1, 2004• 5 citations
Publisher

Abstract

Photon correlation measurements reveal the response of the conditional evolution of the cavity QED system to a novel quantum feedback protocol. A photodetection collapses the state of the system and triggers a feedback pulse with an adjustable delay and amplitude that alters the intensity driving the system. The conditional evolution of the system freezes into a new steady state where it resides until, after an amount of time determined by the experimenter, it re-equilibrates into the original steady state. We carry out a sensitivity analysis using a theoretical model with atomic detuning and make quantitative comparisons with measured results.

DOI
10.1088/1464-4266/6/2/002

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Photon correlation measurements reveal the response of the conditional evolution of the cavity QED system to a novel quantum feedback protocol. A photodetection collapses the state of the system and triggers a feedback pulse with an adjustable delay and amplitude that alters the intensity driving the system. The conditional evolution of the system freezes into a new steady state where it resides until, after an amount of time determined by the experimenter, it re-equilibrates into the original steady state. We carry out a sensitivity analysis using a theoretical model with atomic detuning and make quantitative comparisons with measured results.
Quantum feedback in strongly coupled systems can probe a regime where one quantum of excitation is a large fluctuation. We present theoretical and experimental studies of quantum feedback in an optical cavity QED system. The time evolution of the conditional state, following a photodetection, can be modified by changing the drive of the cavity. For the appropriate feedback, the conditional state can be captured in a new steady state and then released. The feedback protocol requires resonance operation, and proper amplitude and delay for the change in the drive. We demonstrate the successful use of feedback in the suppression of the vacuum Rabi oscillations for the length of the feedback pulse and their subsequent return to steady state. The feedback works only because we have an entangled quantum system, rather than an analogous correlated classical system.
We probe a ground-state superposition that produces a quantum beat in the intensity correlation of a two-mode cavity QED system. We mix drive with scattered light from an atomic beam traversing the cavity, and effectively measure the interference between the drive and the light from the atom. When a photon escapes the cavity, and upon detection, it triggers our feedback which modulates the drive at the same beat frequency but opposite phase for a given time window. This results in a partial interruption of the beat oscillation in the correlation function, that then returns to oscillate.
Abstract. We probe a ground-state superposition that produces a quantum beat in the intensity correlation of a two-mode cavity QED system. We mix drive with scattered light from an atomic beam traversing the cavity, and effectively measure the interference between the drive and the light from the atom. When a photon escapes the cavity, and upon detection, it triggers our feedback which modulates the drive at the same beat frequency but opposite phase for a given time window. This results in a partial interruption of the beat oscillation in the correlation function, that then returns to oscillate. 1.
We explore experimentally the fundamental projective properties of a quantum measurement and their application in the control of a system's evolution. We perform quantum non-demolition (QND) photon counting on a microwave field trapped in a very-high- Q superconducting cavity, employing circular Rydberg atoms as non-absorbing probes of light. By repeated measurement of the cavity field we demonstrated the freeze of its initially coherent evolution, illustrating the back action of the photon number measurement on the field's phase. On the contrary, by utilizing a weak QND measurement in combination with the control injection of coherent pulses, we plan to force the field to deterministically evolve towards any target photon-number state. This quantum feedback procedure will enable us to prepare and protect photon-number states against decoherence.
We describe an efficient approach to modeling cavity quantum electrodynamics (QED) with a time-delayed coherent feedback using quantum trajectory simulations. An analytical set of equations is derived to exploit the advantages of trajectories in the presence of the non-Markovian dynamics, where adjustments to the standard stochastic dynamics are discussed. In the weak excitation regime, we first verify that our approach recovers known results obtained with other simulation methods and demonstrate how a coherent feedback loop can increase the photon lifetime in typical cavity-QED systems. We also highlight the underlying stochastic dynamics. We then explore the nonlinear few-photon regime of cavity QED, under the restriction of at most one photon at a time in the feedback loop. In particular, we show how feedback affects the cavity photoluminescence (populations versus laser detuning) and describe how one must account for conditioning in the presence of feedback; specifically, the system observables must be conditioned on no photon detections at the feedback output channel occurring.