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arXiv:quant-ph/9902077 · Physical Review A

Non-Markovian quantum feedback from homodyne measurements: The effect of a nonzero feedback delay time

Vittorio Giovannetti, P. Tombesi, David Vitali

Published Aug 1, 1999• 52 citations
arXivHTMLPDF

Abstract

We solve exactly the non-Markovian dynamics of a cavity mode in the presence of a feedback loop based on homodyne measurements, in the case of a nonzero feedback delay time. With an appropriate choice of the feedback parameters, this scheme is able to significantly increase the decoherence time of the cavity mode, even for delay times not much smaller than the decoherence time itself.

DOI
10.1103/physreva.60.1549

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We solve exactly the non-Markovian dynamics of a cavity mode in the presence of a feedback loop based on homodyne measurements, in the case of a nonzero feedback delay time. With an appropriate choice of the feedback parameters, this scheme is able to significantly increase the decoherence time of the cavity mode, even for delay times not much smaller than the decoherence time itself.
Abstract The no-knowledge quantum feedback, introduced in Phys. Rev. Lett., 113, 020407 (2014), is a measurement-based feedback protocol for decoherence suppression in a quantum system coupled to noisy environment. By continuously measuring the environmental noise, without directly gathering any information about the system, the decoherence effect can be suppressed by feeding back quantum controls proportional to the measured signal. In the original work, the feedback control was assumed instantaneous, leading to perfect cancellation of noise backaction on the quantum system. However, the instantaneous feedback is difficult to achieve in practice, and close-loop feedback protocols are always associated with finite delayed time. Therefore, in this work, we consider the effects of the delay between the time at which the measurement signal is acquired and the time that such signal is fed back to the system. We investigate the problem with an example of a two-level system (qubit) coupled to a Markovian reservoir, via a Hermitian coupling operator, where a homodyne detection is used to measure the environmental noise. We numerically simulate quantum stochastic trajectories of the qubit and analyse their averaged dynamics. We find that the feedback control with time delay can either enhance or reduce the decoherence effects, depending on whether the delayed time is in-phase or out-of-phase with the unitary dynamics of the qubit system.
Quantum feedback can stabilize a two-level atom against decoherence (spontaneous emission), putting it into an arbitrary (specified) pure state. This requires perfect homodyne detection of the atomic emission, and instantaneous feedback. Inefficient detection was considered previously by two of us. Here we allow for a non-zero delay time tau\\tautau in the feedback circuit. Because a two-level atom is a nonlinear optical system, an analytical solution is not possible. However, quantum trajectories allow a simple numerical simulation of the resulting non-Markovian process. We find the effect of the time delay to be qualitatively similar to that of inefficient detection. The solution of the non-Markovian quantum trajectory will not remain fixed, so that the time-averaged state will be mixed, not pure. In the case where one tries to stabilize the atom in the excited state, an approximate analytical solution to the quantum trajectory is possible. The result, that the purity (P=2rmTr[rho2]−1P=2{\\rm Tr}[\\rho^{2}]-1P=2rmTr[rho2]−1) of the average state is given by P=1−4gammatauP=1-4\\gamma\\tauP=1−4gammatau (where gamma\\gammagamma is the spontaneous emission rate) is found to agree very well with the numerical results.
The no-knowledge quantum feedback was proposed by Szigeti et al. [Phys. Rev. Lett. 113, 020407 (2014)] as a measurement-based feedback protocol for decoherence suppression for an open quantum system. By continuously measuring environmental noises and feeding back controls on the system, the protocol can completely reverse the measurement back-action and therefore suppress the system's decoherence. However, the complete decoherence cancellation was shown only for the instantaneous feedback, which is impractical in real experiments. Therefore, in this work, we generalize the original work and investigate how the decoherence suppression can be degraded with unavoidable delay times, by analyzing non-Markovian average dynamics. We present analytical expressions for the average dynamics and numerically analyze the effects of the delayed feedback for a coherently driven two-level system, coupled to a bosonic bath via a Hermitian coupling operator. We also find that, when the qubit's unitary dynamics does not commute with the measurement and feedback controls, the decoherence rate can be either suppressed or amplified, depending on the delay time.
This paper explores the control of non-Markovian systems via coherent quantum feedback. In the spirit of classical control theory that is widely used in engineering, we acquire completely new insights in the closed-loop design from the frequency domain point of view, which appears missing in quantum control theory. Based on the non-Markovian Langevin equation for the closed-loop quantum dynamics, it is found that, in contrast to the existing time-domain design methods, the frequency domain analysis is more natural on the memory kernel function, which can be reshaped by feedback to suppress the non-Markovian decoherence. For illustration, we consider the case that the coupling strength in the feedback loop is constant. The analysis shows that the coherent feedback shifts the two components in the original noise spectral function towards two opposite directions, thereby makes it possible to suppress the noise near the system's working frequency. When the system to be controlled is Markovian, this simple scheme needs to be replaced by more careful design due to the flatness of the noise spectrum. As an example, the effectiveness of our scheme is demonstrated in photonic crystal systems.
A general theory of quantum-limited feedback for continuously monitored systems is presented. Two approaches are used, one based on quantum measurement theory and one on Hamiltonian system-bath interactions. The former gives rise to a stochastic non-Markovian evolution equation for the density operator, and the latter a non-Markovian quantum Langevin equation. In the limit that the time delay in the feedback loop is negligible, a simple deterministic Markovian master equation can be derived from either approach. Two special cases of interest are treated: feedback mediated by optical homodyne detection and self-excited quantum point processes.