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arXiv:1608.05802 · Scientific Reports

Stochastic quantum Zeno-based detection of noise correlations

Matthias M. Müller, Stefano Gherardini, Filippo Caruso

Published Dec 12, 2016• 23 citations
arXivHTMLPDF

Abstract

A system under constant observation is practically freezed to the measurement subspace. If the system driving is a random classical field, the survival probability of the system in the subspace becomes a random variable described by the Stochastic Quantum Zeno Dynamics (SQZD) formalism. Here, we study the time and ensemble average of this random survival probability and demonstrate how time correlations in the noisy environment determine whether the two averages do coincide or not. These environment time correlations can potentially generate non-Markovian dynamics of the quantum system depending on the structure and energy scale of the system Hamiltonian. We thus propose a way to detect time correlations of the environment by coupling a quantum probe system to it and observing the survival probability of the quantum probe in a measurement subspace. This will further contribute to the development of new schemes for quantum sensing technologies, where nanodevices may be exploited to image external structures or biological molecules via the surface field they generate.

DOI
10.1038/srep38650

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A system under constant observation is practically freezed to the measurement subspace. If the system driving is a random classical field, the survival probability of the system in the subspace becomes a random variable described by the Stochastic Quantum Zeno Dynamics (SQZD) formalism. Here, we study the time and ensemble average of this random survival probability and demonstrate how time correlations in the noisy environment determine whether the two averages do coincide or not. These environment time correlations can potentially generate non-Markovian dynamics of the quantum system depending on the structure and energy scale of the system Hamiltonian. We thus propose a way to detect time correlations of the environment by coupling a quantum probe system to it and observing the survival probability of the quantum probe in a measurement subspace. This will further contribute to the development of new schemes for quantum sensing technologies, where nanodevices may be exploited to image external structures or biological molecules via the surface field they generate.
The unavoidable interaction between a quantum system and the external noisy environment can be mimicked by a sequence of stochastic measurements whose outcomes are neglected. Here we investigate how this stochasticity is reflected in the survival probability to find the system in a given Hilbert subspace at the end of the dynamical evolution. In particular, we analytically study the distinguishability of two different stochastic measurement sequences in terms of a Fisher information measure depending on the variation of a function, instead of a finite set of parameters. We find a characterization of Zeno phenomena as the physical result of the random observation of the quantum system, linked to the sensitivity of the survival probability to an arbitrary small perturbation of the measurement stochasticity. Finally, the implications on the Cram\'er-Rao bound and the Zeno time are discussed, together with a numerical example.
Abstract Quantum measurements play a crucial role in quantum mechanics since they perturb, unavoidably and irreversibly, the state of the measured quantum system. More extremely, the constant observation of a quantum system can even freeze its dynamics to a subspace, effectively truncating the Hilbert space of the system. It represents the quantum version of the famous flying arrow Zeno paradox, and is called quantum Zeno dynamics. In general, it can be obtained by applying frequent consecutive quantum measurements that are equally spaced in time. Here, we introduce time disorder in the measurement sequence, and analytically investigate how this temporal stochasticity may affect the confinement probability of the system in the subspace. As main result, we then exploit how different dissipative and coherent Zeno protocols can be generalized to this stochastic scenario. Finally, our analytical predictions are numerically tested on a paradigmatic spin chain where we find a trade‐off between a probabilistic scheme with high fidelity (compared to perfect subspace dynamics) and a deterministic one with a slightly lower fidelity, moving further steps towards new schemes of Zeno‐based control for future quantum technologies.
Quantum measurements are crucial for observing the properties of a quantum system, which, however, unavoidably perturb its state and dynamics in an irreversible way. Here we study the dynamics of a quantum system being subjected to a sequence of projective measurements applied at random times. In the case of independent and identically distributed intervals of time between consecutive measurements, we analytically demonstrate that the survival probability of the system to remain in the projected state assumes a large deviation (exponentially decaying) form in the limit of an infinite number of measurements. This allows us to estimate the typical value of the survival probability, which can therefore be tuned by controlling the probability distribution of the random time intervals. Our analytical results are numerically tested for Zeno-protected entangled states, which also demonstrate that the presence of disorder in the measurement sequence further enhances the survival probability when the Zeno limit is not reached (as it happens in experiments). Our studies provide a new tool for protecting and controlling the amount of quantum coherence in open complex quantum systems by means of tunable stochastic measurements.
Quantum control is a fundamental tool for quantum technologies, and quantum Zeno (QZE) and anti-Zeno (AZE) effects, respectively denoting the slowdown and speedup of quantum system evolution by frequent interruptions/operations, represent quantum control paradigms. Indeed, they allow either protecting (QZE) or steering (AZE) the state of a quantum system via the interplay between frequent operations (system control) and the coupling of the system to its environment. Here we present two experiments demonstrating the possibility to exploit QZE and AZE to extract information on noise processes occurring in a quantum channel. In the first experiment, we realize the first noise diagnostics scheme based on repeated quantum measurements, showing how a single photon undergoing a noise process (in our case, random polarization fluctuations) can quantify non-Markovian temporal correlations within such a noise. In the second experiment, we show how a QZE-protection mechanism applied to a quantum state can become a noise sensing technique, allowing to estimate the statistical distribution of decoherence effects occurring on a photonic qubit during its propagation in a noisy quantum channel.
We study the quantum Zeno effect (QZE) induced by continuous partial measurement in the presence of short-correlated noise in the system Hamiltonian.We study the survival probability and the onset of the QZE as a function of the measurement strength and find that, depending on the noise parameters, the quantum Zeno effect can be enhanced or suppressed by the noise in different regions of parameter space.Notably, the conditions for the enhancement of the QZE are different when determined by the short-time or long-time behavior of the survival probability, or by the measurement strength marking the onset of the quantum Zeno regime.
We study the quantum Zeno effect (QZE) induced by continuous partial measurement in the presence of short-correlated noise in the system Hamiltonian. We study the survival probability and the onset of the QZE as a function of the measurement strength and find that, depending on the noise parameters, the quantum Zeno effect can be enhanced or suppressed by the noise in different regions of parameter space. Notably, the conditions for the enhancement of the QZE are different when determined by the short-time or long-time behavior of the survival probability, or by the measurement strength marking the onset of the quantum Zeno regime.