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arXiv:1808.10345

Protecting temporal correlations of two-qubit states using quantum channels with memory

Shounak Datta, Shiladitya Mal, A. S. Majumdar

Published Aug 30, 2018• 3 citations
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Abstract

Quantum temporal correlations exhibited by violations of Leggett-Garg Inequality (LGI) and Temporal Steering Inequality (TSI) are in general found to be non-increasing under decoherence channels when probed on two-qubit pure entangled states. We study the action of decoherence channels, such as amplitude damping, phase-damping and depolarising channels when partial memory is introduced in a way such that two consecutive uses of the channels are time-correlated. We show that temporal correlations demonstrated by violations of the above temporal inequalities can be protected against decoherence using the effect of memory.

DOI
10.48550/arxiv.1808.10345

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Quantum temporal correlations exhibited by violations of Leggett-Garg Inequality (LGI) and Temporal Steering Inequality (TSI) are in general found to be non-increasing under decoherence channels when probed on two-qubit pure entangled states. We study the action of decoherence channels, such as amplitude damping, phase-damping and depolarising channels when partial memory is introduced in a way such that two consecutive uses of the channels are time-correlated. We show that temporal correlations demonstrated by violations of the above temporal inequalities can be protected against decoherence using the effect of memory.
The combined effects of memory effects due to correlated actions of the amplitude damping channel on two qubits and temporal correlations in the time evolution of each qubit on the average steered coherence (ASC) are explored. For the input Bell states, it is shown that the two kinds of memory effects are beneficial for enhancing ASC of the output states in a wide time region. In particular, the ASC is immune to decoherence for some input states if the actions of the amplitude damping channel are fully correlated. Moreover, in the infinite-time limit, the positive role of the non-Markovian effect on the steady-state ASC disappears, whereas the correlated actions of the channel are still beneficial to certain input states.
In this paper, we attempt to establish the relationship between quantum coherence and the violation of the Leggett-Garg inequality. In particular, employing the Lindblad equation, we obtain the pseudo-density matrix for a damping system to study the effect of environment interaction on the violation of this inequality in a two-state quantum system. It is shown that the violation of the Leggett-Garg inequality can be observed as long as temporal evolution does not induce decoherence. This statement is independent of the initial state of the system. Furthermore, similar to the Horodecki criterion for the CHSH inequality (R. Horodecki et al. Phys. Lett. {\bf A200}, 340), we study necessary and sufficient conditions for violating the Leggett-Garg inequality. Hereby, under the circumstance that the inequality violation occurs, an upper bound for the time interval between consecutive measurements with respect to the time scale of interaction with the environment (the relaxation time) is obtained.
In this work we study temporal quantum correlations, quantified by Leggett-Garg (LG) and LG-type inequalities, in the BBB and KKK meson systems. We use the tools of open quantum systems to incorporate the effect of decoherence which is quantified by a single phenomenological parameter. The effect of CPCPCP violation is also included in our analysis. We find that the LG inequality is violated for both BBB and KKK meson systems, the violation being most prominent in the case of KKK mesons and least for Bs{B}_{s}Bs​ system. Since the systems with no coherence do not violate LGI, incorporating decoherence is expected to decrease the extent of violation of LGI and is clearly brought out in our results. We show that the expression for the LG functions depends upon an additional term, apart from the experimentally measurable meson transition probabilities. This term vanishes in the limit of zero decoherence. On the other hand, the LG-type parameter can be directly expressed in terms of transition probabilities, making it a more appropriate observable for studying temporal quantum correlations in neutral meson systems.
We investigate the average steered coherence (ASC) and maximal steered coherence (MSC) of two qubits in a squeezed generalized amplitude damping channel modeled by a squeezed thermal bath with dissipation and memory. By considering the input Bell states, it is shown that while the dissipation and thermal effects induce decay of the ASC and MSC in the output states, the squeezing effect suppresses their decay in the strong (full) squeezing region for the input even-parity (odd-parity) Bell states. Besides, the memory effect due to correlated actions of the channel can also be exploited for suppressing decay of the ASC and MSC.
Leggett-Garg inequalities (LGI) test the correlations of a single system measured at different times. Violation of LGI implies either the absence of a realistic description of the system or the impossibility of measuring the system without disturbing it. We investigate the violation of the Leggett-Garg inequality for a two level system under decoherence in a non-Markovian dephasing environment. We discuss the non-Markovian dynamics of the violation of LGI at zero temperature and also at finite temperature for different structured environments. An enhanced quantum coherence is shown through the violation of Leggett-Garg inequality in the strong non-Markovian regime of the environment.
We consider maximal violations of the Leggett-Garg inequality, obtained by maximizing over all possible measurement operators, in relation to nonunitary aspects of the system dynamics. We model the action of an environment on a qubit in terms of generic quantum channels and relate the maximal value of the Leggett-Garg correlator to the channel parameters. We focus on unital channels and hence on decoherence. In certain important cases, exact relations between the channel parameters and maximal violations can be found. Moreover, we demonstrate the existence of distinct thresholds for the channel parameters, below which no violation of the Leggett-Garg inequality can occur.