Leggett-Garg inequality in Markovian quantum dynamics: role of temporal sequencing of coupling to bath

Sayan Ghosh; Anant V. Varma; Sourin Das

doi:10.1088/1751-8121/acc912

paper

Leggett-Garg inequality in Markovian quantum dynamics: role of temporal sequencing of coupling to bath

arXiv:2110.10696 · doi:10.1088/1751-8121/acc912

Abstract

We study Leggett-Garg inequalities (LGIs) for a two level system (TLS) undergoing Markovian dynamics described by unital maps. We find analytic expression of LG parameter $K_{3}$ (simplest variant of LGIs) in terms of the parameters of two distinct unital maps representing time evolution for intervals: $t_{1}$ to $t_{2}$ and $t_{2}$ to $t_{3}$ . We show that the maximum violation of LGI for these maps can never exceed well known Lüders bound of $K_{3}^{L u ¨ d er s} = 3/2$ over the full parameter space. We further show that if the map for the time interval $t_{1}$ to $t_{2}$ is non-unitary unital then irrespective of the choice of the map for interval $t_{2}$ to $t_{3}$ we can never reach Lüders bound. On the other hand, if the measurement operator eigenstates remain pure upon evolution from $t_{1}$ to $t_{2}$ , then depending on the degree of decoherence induced by the unital map for the interval $t_{2}$ to $t_{3}$ we may or may not obtain Lüders bound. Specifically, we find that if the unital map for interval $t_{2}$ to $t_{3}$ leads to the shrinking of the Bloch vector beyond half of its unit length, then achieving the bound $K_{3}^{L u ¨ d er s}$ is not possible. Hence our findings not only establish a threshold for decoherence which will allow for $K_{3} = K_{3}^{L u ¨ d er s}$ , but also demonstrate the importance of temporal sequencing of the exposure of a TLS to Markovian baths in obtaining Lüders bound.

arXiv comment9 pages, 2 figures

Leggett-Garg inequality in Markovian quantum dynamics: role of temporal sequencing of coupling to bath

Abstract

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