Temporal nonclassicality in continuous-time quantum walks

TL;DR

This paper explores quantum nonclassicality in continuous-time quantum walks using single-time and multi-time measures, revealing how topology and decoherence influence quantum features over time.

Contribution

It introduces a combined analysis of dynamical distance and sequential measurement quantifiers, highlighting their different scaling and topology-dependent behaviors.

Findings

Quadratic short-time scaling of the multi-time quantifier differs from linear scaling of the dynamical distance.

Topology strongly affects the multi-time quantifier at longer times, suppressing it on complete graphs and oscillating on cycles.

Dephasing in different bases leads to distinct asymptotic behaviors of the quantifiers, with site dephasing vanishing them and energy dephasing preserving finite values.

Abstract

We investigate the genuinely quantum features of continuous-time quantum walks by combining a single-time and a multi-time quantifier of nonclassicality. On the one hand, we consider the quantum-classical dynamical distance $D_{\mathrm{QC}}(t)$, which measures the departure of the time-evolved quantum state of a continuous-time quantum walk from the classical state of a random walk on the same graph. On the other, we analyse the joint probability distributions associated with sequential measurements of the walker's position, assessing their violation of the classical Kolmogorov consistency conditions via a dedicated quantifier $\bar{K}(t)$. We demonstrate a quadratic short-time scaling of $\bar{K}(t)$, which differs from the known linear scaling of $D_{\mathrm{QC}}(t)$, but, as the latter, is fully determined by the degree of the initially occupied node and is independent of the global graph topology. At longer times, instead, $\bar{K}(t)$ exhibits a pronounced topology-driven behavior: it is strongly suppressed on complete graphs while remaining finite and oscillatory on cycles, in contrast with the almost topology-independent asymptotics of $D_{\mathrm{QC}}(t)$. We then extend the analysis to Markovian open-system dynamics, focusing on dephasing in the position basis (Haken-Strobl model) and in the energy basis (intrinsic decoherence). Site dephasing drives both quantifiers to zero, with the decay of $\bar{K}(t)$ controlled by the spectral gap of the corresponding Lindblad generator. By contrast, energy-basis dephasing preserves a finite asymptotic value of $\bar{K}(t)$, depending on the overlap structure of the Laplacian eigenspaces with the site basis.