Temporal nonclassicality in continuous-time quantum walks

Quantum walks represent a paradigmatic framework to explore and manipulate quantum behaviors. In this
paper, we investigate the genuinely quantum features of continuous-time quantum walks by combining a
single-time and a multitime quantifier of nonclassicality. On the one hand, we consider the quantum-classical dynamical distance DQC(t ), which measures the departure of the time-evolved quantum state of a continuoustime
quantum walk from the classical state of a random walk on the same graph. On the other hand, we
analyze 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 ¯K (t ).
We demonstrate a quadratic short-time scaling of ¯K (t ), which differs from the known linear scaling of DQC(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, ¯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 DQC(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 ¯K (t ) controlled by the spectral gap of the corresponding Lindblad generator. By contrast, energy-basis dephasing preserves a finite asymptotic value of ¯K (t ), depending on the overlap structure of the Laplacian eigenspaces with the site basis. Our results
demonstrate that distinct notions of nonclassicality lead to qualitatively and quantitatively different assessments
of how quantum a given walk is, pointing at the intricate nature of temporal quantum correlations in networked quantum systems under realistic decoherence.