Measurement induced quantum walks

TL;DR

This paper studies measurement-induced quantum walks on a graph, revealing how measurements influence the walk's behavior, including Gaussian spreading, quantum corrections, and the impact of sampling rates on first passage times and ergodicity-like phenomena.

Contribution

It introduces a detailed analysis of measurement effects on quantum walks, including quantum corrections, first passage properties, and the influence of sampling rates, highlighting phenomena like ergodicity breaking.

Findings

Quantum walks converge to Gaussian statistics away from the Zeno limit.

Quantum corrections modify the normal distribution behavior.

Sampling rates can cause divergence in mean detection times and phase space decomposition.

Abstract

We investigate a tight binding quantum walk on a graph. Repeated stroboscopic measurements of the position of the particle yield a measured "trajectory", and a combination of classical and quantum mechanical properties for the walk are observed. We explore the effects of the measurements on the spreading of the packet on a one dimensional line, showing that except for the Zeno limit, the system converges to Gaussian statistics similarly to a classical random walk. A large deviation analysis and an Edgeworth expansion yield quantum corrections to this normal behavior. We then explore the first passage time to a target state using a generating function method, yielding properties like the quantization of the mean first return time. In particular, we study the effects of certain sampling rates which cause remarkable change in the behavior in the system, like divergence of the mean detection time in finite systems and a decomposition of the phase space into mutually exclusive regions, an effect that mimics ergodicity breaking, whose origin here is the destructive interference in quantum mechanics. For a quantum walk on a line we show that in our system the first detection probability decays classically like $(\text{time})^{-3/2}$, this is dramatically different compared to local measurements which yield a decay rate of $(\text{time})^{-3}$, indicating that the exponents of the first passage time depends on the type of measurements used.