Decoherence Models for Discrete-Time Quantum Walks and their Application to Neutral Atom Experiments

TL;DR

This paper models decoherence in discrete-time quantum walks with neutral atoms, identifying dominant mechanisms, quantifying decoherence, and introducing a coherence length concept, with implications for experimental quantum coherence control.

Contribution

It introduces a phenomenological decoherence model distinguishing spin and spatial effects, and develops a phase space formalism for visualizing decoherence in quantum walks.

Findings

Spin decoherence is the main loss mechanism.

Ballistic expansion does not reliably indicate coherence.

Decoherence affects spatial coherences without altering local properties.

Abstract

We discuss decoherence in discrete-time quantum walks in terms of a phenomenological model that distinguishes spin and spatial decoherence. We identify the dominating mechanisms that affect quantum walk experiments realized with neutral atoms walking in an optical lattice. From the measured spatial distributions, we determine with good precision the amount of decoherence per step, which provides a quantitative indication of the quality of our quantum walks. In particular, we find that spin decoherence is the main mechanism responsible for the loss of coherence in our experiment. We also find that the sole observation of ballistic instead of diffusive expansion in position space is not a good indicator for the range of coherent delocalization. We provide further physical insight by distinguishing the effects of short and long time spin dephasing mechanisms. We introduce the concept of coherence length in the discrete-time quantum walk, which quantifies the range of spatial coherences. Unexpectedly, we find that quasi-stationary dephasing does not modify the local properties of the quantum walk, but instead affects spatial coherences. For a visual representation of decoherence phenomena in phase space, we have developed a formalism based on a discrete analogue of the Wigner function. We show that the effects of spin and spatial decoherence differ dramatically in momentum space.