Quantum walk on a toral phase space

TL;DR

This paper investigates a quantum walk on a toral phase space, revealing exact solvability, novel periodicity, and phase-space cat-states, with implications for understanding quantum delocalization and entanglement dynamics.

Contribution

It introduces an exactly solvable quantum walk model on a toral phase space, highlighting unique spectral properties, phase-space phenomena, and entanglement behavior.

Findings

Eigenangles are equally spaced for odd lattices, independent of the coin.

The walk exhibits exact periodicity and phase-space cat-states.

Quantum participation ratio shows power-law growth with exponent 0.825.

Abstract

A quantum walk on a toral phase space involving translations in position and its conjugate momentum is studied in the simple context of a coined walker in discrete time. The resultant walk, with a family of coins parametrized by an angle is such that its spectrum is exactly solvable with eigenangles for odd parity lattices being equally spaced, a feature that is remarkably independent of the coin. The eigenvectors are naturally specified in terms the $q-$Pochhammer symbol, but can also be written in terms of elementary functions, and their entanglement can be analytically found. While the phase space walker shares many features in common with the well-studied case of a coined walker in discrete time and space, such as ballistic growth of the walker position, it also presents novel features such as exact periodicity, and formation of cat-states in phase-space. Participation ratio (PR) a measure of delocalization in walker space is studied in the context of both kinds of quantum walks; while the classical PR increases as $\sqrt{t}$ there is a time interval during which the quantum walks display a power-law growth $\sim t^{0.825}$. Studying the evolution of coherent states in phase space under the walk enables us to identify an Ehrenfest time after which the coin-walker entanglement saturates.