Influence of coin symmetry on infinite hitting times in quantum walks

TL;DR

This paper investigates how the symmetry properties of coins in quantum walks influence the occurrence of infinite hitting times, revealing that highly symmetric coins can lead to non-zero probabilities of never reaching target vertices.

Contribution

It introduces the concept of coin-permutation symmetries and analyzes their impact on infinite hitting times in quantum walks, extending understanding beyond graph symmetry alone.

Findings

Highly symmetric coins increase the IHT subspace size.

Certain symmetric coins can cause quantum walks to never reach targets.

Graph symmetry alone does not fully determine infinite hitting times.

Abstract

Classical random walks on finite graphs have an underrated property: a walk from any vertex can reach every other vertex in finite time, provided they are connected. Discrete-time quantum walks on finite connected graphs however, can have infinite hitting times. This phenomenon is related to graph symmetry, as previously characterized by the group of direction-preserving graph automorphisms that trivially affect the coin Hilbert space. If a graph is symmetric enough (in a particular sense) then the associated quantum walk unitary will contain eigenvectors that do not overlap a set of target vertices, for any coin flip operator. These eigenvectors span the Infinite Hitting Time (IHT) subspace. Quantum states in the IHT subspace never reach the target vertices, leading to infinite hitting times. However, this is not the whole story: the graph of the 3D cube does not satisfy this symmetry constraint, yet quantum walks on this graph with certain symmetric coins can exhibit infinite hitting times. We study the effect of coin symmetry by analyzing the group of coin-permutation symmetries (CPS): graph automorphisms that act nontrivially on the coin Hilbert space but leave the coin operator invariant. Unitaries using highly symmetric coins with large CPS groups, such as the permutation-invariant Grover coin, are associated with higher probabilities of never arriving, as a result of their larger IHT subspaces.