Quantum Walk Search with Time-Reversal Symmetry Breaking

TL;DR

This paper explores how breaking time-reversal symmetry in quantum walks affects Grover's search algorithm, showing robustness and different state evolutions without changing the search runtime.

Contribution

It introduces a chiral quantum walk formulation of Grover's algorithm, analyzing the effects of phase-induced time-reversal symmetry breaking on its dynamics.

Findings

Small phases do not significantly alter the search time.

Increasing phases change the excited states involved in evolution.

The quantum search remains robust against symmetry breaking.

Abstract

We formulate Grover's unstructured search algorithm as a chiral quantum walk, where transitioning in one direction has a phase conjugate to transitioning in the opposite direction. For small phases, this breaking of time-reversal symmetry is too small to significantly affect the evolution: the system still approximately evolves in its ground and first excited states, rotating to the marked vertex in time $\pi \sqrt{N} / 2$. Increasing the phase does not change the runtime, but rather changes the support for the 2D subspace, so the system evolves in its first and second excited states, or its second and third excited states, and so forth. Apart from the critical phases corresponding to these transitions in the support, which become more frequent as the phase grows, this reveals that our model of quantum search is robust against time-reversal symmetry breaking.