Key graph properties affecting transport efficiency of flip-flop Grover percolated quantum walks

TL;DR

This paper investigates how specific graph properties influence the transport efficiency of flip-flop Grover quantum walks, revealing how graph modifications can unexpectedly enhance quantum transport and providing a general framework for analyzing trapping probabilities.

Contribution

It introduces a comprehensive method to determine trapping probabilities in percolated quantum walks on arbitrary graphs, extending previous results beyond planar 3-regular graphs.

Findings

Graph modifications can unexpectedly improve quantum transport efficiency.

A complete basis for trapped states enables calculation of asymptotic trapping probabilities.

Closed-form formulas for transport probabilities are derived for certain graph structures.

Abstract

Quantum walks exhibit properties without classical analogues. One of those is the phenomenon of asymptotic trapping -- there can be non-zero probability of the quantum walker being localised in a finite part of the underlying graph indefinitely even though locally all directions of movement are assigned non-zero amplitudes at each step. We study quantum walks with the flip-flop shift operator and the Grover coin, where this effect has been identified previously. For the version of the walk further modified by a random dynamical disruption of the graph (percolated quantum walks) we provide a recipe for the construction of a complete basis of the subspace of trapped states allowing to determine the asymptotic probability of trapping for arbitrary finite connected simple graphs, thus significantly generalizing the previously known result restricted to planar 3-regular graphs. We show how the position of the source and sink together with the graph geometry and its modifications affect the excitation transport. This gives us a deep insight into processes where elongation or addition of dead-end subgraphs may surprisingly result in enhanced transport and we design graphs exhibiting this pronounced behavior. In some cases this even provides closed-form formulas for the asymptotic transport probability in dependence on some structure parameters of the graphs.