Chiral Quantum Walks

TL;DR

This paper introduces a network-based framework for understanding time-reversal symmetry in quantum circuits, demonstrating how time-asymmetry can be controlled and used to enhance quantum transport, with experimental validation.

Contribution

It develops a classification of quantum circuits based on time-reversal symmetry and shows how to control asymmetry using local gates, with experimental demonstration of transport enhancement.

Findings

Many quantum circuits exhibit time-asymmetry.

Time-asymmetry can be controlled via local gates.

Controlled asymmetry improves quantum transport.

Abstract

Given its importance to many other areas of physics, from condensed matter physics to thermodynamics, time-reversal symmetry has had relatively little influence on quantum information science. Here we develop a network-based picture of time-reversal theory, classifying Hamiltonians and quantum circuits as time-symmetric or not in terms of the elements and geometries of their underlying networks. Many of the typical circuits of quantum information science are found to exhibit time-asymmetry. Moreover, we show that time-asymmetry in circuits can be controlled using local gates only, and can simulate time-asymmetry in Hamiltonian evolution. We experimentally implement a fundamental example in which controlled time-reversal asymmetry in a palindromic quantum circuit leads to near-perfect transport. Our results pave the way for using time-symmetry breaking to control coherent transport, and imply that time-asymmetry represents an omnipresent yet poorly understood effect in quantum information science.