# Topological classification of time-asymmetry in unitary quantum processes

## TL;DR

This paper classifies when quantum processes are symmetric or asymmetric in time based on the structure of their Hamiltonians and gauge fields, revealing conditions for time-symmetry and asymmetry in quantum systems.

## Contribution

It provides a comprehensive topological classification of time-asymmetry in quantum processes, linking graph structure and gauge fields to time-reversal symmetry.

## Key findings

- Quantum processes are time-symmetric if the support graph is bipartite or no Aharonov-Bohm phases are present.
- Certain bipartite graphs show transition probability suppression without breaking time-reversal symmetry.
- Develops a framework for gauge potentials on combinatorial graphs in quantum systems.

## Abstract

Understanding which physical processes are symmetric with respect to time inversion is a ubiquitous problem in physics. In quantum physics, effective gauge fields allow emulation of matter under strong magnetic fields, realizing the Harper-Hofstadter, the Haldane models, demonstrating one-way waveguides and topologically protected edge states. Central to these discoveries is the chirality induced by time-symmetry breaking. In quantum walk algorithms, recent work has discovered implications time-reversal symmetry breaking has on the transport of quantum states which has enabled a host of new experimental implementations. We provide a full topological classification of the Hamiltonians of operators breaking time-reversal symmetry in their induced transition probabilities between elements in a preferred site-basis. We prove that a quantum process is necessarily time-symmetric for any choice of time-independent Hamiltonian precisely when the underlying support graph is bipartite or no Aharonov-Bohm phases are present in the gauge field. We further prove that certain bipartite graphs exhibit transition probability suppression, but not broken time-reversal symmetry. Furthermore, our development of a general framework characterizes gauge potentials on combinatorial graphs. These results and techniques fill an important missing gap in understanding the role this omnipresent effect has in quantum information and computation.