Coherent quantum dynamics launched by incoherent relaxation in a quantum circuit simulator of a light-harvesting complex

TL;DR

This paper demonstrates that incoherent relaxation processes in a quantum circuit simulator can spontaneously generate electronic coherences and oscillatory motion, offering new insights into light-harvesting complex dynamics without the need for cold or coherent initial states.

Contribution

It introduces a simulation of exciton dynamics in a quantum circuit that reveals coherence generation from incoherent relaxation, a novel mechanism for quantum coherence in open systems.

Findings

Incoherent relaxation can generate electronic coherences and quantum beats.

Environmental noise at high temperature does not destroy coherence.

Transient violation of detailed balance observed in population relaxation.

Abstract

Engineering and harnessing coherent excitonic transport in organic nanostructures has recently been suggested as a promising way towards improving man-made light harvesting materials. However, realising and testing the dissipative system-environment models underlying these proposals is presently very challenging in supramolecular materials. A promising alternative is to use simpler and highly tunable quantum simulators built from programmable qubits, as recently achieved in a superconducting circuit by Potocnik et al. In this article, we simulate the real-time dynamics of an exciton coupled to a quantum bath as it moves through a network based on the quantum circuit of Potocnik et al. Using the numerically exact hierarchical equations of motion to capture the open quantum system dynamics, we find that an ultrafast but completely incoherent relaxation from a high-lying bright exciton into a doublet of closely spaced dark excitons can spontaneously generate electronic coherences and oscillatory real-space motion across the network (quantum beats). Importantly, we show that this behaviour also survives when the environmental noise is classically stochastic (effectively high temperature), as in present experiments, and also leads to a novel, transient violation of detailed balance in the population relaxation. These predictions highlight the possibilities of designing matched electronic and spectral noise structures for robust coherence generation that does not require coherent excitation or cold environments.