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

Engineering and harnessing coherent excitonic transport in organic nanostructures has recently been suggested
as a promising way towards improving manmade light-harvesting materials. However, realizing 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 Potočnik et al. [A. Potočnik et al.,
Nat. Commun. 9, 904 (2018)]. 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 Potočnik 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 showthat this behavior also surviveswhen the environmental noise is classically
stochastic (effectively high temperature), as in present experiments. These predictions highlight the possibilities
of designing matched electronic and spectral noise structures for robust coherence generation that do not require
coherent excitation or cold environments.