Exciton coherence propagation measured with non-local four-wave mixing micro-spectroscopy

TL;DR

This study demonstrates that excitons in semiconductor quantum wells can transfer optical coherence over mesoscopic distances, enabling potential applications in quantum information processing with compact excitonic circuits.

Contribution

It provides the first experimental evidence of exciton-mediated non-local coherence transfer in semiconductors using micro-spectroscopy.

Findings

Excitons can diffuse over distances up to 10 μm before recombination.

Excitons inherit phase modulation from optical excitation, enabling coherent links.

The method offers a pathway for miniaturized quantum coherence devices.

Abstract

Coherence transfer is a multi-disciplinary topic of interest, including chemistry, biology and physics. In quantum technologies, achieving non-local coherent coupling between solid-state qubits is of the utmost importance. Here, we demonstrate that excitons - i.e. electron-hole pairs bound by the Coulomb force within a quantum well - can act as a medium for mesoscopic optical coherence transfer in semiconductors. To this end, we use a femtosecond laser pulse to resonantly generate excitons within the light cone. These excitons can then either recombine radiatively or scatter out of the light cone, gaining an in-plane momentum in the process. In samples without disorder, such as the CdTe quantum wells used here, the resulting fast excitons can diffuse over mesoscopic distances before recombining radiatively. Using coherent nonlinear micro-spectroscopy, we carry out exciton time-of-flight measurements. Specifically, we monitor the spatio-temporal propagation of launched exciton wave packets, selectively observing their coherence or density on a scale of up to 10$\,\mu$m. Our proof-of-principle experiment demonstrates that free excitons inherit a phase modulation from the optical pulsed excitation and can generate coherent links within excitonic circuits, offerring a higher level of miniaturisation and compactness than photonic or polaritonic architectures.