East China Normal University team reports in National Science Review: first direct observation of femtosecond-scale quantum coherence transfer

Microcavity exciton–polaritons are quantum oscillation states formed under strong coupling between microcavity photons and semiconductor excitons. As bosonic quasiparticles with a “half-light, half-matter” character, they possess an extremely small effective mass and strong nonlinearity, providing an ideal platform for studying macroscopic quantum coherence and nonequilibrium Bose–Einstein condensation at room temperature. The establishment of coherence is central to understanding the formation mechanism of exciton–polariton condensation. However, the process by which the coherence of an externally applied resonant driving field is transferred to exciton–polaritons has so far lacked direct experimental verification.

Using a home-built femtosecond angle-resolved spectroscopic imaging system, the research team first carried out systematic characterization of resonantly injected exciton–polaritons at low pump power. The results show that the resonantly injected exciton–polaritons are generated and decay almost simultaneously with the external pump laser, and are scattered to large in-plane momenta, with lifetimes on the order of a few hundred femtoseconds. By precisely measuring the first-order spatial coherence function g(1), the team observed clear interference fringes between the resonant exciton–polaritons and the pump laser, directly demonstrating that the polaritons faithfully inherit and preserve the coherence of the laser field.

When the pump power is increased, in addition to the signal from resonantly injected exciton–polaritons, new exciton–polaritons emerge in a higher-energy, non-resonant region. The dynamical behavior of this component closely resembles that previously observed under conventional non-resonant excitation conditions, with lifetimes on the picosecond scale. However, these subsequently generated non-resonant exciton–polaritons no longer produce interference with the injection laser, indicating that the initial coherence of the excitation light has been completely lost in the process.

Based on these observations, the team proposed a dynamical model comprising four key stages, and achieved quantitative agreement with experiment through theoretical simulations:

1. The pump laser resonantly excites a single exciton–polariton dispersion branch, directly injecting exciton–polaritons into the system while effectively preserving the original coherence of the excitation light.
2. The resonantly injected exciton–polaritons are scattered to large-angle states in adjacent dispersion branches via particle–particle interactions, during which the coherence is still maintained.
3. When the resonant exciton–polaritons are formed, the system is in the strong-coupling regime, and the exciton reservoir is populated through the Rabi oscillation process.
4. Excitons in the reservoir relax via a decoherent stimulated-emission process, then strongly couple to cavity photons again to form non-resonant exciton–polaritons. Because the coherence of the excitation light is lost during the decoherence of the exciton reservoir, the finally generated non-resonant exciton–polaritons no longer produce interference fringes with the pump laser.

To further validate this dynamical picture, the team constructed a coupled-oscillator model based on an open-dissipative Gross–Pitaevskii equation and performed numerical simulations under experimental conditions. The simulations not only reproduced the femtosecond-timescale dynamical evolution observed in the measurements, but also firmly confirmed that decoherence in the exciton reservoir is the key mechanism responsible for the loss of the original coherence of the excitation light.

This work provides the first direct evidence of the transfer and evolution of coherence from an external laser field to a microcavity exciton–polariton system, offering a clear physical picture for the coherent origin of exciton–polariton condensation. It also lays an important foundation for future efforts to precisely control quantum states via tailored optical fields and to design and develop new quantum photonic and quantum information devices.

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