Scientists have reported the first direct evidence that coherence from an external laser field can be transferred to microcavity exciton–polaritons, a hybrid quantum state combining light and matter.

Using a custom-built femtosecond spectroscopic imaging system, the team observed that exciton–polaritons injected at low pump power inherited and preserved the coherence of the driving laser. The particles, which decay within a few hundred femtoseconds, produced clear interference fringes with the pump beam, confirming the transfer of coherence.

At higher pump power, additional exciton–polaritons appeared in a non-resonant region with picosecond lifetimes. Unlike the resonant states, these did not interfere with the laser, showing that coherence was lost during relaxation in the exciton reservoir.

The researchers proposed a four-stage dynamical model to explain the process, supported by simulations based on a coupled-oscillator framework. The work identifies decoherence in the exciton reservoir as the key mechanism behind the loss of coherence.

The findings provide a physical picture for the coherent origin of exciton–polariton condensation and could help advance efforts to control quantum states with tailored optical fields, paving the way for new quantum photonic and information technologies.