The Walther group published a new paper in Nature Communications!

05.05.2026

Asymmetric two-photon response of an incoherently driven quantum emitter

Single-photon sources are central to advances in both fundamental quantum optics and quantum information technologies. As the name implies, they are designed to emit exactly one photon when triggered by a short laser pulse. In practice, however, the finite duration of the pulse leaves a small chance that a second photon will be produced. Crucially, this is not just a technical imperfection—it arises from the intrinsic way light interacts with matter in these quantum systems.

This work reports the first experimental insight to the time and frequency profiles of this two-photon emission process and reveals tempo-spectral correlations. Using an incoherent excitation scheme, the optical response becomes asymmetric in frequency. Building on this insight, the study demonstrates a filtering technique that leverages the asymmetry to selectively suppress only one of the two photons, thereby restoring high single-photon purity. These results deepen our understanding of light–matter interactions in few-level emitters and directly improve the performance of single-photon sources for quantum communication and quantum computing.

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The first and second photon generated in a re-excitation process of a quantum dot feature distinct temporal and spectral profiles, which have been measured here for the first time. The phonon-assisted incoherent driving scheme introduces a strong frequency shift to the first photon, making this approach uniquely suited to supress the early photon using straightforward spectral filtering and thus restore the performance of a high-quality single-photon source.