Quantum emitters promise to emit exactly one photon when pumped by a laser pulse. However, even in ideal systems, re-excitation during the laser pulse duration causes the consecutive emission of two photons, thus limiting the single-photon purity. Although the probability and characteristics of re-excitation are largely determined by the excitation scheme, until now, only resonant driving has been studied. Here, we demonstrate qualitative differences for phonon-assisted excitation and resolve the distinct temporal and spectral profile of each of the photons. These uncover correlations between the emission time and wavelength, resulting in an asymmetric two-photon spectrum. On a fundamental level, we show that this spectrum provides direct access to the Rabi frequency of an incoherently driven quantum dot. On the application side, we leverage the asymmetry to selectively suppress multiphoton noise, ensuring high single-photon purity regardless of the laser pulse length and thus enhancing implementations across quantum cryptography and quantum computing. Re-excitation, whereby a single excitation pulse leads to the emission of multiple photons, limits the single-photon purity of quantum emitters. Here, by examining the re-excitation dynamics of a quantum dot under incoherent excitation, the authors resolve spectro-temporal correlations in the emission that can be used to suppress multiphoton noise and boost performance of a single-photon source.

Nature is a scientific journal publishing research articles, reviews, and commentary across a wide range of scientific disciplines. Readers can find articles on topics such as biology, chemistry, physics, and environmental science. Additionally, they can learn about  research, scientific discoveries, and advances in understanding the natural world.

Nature

Researchers demonstrate qualitative differences in re-excitation dynamics of quantum dots under phonon-assisted (incoherent) excitation compared to resonant driving. By resolving distinct temporal and spectral profiles of emitted photons, they uncover correlations between emission time and wavelength, producing an asymmetric two-photon spectrum. This spectrum provides direct access to the Rabi frequency of an incoherently driven quantum dot. Practically, the asymmetry is leveraged to selectively suppress multiphoton noise, achieving high single-photon purity independent of laser pulse length — with implications for quantum cryptography and quantum computing.

Asymmetric two-photon response of an incoherently driven quantum emitter