Selective filtering of multi-photon events from a single-photon emitter

TL;DR

This paper demonstrates a method to improve single-photon source purity by spectrally filtering out multi-photon errors, significantly reducing multi-photon events without harming single-photon emission quality.

Contribution

The study introduces a spectral filtering technique based on the distinct properties of photons emitted during short excitation pulses, surpassing fundamental emission limits.

Findings

Spectral filtering reduces multi-photon error rate by nearly an order of magnitude.

Shorter driving pulses produce photons with broader spectral shapes.

Filtering does not negatively impact single-photon emission efficiency.

Abstract

Single-photon purity is one of the most important key metrics of many quantum states of light. For applications in photonic quantum technologies, e.g. quantum communication and linear optical quantum computing, a minimization of the multi-photon error rate is required because of its error-introducing nature. Ultimately, the purity of state-of-the-art single-photon sources was found to be limited by spontaneous emission and subsequent reexcitation during the interaction with the driving field. Here, we demonstrate that even this fundamental limit to the single-photon purity can be overcome due to the distinct spectro-temporal properties of the individual photons forming multi-photon errors. For driving pulses shorter than the emitter lifetime, we find that photons emitted during the pulse exhibit a significantly broader spectral shape than the emitter's natural linewidth. Thus, we can selectively filter out the majority of this instantaneously emitted photon by employing narrowband spectral filters which reduces the measured degree of second-order coherence at zero time delay by almost one order of magnitude. This enables a significant suppression of the multi-photon error rate without detrimental effects on the desired single-photon emission.