# Optimal Coherent Filtering for Single Noisy Photons

## TL;DR

This paper presents a quantum buffer-based filter that optimally removes noise from single photons, enhancing their indistinguishability and brightness, with experimental validation in warm atomic systems.

## Contribution

The paper introduces a novel quantum buffer filter capable of optimal noise filtering and mode conversion, outperforming traditional intensity filtering methods.

## Key findings

- Quantum buffer filter effectively removes temporal-spectral noise.
- Experimental demonstration shows improved photon indistinguishability.
- Filter is well matched to quantum dot sources.

## Abstract

We introduce a filter using a noise-free quantum buffer with large optical bandwidth that can both filter temporal-spectral modes, as well as inter-convert them and change their frequency. We show that such quantum buffers optimally filter out temporal-spectral noise; producing identical single-photons from many distinguishable noisy single-photon sources with the minimum required reduction in brightness. We then experimentally demonstrate a noise-free quantum buffer in a warm atomic system that is well matched to quantum dots and can outperform all intensity (incoherent) filtering schemes for increasing indistinguishability.