# Single-photon interferometry and spectroscopy with two laser frequency combs

## TL;DR

This paper demonstrates a novel single-photon interferometry technique using two laser frequency combs, enabling high-resolution spectroscopy at extremely low light levels, with potential applications in sensitive spectral diagnostics.

## Contribution

It introduces a new method of dual-comb Fourier transform spectroscopy at the single-photon level, expanding the capabilities of spectroscopic measurements in light-starved conditions.

## Key findings

- High-resolution dual-comb spectroscopy at billion-fold weaker light levels.
- Observation of single-photon interference in time domain.
- Potential for broad spectral diagnostics with minimal photon flux.

## Abstract

We demonstrate single-photon time-domain interference in a new realm. We observe interferences in the photon counting statistics with two separate mode-locked femtosecond lasers of slightly different repetition frequencies, each emitting a comb of evenly spaced spectral lines over a wide spectral span. We exploit the interference pattern for spectroscopic diagnostics over a broad spectral range. An experimental proof-of-concept shows that the emerging technique of high-resolution dual-comb Fourier transform spectroscopy can be performed at light powers that are a billion-fold weaker than those commonly employed. Our experiments challenge the intuitive concept that a photon exists before detection and they open the prospect of precise spectroscopy over broad spectral bandwidth in light-starved conditions.

---
Source: https://tomesphere.com/paper/1906.03706