Quantum cascade laser frequency stabilisation at the sub-Hz level

TL;DR

This paper demonstrates the stabilization of a Quantum Cascade Laser to an optical frequency comb achieving sub-Hz stability and high accuracy, enabling ultra-high precision molecular spectroscopy and potential fundamental physics tests.

Contribution

It introduces a method to stabilize QCLs to an optical frequency comb with unprecedented stability and accuracy, surpassing previous benchmarks.

Findings

Achieved 2x10^-15 relative stability of QCL

Demonstrated 10^-14 accuracy in frequency measurement

Performed high-resolution molecular spectroscopy with 8x10^-13 uncertainty

Abstract

Quantum Cascade Lasers (QCL) are increasingly being used to probe the mid-infrared "molecular fingerprint" region. This prompted efforts towards improving their spectral performance, in order to reach ever-higher resolution and precision. Here, we report the stabilisation of a QCL onto an optical frequency comb. We demonstrate a relative stability and accuracy of 2x10-15 and 10-14, respectively. The comb is stabilised to a remote near-infrared ultra-stable laser referenced to frequency primary standards, whose signal is transferred via an optical fibre link. The stability and frequency traceability of our QCL exceed those demonstrated so far by two orders of magnitude. As a demonstration of its capability, we then use it to perform high-resolution molecular spectroscopy. We measure absorption frequencies with an 8x10-13 relative uncertainty. This confirms the potential of this setup for ultra-high precision measurements with molecules, such as our ongoing effort towards testing the parity symmetry by probing chiral species.