Quantum cascade laser frequency stabilization at the sub-Hz level

High-precision measurements with molecules may refine our knowledge of various fields of physics, from atmospheric and interstellar physics to the standard model or physics beyond it. Most of them can be cast as absorption frequency measurements, particularly in the mid-infrared ‘molecular fingerprint’ region, creating the need for narrow-linewidth lasers of well-controlled frequency. Quantum cascade lasers provide a wide spectral coverage anywhere in the mid-infrared, but show substantial free-running frequency fluctuations. Here, we demonstrate that the excellent stability and accuracy of an ultra-stable near-infrared laser, transferred from a metrological institute through a fibre link, can be copied to a quantum cascade laser using an optical frequency comb. The obtained relative stability and accuracy of 2 × 10 −15 and 10 −14 exceed those demonstrated so far with quantum cascade lasers by almost two orders of magnitude. This set-up enables us to measure molecular absorption frequencies with state-of-the-art uncertainties, confirming its potential for ultra-high-precision spectroscopy. A 10 μm quantum cascade laser is phase-locked to a remote ultrastable laser referenced to primary frequency standards using an optical frequency comb. The obtained relative stability of 2 × 10 −15 is record-breaking in the mid-infrared region.