Cavity-enhanced spectroscopy in the deep cryogenic regime -- new hydrogen technologies for quantum sensing

TL;DR

This paper presents a novel cavity-enhanced spectrometer operating at 4 K, enabling precise cryogenic hydrogen spectroscopy for fundamental physics, standards, and molecular studies, overcoming technological challenges of deep cryogenic cooling.

Contribution

The authors developed a cryogenic cavity-enhanced spectrometer that operates fully at 4 K, allowing for advanced hydrogen spectroscopy and related fundamental and practical applications.

Findings

Accurate spectroscopy of cryogenic hydrogen molecules.

Validation of quantum electrodynamics for molecules.

Establishment of primary SI standards in the deep cryogenic regime.

Abstract

Spectrometers based on high-finesse optical cavities have proven to be powerful tools for applied and fundamental studies. Extending this technology to the deep cryogenic regime is beneficial in many ways: Doppler broadening is reduced, peak absorption is enhanced, the Boltzmann distribution of rotational states is narrowed, all unwanted molecular species disturbing the spectra are frozen out, and dense spectra of complex polyatomic molecules become easier to assign. We demonstrate a cavity-enhanced spectrometer fully operating in the deep cryogenic regime down to 4 K. We solved several technological challenges that allowed us to uniformly cool not only the sample but also the entire cavity, including the mirrors and cavity length actuator, which ensures the thermodynamic equilibrium of a gas sample. Our technology well isolates the cavity from external noise and cryocooler vibrations. This instrument enables a variety of fundamental and practical applications. We demonstrate a few examples based on accurate spectroscopy of cryogenic hydrogen molecules: accurate test of the quantum electrodynamics for molecules; realization of the primary SI standards for temperature, concentration and pressure in the deep cryogenic regime; measurement of the H$_{2}$ phase diagram; and determination of the ortho-para spin isomer conversion rate.