An apparatus for detecting ultralight dark matter with cryogenic Fabry-Perot resonators*

A signature of scalar virialized ultralight fields (VULFs), a promising class of dark matter candidates, is the induction of oscillatory strains in solids as a result of temporal oscillations of the Bohr radius at their Compton frequency arising from their coupling to standard model fields. Here we report on the development of a cryogenic apparatus which exploits this effect for a broadband VULF search based on differential measurements of frequency shifts in two ultra-stable optical cavities designed to respond differently to the presence of a VULF field. The first one, with its mirrors connected by a rigid spacer, will experience length oscillations. The second one consists of two mirrors decoupled from each other and from the surroundings through a mechanical suspension which suppresses VULF-induced strains. This experiment is expected to improve current bounds on VULF couplings by an estimated two orders of magnitude in a range within the audio 0.1-10 kHz bandwidth (10^-13-10^-11 eV/c²).