Detectors for Fundamental Physics

Conventional gravitational wave astronomy occupies the 10–1000 Hz band of LIGO and Virgo. Above this, the kilohertz-to-megahertz band remains essentially unobserved — despite hosting astrophysical sources such as the post-merger remnants of binary neutron stars, and cosmological backgrounds from phase transitions in the early universe and from primordial black-hole populations. Detectors operating in this regime need not be kilometre-scale: the required sensitivity can be approached with table-top instruments built from microwave cavities and high-Q mechanical modes.

We are developing high-frequency gravitational wave detectors based on superfluid ⁴He inside three-dimensional microwave re-entrant cavities. A passing gravitational wave modulates the geometry of the cavity-helium system, imprinting a signature on the microwave field that can be read out near the standard quantum limit using superconducting amplifiers. The combination of a millikelvin operating environment, a high-quality-factor microwave resonator, and a superfluid medium with extraordinarily low acoustic loss is well-matched to the kilohertz target band.

This effort is closely tied to our work on cavity optomechanics with superfluids — the underlying physics is the same — and it benefits from the broader detector-development community working on bulk acoustic resonators, levitated objects, and high-Q microwave cavities for axion and gravitational wave searches. The peripheral hardware that makes such measurements possible is the focus of our superconducting quantum technologies programme.