An Ultra-Cold Mechanical Quantum Sensor for Tests of New Physics

Preparing quantum systems in their ground state is essential for quantum information processing and quantum sensing protocols. Excitations out of the ground state degrade the fidelity of quantum processors and add noise to quantum sensors, limiting their ability to detect weak signals. In this presentation, I will discuss direct measurement of excited-state populations of GHz-frequency modes in a high-overtone bulk acoustic wave resonator (HBAR). The measured populations can be as low as (1.2±5.5)x10^(-5), which corresponds to an effective temperature of about 25 mK–lower than in other MHz- or GHz-frequency quantum systems. Together with its large effective mass, this makes the HBAR an excellent platform for tests of new physics. I will present the implications of our results on possible signatures of gravitational waves and dark photons.