Ultracold Mechanical Quantum Sensor for Tests of New Physics

TL;DR

This paper demonstrates ultracold mechanical modes in a high-overtone bulk acoustic wave resonator, enabling advanced quantum sensing and tests of fundamental physics such as dark matter and gravitational waves.

Contribution

It reports the lowest excited-state populations in GHz mechanical modes and uses these measurements to constrain new physics models.

Findings

First excited state population as low as 1.2e-5

Effective temperature of 25.2 mK for the mechanical modes

Constraints on high-frequency gravitational waves and dark matter interactions

Abstract

Initialization of mechanical modes in the quantum ground state is crucial for their use in quantum information and quantum sensing protocols. In quantum processors, impurity of the modes' initial state affects the infidelity of subsequent quantum algorithms. In quantum sensors, excitations out of the ground state contribute to the noise of the detector, and their prevalence puts a bound on rare events that deposit energy into the mechanical modes. In this work, we measure the excited-state populations of GHz-frequency modes in a high-overtone bulk acoustic wave resonator (HBAR). We find that the population of the first excited state can be as low as $P_p$=(1.2$\pm$5.5)$\times10^{-5}$, corresponding to an effective temperature of 25.2 mK, which are upper bounds limited by imperfections in the measurement process. These results compare favorably to the lowest populations measured in superconducting circuits. Finally, we use the measured populations to constrain the amplitude of high-frequency gravitational waves, the kinetic mixing strength of ultra-light dark matter, and non-linear modifications of the Schr\"{o}dinger equation describing wavefunction collapse mechanisms. Our work establishes HBARs as a versatile resource for quantum state initialization and studies of fundamental physics.