Observation of the Fundamental Nyquist Noise Limit in an Ultra-High $Q$-Factor Cryogenic Bulk Acoustic Wave Cavity

TL;DR

This study demonstrates the observation of thermal Nyquist noise in ultra-high Q-factor cryogenic bulk acoustic wave cavities, confirming the device's low noise and high quality factor at quantum-limited conditions.

Contribution

It provides the first experimental verification of Nyquist noise limits in ultra-high Q BAW cavities at cryogenic temperatures, validating the models and showing potential for quantum-limited operation.

Findings

Nyquist noise observed at cryogenic temperatures with high SNR

Effective temperature matches physical temperature, confirming model accuracy

Q factor remains above 10^8 at low system motion levels

Abstract

Thermal Nyquist noise fluctuations of high-$Q$ Bulk Acoustic Wave (BAW) cavities have been observed at cryogenic temperatures with a DC Superconducting Quantum Interference Device (SQUID) amplifier. High $Q$ modes with bandwidths of few tens of milliHz produce thermal fluctuations with a Signal-To-Noise ratio of up to 23dB. The estimated effective temperature from the Nyquist noise is in good agreement with the physical temperature of the device, confirming the validity of the equivalent circuit model and the non-existence of any excess resonator self-noise. The measurements also confirm that the quality factor remains extremely high ($Q>10^8$ at low order overtones) for very weak (thermal) system motion at low temperatures, when compared to values measured with relatively strong external excitation. This result represents an enabling step towards operating such a high-Q acoustic device at the standard quantum limit.