Feedback cooling of the normal modes of a massive electromechanical system to submillikelvin temperature

TL;DR

This paper demonstrates feedback cooling of a large electromechanical system's normal modes to submillikelvin temperatures, advancing toward quantum ground state preparation for massive resonators.

Contribution

It introduces a feedback cooling method for a ton-scale gravitational wave detector's normal modes using a SQUID-based measurement system.

Findings

Achieved a minimum temperature of 0.17 mK for the normal modes.

Demonstrated cooling starting from 4.2 K bath temperature.

Potential to reach quantum ground state with further improvements.

Abstract

We apply a feedback cooling technique to simultaneously cool the three electromechanical normal modes of the ton-scale resonant-bar gravitational wave detector AURIGA. The measuring system is based on a dc Superconducting Quantum Interference Device (SQUID) amplifier, and the feedback cooling is applied electronically to the input circuit of the SQUID. Starting from a bath temperature of 4.2 K, we achieve a minimum temperature of 0.17 mK for the coolest normal mode. The same technique, implemented in a dedicated experiment at subkelvin bath temperature and with a quantum limited SQUID, could allow to approach the quantum ground state of a kilogram-scale mechanical resonator.