Quantum bath suppression in a superconducting circuit by immersion cooling

TL;DR

This paper demonstrates that immersing superconducting circuits in liquid helium-3 can significantly lower environmental temperature, reducing decoherence and improving quantum device performance by effectively cooling the quantum bath.

Contribution

The study introduces a method of immersing superconducting circuits in liquid helium-3 to suppress the quantum bath and achieve lower effective temperatures, enhancing coherence.

Findings

Continuous physical quantity changes down to sub-mK temperatures.

Energy relaxation rate increased by a factor of a thousand.

No additional circuit losses or noise introduced.

Abstract

Quantum circuits interact with the environment via several temperature-dependent degrees of freedom. Yet, multiple experiments to-date have shown that most properties of superconducting devices appear to plateau out at $T\approx 50$ mK -- far above the refrigerator base temperature. This is for example reflected in the thermal state population of qubits, in excess numbers of quasiparticles, and polarisation of surface spins -- factors contributing to reduced coherence. We demonstrate how to remove this thermal constraint by operating a circuit immersed in liquid $^3$He. This allows to efficiently cool the decohering environment of a superconducting resonator, and we see a continuous change in measured physical quantities down to previously unexplored sub-mK temperatures. The $^3$He acts as a heat sink which increases the energy relaxation rate of the quantum bath coupled to the circuit a thousand times, yet the suppressed bath does not introduce additional circuit losses or noise. Such quantum bath suppression can reduce decoherence in quantum circuits and opens a route for both thermal and coherence management in quantum processors.