# Integrating superfluids with superconducting qubit systems

## TL;DR

This study explores the integration of superfluid helium-4 with superconducting transmon qubits, demonstrating minimal impact on energy relaxation and a modest increase in dephasing time, highlighting potential for improved thermal management in quantum systems.

## Contribution

It provides the first controlled immersion of a superconducting qubit in superfluid helium-4 and analyzes the effects within the cQED framework, revealing how superfluid modifies system properties.

## Key findings

- Superfluid helium-4 does not significantly affect qubit energy relaxation times.
- Pure dephasing times of the qubit are modestly increased by the superfluid.
- Superfluid modifies the cavity, qubit, and their coupling properties.

## Abstract

Superfluid helium's low-loss dielectric properties, excellent thermal conductivity, and unique collective excitations make it an attractive candidate to incorporate into superconducting qubit systems. We controllably immerse a three-dimensional superconducting transmon qubit in superfluid helium-4 and measure the spectroscopic and coherence properties of the system. We find that the cavity, the qubit, and their coupling are all modified by the superfluid, which we analyze within the framework of circuit quantum electrodynamics (cQED). At at temperatures relevant to quantum computing experiments, the energy relaxation time of the qubit is not significantly changed by the presence of the superfluid, while the pure dephasing time modestly increases, which we attribute to improved thermalization of the microwave environment via the superfluid.