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Superfluid-Cooled Transmon Qubits under Optical Excitation

Chunzhen Li1,*, Yufeng Wu1,*, Manuel C. C. Pace1, Matthew D. LaHaye2, Michael Senatore2,†, and Hong X. Tang1,‡

  • *These authors contributed equally to this work.
  • †Contact author: michael.senatore.1@us.af.mil
  • ‡Contact author: hong.tang@yale.edu
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PRX Quantum 6, 030303 – Published 7 July, 2025

DOI: https://doi.org/10.1103/q99s-lrnv

Abstract

Microwave-to-optical quantum state transfer enables the interconnection of remote superconducting quantum processors via optical-fiber links. Achieving high conversion efficiency necessitates an intense optical pump, yet direct optical exposure of superconducting circuits often degrades their performance. Efficient and fast thermalization of superconducting qubits is therefore essential. To achieve this, we immerse a laser-illuminated transmon qubit in superfluid helium-4 and investigate its behavior. We observe that the qubit recovers significantly faster in superfluid helium than in vacuum, with an improvement in power handling by over 10 dB. This enhanced cooling capacity supports higher optical pump powers, shortening recovery times after high-power optical pulses. These findings open a promising pathway for future efficient implementation of microwave-to-optical quantum transduction devices.

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Popular Summary

To build large-scale quantum networks, scientists are developing ways to link superconducting quantum computers, which operate at microwave frequency, via laser light transmitted through optical fibers, a strategy that minimizes loss over long distances. However, superconducting circuits are highly sensitive to heat and the intense laser light required for high-efficiency quantum communication can easily degrade the fragile superconducting circuits. One promising solution involves superfluid helium-4, a special form of liquid helium known for its remarkable cooling capacity.

In this work, we placed a superconducting qubit, a fundamental building block of quantum processors, into superfluid helium-4 and subjected it to laser pulses. Compared to the traditional vacuum environment, the qubit recovered far more quickly from laser-induced effects, showing an over tenfold increase in tolerance to laser power. This allows us to apply stronger laser power without harming the quantum circuitry, a critical advancement for devices that convert quantum signals between microwave and optical frequencies.

These findings highlight the potential of superfluid helium-4 as a thermal buffer for protecting quantum devices from laser-induced heating. Looking ahead, this approach paves the way for more resilient hybrid quantum systems capable of supporting long-range quantum communication, bringing us a step closer to the goal of a global quantum Internet.

Article Text

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PRX Quantum 6, 030303– Published 7 July, 2025
  • Received 11 February 2025
  • Accepted 9 June 2025
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