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Strong-coupling quantum thermodynamics using a superconducting flux qubit

Rishabh Upadhyay1,2,*, Bayan Karimi1,3, Diego Subero1, Christoforus Dimas Satrya1, Joonas T. Peltonen1, Yu-Cheng Chang1, and Jukka P. Pekola1

  • *Contact author: rishabh.upadhyay@aalto.fi
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Phys. Rev. Applied 25, 044030 – Published 13 April, 2026

DOI: https://doi.org/10.1103/3v35-y762

Abstract

Thermodynamics in quantum circuits aims to find improved functionalities of thermal machines, highlight fundamental phenomena peculiar to quantum nature in thermodynamics, and point out limitations in quantum information processing due to the coupling of the system to its environment. An important aspect in achieving some of these goals is the regime of strong coupling, which has remained until now a domain of theoretical works only. Our aim is to demonstrate strong coupling features in heat transport using a superconducting flux qubit, which is capable of reaching strong- to deep-ultrastrong-coupling regimes, as shown in previous studies. Here, we show experimental evidence of strong coupling by observing a hybridized state of the qubit with two cavities coupled to it, leading to a triplet-like thermal transport via this combined system around the minimum energy of the qubit at power levels of tens of femtowatts, exceeding by an order of magnitude those in earlier experiments. We also demonstrate a nearly 100% on-off switching ratio of heat current mediated by photons by applying magnetic flux to the qubit. Our experiment opens a way towards testing debated questions in strong-coupling thermodynamics, such as what heat is in this regime. We also present a theoretical model that aligns with our experimental findings and explains the mechanism behind heat transport in our device. Furthermore, our experiment opens possibilities for quantum thermodynamics, aiming to realize true quantum heat engines and refrigerators with enhanced power and efficiency, by leveraging ultrastrong coupling between the system and its environment in future experiments.

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Phys. Rev. Applied 25, 044030– Published 13 April, 2026
  • Received 8 December 2024
  • Revised 27 January 2026
  • Accepted 10 February 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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