Superconducting flux qubit for quantum thermodynamics experiments

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School of Science | Doctoral thesis (article-based) | Defence date: 2025-10-31
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Authors

Date

2025

Department

Teknillisen fysiikan laitos
Department of Applied Physics

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Mcode

Degree programme

Language

en

Pages

113 +app. 88

Series

Aalto University publication series Doctoral Theses, 197/2025

Abstract

Advancements in the field of cavity quantum electrodynamics (c-QED) have deepened understanding of quantum phenomena, enabled reliable Josephson junction-based devices, and facilitated their integration with other quantum technologies to explore new physics and applications. Motivated by these developments, this thesis aims to construct a testbed for studying quantum thermodynamics using a superconducting flux qubit. The flux qubit is particularly suited for thermodynamic experiments due to its strong anharmonicity and its galvanic coupling architectures that facilitate efficient energy transport. In this thesis, the on-chip components include harmonic oscillators realized as superconducting resonators, a strongly nonlinear superconducting flux qubit, two copper thermal reservoirs modelled as Johnson–Nyquist noise sources and NIS thermometers, integrated on a highly resistive silicon platform that acts as a phonon bath. These components enable the study and optimization of heat transport between the metallic reservoirs, mediated by the quantum circuit, with the flux qubit as the central, flux-tunable working substance. Heat transport through the qubit occurs via sequential excitation and relaxation cycles between its ground and excited states, where excitation extracts heat equal to the qubit’s energy gap from the hot reservoir, while relaxation releases the same energy into the cold reservoir. Enhancing the thermal device’s performance critically depends on the coupling strength between on-chip components, particularly between the qubit and resonator. This thesis commences with the investigation and demonstration of a robust qubit–resonator coupling design that achieves the ultrastrong regime experimentally. Building on this result, the developed architecture is extended to connect the qubit with two identical resonators, symmetrically positioned on the two sides. These resonators are, in turn, coupled to metallic reservoirs, enabling the first ever demonstration of flux qubit-controlled heat transport between the two reservoirs with excellent device performance. Added geometric asymmetry in the coupler, combined with the flux qubit’s strong anharmonicity, enables the successful demonstration--presented in this thesis--of a non-reciprocal device such as the Microwave Quantum Diode, which, together with the reservoirs, could potentially form an efficient thermal diode. Finally, the thesis concludes with a brief outlook on future directions and ongoing experiments that build on the flux qubit–based testbed developed here for exploring quantum thermodynamic phenomena and device applications.

Description

Supervising professor

Pekola, Jukka, Prof., Aalto University, Department of Applied Physics, Finland

Thesis advisor

Peltonen, Joonas, Dr., Aalto University, OtaNano, Finland

Keywords

quantum thermodynamics, quantum heat transport, superconducting flux qubit, quantum electrodynamics, superconducting circuits, mesoscopic devices, thermal diode

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Parts

  • [Publication 1]: Rishabh Upadhyay, George Thomas, Yu-Cheng Chang, Dmitry S. Golubev, Andrew Guthrie, Azat Gubaydullin, Joonas T. Peltonen and Jukka P. Pekola. Robust strong-coupling architecture in circuit quantum electrodynamics. Phys. Rev. Applied, Vol 16, Issue 4, Pages 11, Published October 2021.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-2021111710263
    DOI: 10.1103/PhysRevApplied.16.044045 View at publisher
  • [Publication 2]: Rishabh Upadhyay, Bayan Karimi, Diego Subero, Christoforus Dimas Satrya, Joonas T. Peltonen, Yu-Cheng Chang and Jukka P. Pekola. Towards strong-coupling quantum thermodynamics using a superconducting flux qubit. Submitted to Phys. Rev. X., December 2024
  • [Publication 3]: Rishabh Upadhyay, Dmitry S. Golubev, Yu-Cheng Chang, George Thomas, Andrew Guthrie, Joonas T. Peltonen and Jukka P. Pekola. Microwave quantum diode. Nat. Commun., Vol 15, Issue 630, Pages 9, Published January 2024.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202402072361
    DOI: 10.1038/s41467-024-44908-w View at publisher
  • [Publication 4]: S. A. Lemziakov, B. Karimi, S. Nakamura, D. S. Lvov, R. Upadhyay, C. D. Satrya, Z.-Y. Chen, D. Subero, Y.-C. Chang, L. B. Wang and J. P. Pekola. Applications of Superconductor–Normal Metal Interfaces. J Low Temp Phys, Vol 217, Pages 54, Published March 2024.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202410306982
    DOI: 10.1007/s10909-024-03144-8 View at publisher
  • [Publication 5]: Christoforus Dimas Satrya, Yu-Cheng Chang, Aleksandr S. Strelnikov, Rishabh Upadhyay, Ilari K. Mäkinen, Joonas T. Peltonen, Bayan Karimi and Jukka P. Pekola. Thermal spectrometer for superconducting circuits. Nat. Commun., Vol 16, Issue 4435, Pages 7, Published May 2025.
    Full text in Acris/Aaltodoc: https://urn.fi/URN:NBN:fi:aalto-202505284105
    DOI: 10.1038/s41467-025-58919-8 View at publisher

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https://urn.fi/URN:ISBN:978-952-64-2764-5

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[diss] Perustieteiden korkeakoulu / SCI