309th RQC Seminar

• 講演者

  Mr. Simon Sundelin
  ( Charmers University of Technology, Sweden )

• 日程

  2026年10月9日(金), 13:00 - 14:00 (1:00pm - 2:00pm)

• 開催場所

  ハイブリッド（Zoom,
  和光地区 統合支援施設2階 大会議室 / Large Meeting Room, 2F, Welfare and Conference Building, Wako Campus (C61))

• 講演タイトル

  One artificial molecule, many machines: heat, photons, and current fluctuations

• お問合せ

  yasunobu.nakamura[at]riken.jp

講演概要
One artificial molecule, many machines: heat, photons, and current fluctuations Fluctuations are central to small nonequilibrium systems, where energy is exchanged in discrete quanta and transport is intrinsically noisy. Engineering how a quantum device couples to its environment allows us both to harness these fluctuations and to investigate the discrete processes underlying energy transport. In this talk, I will present an artificial molecule formed by two strongly coupled transmon qubits, with symmetry-selective coupling to microwave waveguides. I will discuss three experiments on this platform that move from using noise as a resource, through resolving individual photon arrivals, to testing the relation between current fluctuations and dissipation.

I will first show how deliberately applied noise induces transitions between molecular states, enabling energy transport against a temperature gradient and powering a refrigerator that cools microwave modes. Moving from average energy flows to individual photons, I will then show how symmetry-selective coupling also enables photon detection. Combining rapid photon absorption with transfer into a long-lived dark state allows an incoming photon to leave a measurable record. Continuous monitoring of this state realizes an always-on microwave single-photon detector and reveals a trade-off between acquiring information and disturbing the absorption process.

Beyond individual detection events, the statistics of photon currents raise a fundamental question: how precisely can a quantum machine operate for a given thermodynamic cost? Suppressing fluctuations in a nonequilibrium current generally requires increased dissipation. Thermodynamic uncertainty relations (TURs) quantify this trade-off for classical stochastic dynamics, but whether quantum devices can surpass the canonical steady-state bound has remained an outstanding experimental question. Operating the artificial molecule as a thermal machine, we measure the first and second moments of the integrated microwave output power, from which we extract both the mean photon current and its fluctuations. We observe a TUR ratio of Q = 1.71 ± 0.17, below the classical bound of 2. This result demonstrates that steady-state quantum transport can achieve a precision beyond that permitted by the canonical relation, opening a path toward quantum thermal devices with enhanced precision at reduced thermodynamic cost.

Together, these results show how symmetry-selective coupling makes an artificial molecule a versatile platform for harnessing fluctuations, resolving individual energy-exchange events, and testing the limits of nonequilibrium quantum transport.

1. M. A. Aamir, C. Castillo Moreno, S. Sundelin, J. Biznárová, M. Scigliuzzo, K. E. Patel, A. Osman, D. P. Lozano, I. Strandberg, and S. Gasparinetti, “Engineering Symmetry-Selective Couplings of a Superconducting Artificial Molecule to Microwave Waveguides,” Physical Review Letters 129, 123604 (2022).
2. S. Sundelin, M. A. Aamir, V. M. Kulkarni, C. Castillo-Moreno, and S. Gasparinetti, “Quantum refrigeration powered by noise in a superconducting circuit,” Nature Communications 17, 359 (2026).
3. V. Kulkarni, M. A. Aamir, S. Sundelin, and S. Gasparinetti, “Always-on, highly efficient microwave photon detector based on a superconducting artificial molecule,” arXiv:2608.12972 (2026).
4. S. Sundelin, L. Nordqvist, K. Mahadeviya, V. Kulkarni, M. A. Aamir, M. T. Mitchison, and S. Gasparinetti, “A quantum thermal machine surpassing the classical thermodynamic limit on precision,” arXiv:2609.37149 (2026).