A quantum thermal machine surpassing the classical thermodynamic limit on precision

  1. Simon Sundelin,
  2. Ludvig Nordqvist,
  3. Khalak Mahadeviya,
  4. Vyom Kulkarni,
  5. Mohammed Ali Aamir,
  6. Mark T. Mitchison,
  7. and Simone Gasparinetti
Precision in nonequilibrium processes comes at a thermodynamic cost: suppressing fluctuations generally requires increased dissipation. Thermodynamic uncertainty relations (TURs) make
this trade-off quantitative by linking current fluctuations to entropy production in classical stochastic dynamics. In the decade since its discovery, the canonical steady-state TUR and its finite-time generalizations have become a cornerstone of non-equilibrium thermodynamics, constraining the performance of molecular machines and allowing heat dissipation to be inferred from observable fluctuations. Whether the canonical TUR can be violated in a controlled quantum device remains an outstanding experimental question, in part because doing so requires resolving extremely small steady-state currents as well as their fluctuations. Here we experimentally show that steady-state quantum transport can surpass the precision permitted by the canonical TUR. We observe this violation in a superconducting quantum thermal machine coupled to a microwave waveguide acting as a cold bath and to a classical noise source providing an effective infinite-temperature bath. We observe a TUR ratio Q=1.71±0.17, in violation of the classical bound Q≥2. Our results demonstrate a fundamental distinction between classical and quantum thermodynamics, paving the way for quantum thermal devices that achieve enhanced precision at reduced energy cost.

Always-on, highly efficient microwave photon detector based on a superconducting artificial molecule

  1. Vyom Kulkarni,
  2. Mohammed Ali Aamir,
  3. Simon Sundelin,
  4. and Simone Gasparinetti
Efficient detection of single microwave photons is a key capability for emerging quantum technologies. Yet, it remains far less developed than its optical domain counterpart. Realizing
detectors that simultaneously achieve high efficiency, low dark counts, and continuous operation has proved challenging. Existing detectors operate cyclically, forcing a trade-off between efficiency and duty cycle. Here, we demonstrate a continuously operated microwave single-photon detector based on a superconducting artificial molecule. In our scheme, an incoming photon is captured by a bright state of the molecule and then transferred to a long-lived dark state via a driven-dissipative process. Photon „clicks“ are revealed as quantum jumps in the continuously monitored dark state. We observe a cyclic detection efficiency of 0.73, and a continuous detection efficiency of 0.47 over a 5MHz instantaneous bandwidth, with a 1μs temporal resolution and a 15μs dead time. By overcoming the trade-off between efficiency and duty cycle, this approach establishes continuous microwave photon detection for quantum sensing, quantum thermodynamics, and fundamental physics.