Thermally driven quantum refrigerator autonomously resets superconducting qubit Mohammed Ali Aamir, 1, ∗ Paul Jamet Suria, 1 Jos´ e Antonio Mar´ ın Guzm´ an, 2 Claudia Castillo-Moreno, 1 Jeffrey M. Epstein, 2, 3 Nicole Yunger Halpern, 2, 3, † and Simone Gasparinetti 1, ‡ 1 Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden 2 Joint Center for Quantum Information and Computer Science, NIST and University of Maryland, College Park, MD 20742, USA 3 Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA (Dated: May 29, 2023) The first thermal machines steered the industrial revolution, but their quantum analogs have yet to prove useful. Here, we demonstrate a useful quantum absorption refrigerator formed from supercon- ducting circuits. We use it to reset a transmon qubit to a temperature lower than that achievable with any one available bath. The process is driven by a thermal gradient and is autonomous— requires no external control. The refrigerator exploits an engineered three-body interaction between the target qubit and two auxiliary qudits coupled to thermal environments. The environments consist of microwave waveguides populated with synthesized thermal photons. The target qubit, if initially fully excited, reaches a steady-state excited-level population of 5 × 10 -4 ± 5 × 10 -4 (an effective temperature of 23.5 mK) in about 1.6 μs. Our results epitomize how quantum thermal machines can be leveraged for quantum information-processing tasks. They also initiate a path toward experimental studies of quantum thermodynamics with superconducting circuits coupled to propagating thermal microwave fields. Quantum thermodynamics should be more useful. The field has yielded fundamental insights, such as extensions of the second law of thermodynamics to small, coherent, and far-from-equilibrium systems [1–13]. Additionally, quantum phenomena have been shown to enhance en- gines [14–19], batteries [20], and refrigeration [21, 22]. These results are progressing gradually from theory to proof-of-principle experiments. However, quantum ther- mal technologies remain experimental curiosities, not practical everyday tools. Key challenges include con- trol [23] and cooling quantum thermal machines to tem- peratures that support quantum phenomena. Both chal- lenges require substantial energy and effort but yield small returns. For example, one would expect a single- atom engine to perform only about an electronvolt of work [24]. Autonomous quantum machines offer hope. First, they operate without external control. Second, they run on heat drawn from thermal baths, which are naturally abundant [25]. A quantum thermal machine would be useful in a context that met three criteria: (i) The ma- chine fulfills a need. (ii) The machine can access natural different-temperature baths. (iii) Maintaining the ma- chine’s coherence costs no extra expense. We identify such a context: qubit reset. Consider a su- perconducting quantum computer starting a calculation. The computer requires qubits initialized to their ground states. If left to thermalize with its environment as thor- oughly as possible, though, the qubit could achieve only an excited-state population of ≈0.01 to 0.03, or an effec- * aamir.ali@chalmers.se † nicoleyh@umd.edu ‡ simoneg@chalmers.se tive temperature of 45 mK to 70 mK [26–29]. Further- more, such passive thermalization takes a few multiples of the qubit’s energy-relaxation time—hundreds of mi- croseconds in state-of-the-art setups—delaying the next computation. A quantum machine cooling the qubits to their ground (minimal-entropy) states fulfills criterion (i). Moreover, superconducting qubits inhabit a dilution refrigerator formed from nested plates, whose temper- atures decrease from the outermost plate to the inner- most. These temperature plates can serve as heat baths, meeting criterion (ii). Finally, the machine can retain its quantum nature if mounted on the coldest plate, next to the quantum processing unit, satisfying criterion (iii). Such an autonomous machine would be a quantum ab- sorption refrigerator. Quantum absorption refrigerators have been widely studied theoretically [30–49]. Reference [50] reported a landmark proof-of-principle experiment performed with trapped ions. However, the heat baths were emu- lated with electric fields and lasers, rather than natu- ral sources. Other quantum refrigerators, motivated by possible applications, have been proposed [51, 52] and tested [53–55] but are not autonomous. We report on a quantum absorption refrigerator re- alized with superconducting circuits. Our quantum re- frigerator cools and therefore resets a target supercon- ducting qubit autonomously. The target qubit’s energy- relaxation time is fully determined by the temperature of a hot bath we can vary. Using this control, we can vary the energy-relaxation time by a factor of > 60. The reset’s fidelity is competitive: The target’s excited-state population reaches as low as 5 × 10 −4 (effective tempera- tures as low as 23.5 mK). In comparison, state-of-the-art reset protocols achieve populations of 8×10 −4 to 2 ×10 −3 (effective temperatures of 40 mK to 49 mK) [28, 29]. arXiv:2305.16710v1 [quant-ph] 26 May 2023