Quantum heat engines provide attractive means in quantum thermodynamics for studying the fundamentals of the flow of heat and work. Previous experimental implementations of heat enginesoperating at the level of a few excitation quanta have utilized external driving, which has made the observation of the produced work challenging. Conversely, autonomous quantum heat engines only require a flow of heat to operate and generate work. However, autonomous quantum heat engines have not yet been experimentally demonstrated in any system despite numerous theoretical investigations. Here, we experimentally realize an autonomous quantum heat engine based on superconducting circuits. We construct the engine circuit implementing an approximate Otto cycle by coupling two superconducting resonators with a superconducting quantum interference device, and coupling this system to spectrally filtered hot and cold reservoirs. By varying the experimental conditions, we observe coherent microwave power generation arising from the internal dynamics of the system driven only by the thermal reservoirs. Our results validate previous theoretical predictions for this circuit and pave the way for detailed studies of quantum effects in heat engines and for using heat-generated coherent microwaves in circuit quantum electrodynamics.
Accurate control of quantum degrees of freedom is promising for sensing, communication, and computing, but building a useful quantum computer faces a central isolation-and-control challenge:qubits must remain well isolated from their environment to preserve coherence, yet also be coupled strongly enough for control, readout, and reset. Existing approaches address this challenge only partially, using separate reset elements, drive schemes, and Purcell filters, often with added complexity and tradeoffs such as heating and crosstalk. Here we introduce and demonstrate a first-generation quantum dial: a device that on demand mediates the coupling of a qubit to selected auxiliary degrees of freedom. Our implementation uses a band-stop filter between a high-coherence transmon qubit and a broadband transmission line, enabling the coupling strength to be tuned by several orders of magnitude on nanosecond timescales without significant Stark shift. In the reset configuration, we reduce the qubit energy relaxation time T1 from >150 μs to about 200 ns and demonstrate reset independent of the initial state. In the control configuration, we obtain 99.99% idle fidelity and 99.9% gate fidelities for 40 ns pulses at about -110 dBm. The same device also enables thermometry of the qubit environment, reaching a noise-equivalent temperature of 0.6 mK/Hz‾‾‾√ at 60 mK and approaching the Cramér-Rao bound at higher temperatures. The quantum dial thus offers fast, on-demand switching between isolation and strong coupling, with potential to reduce noise and errors in future quantum processors.
Propagating photons serve as essential links for distributing quantum information and entanglement across distant nodes. Knowledge of their Wigner functions not only enables their deploymentas active information carriers but also provides error diagnostics when photons passively leak from a quantum processing unit. While well-established for standing waves, characterizing propagating microwave photons requires post-processing of room-temperature signals with excessive amplification noise. Here, we demonstrate cryogenic and amplification-free Wigner function tomography of propagating microwave photons using a superconductor-normal metal-superconductor bolometer based on the resistive heating effect of absorbed radiation. By introducing two-field interference in power detection, the bolometer acts as a sensitive and broadband quadrature detector that samples the input field at selected angles at millikelvin with no added noise. Adapting the principles of computed tomography (CT) in medical imaging, we implement Wigner function CT by combining quadrature histograms across different projection angles and demonstrate it for Gaussian states at the single-photon level with different symmetries. Compressed sensing and neural networks further reduce the projections to three without compromising the reconstruction quality. These results address the long-standing challenge of characterizing propagating microwave photons in a superconducting quantum network and establish a new avenue for real-time quantum error diagnostics and correction.
Inductively shunted superconducting qubits, such as the unimon qubit, combine high anharmonicity with protection from low-frequency charge noise, positioning them as promising candidatesfor the implementation of fault-tolerant superconducting quantum computers. In this work, we develop accurate closed-form approximations for the frequency and anharmonicity of the unimon qubit that are also applicable to any single-mode superconducting qubits with a single-well potential profile, such as the quarton qubit or the kinemon qubit. We use these results to theoretically explore the single-qubit gate fidelity and coherence times across the parameter space of qubits with a single-well potential. We find that the gate fidelity can be optimized by tuning the Hamiltonian to (i) a high qubit mode impedance of 1−2 kΩ, (ii) a low qubit frequency of 1 GHz, (iii) and a perfect cancellation of the linear inductance and the Josephson inductance attained at a flux bias of half flux quantum. According to our theoretical analysis, the proposed qubit parameters have potential to enhance the single-qubit gate fidelity of the unimon beyond 99.99% even without significant improvements to the dielectric quality factor or the flux noise density measured for the first unimon qubits. Furthermore, we compare unimon, transmon and fluxonium qubits in terms of their energy spectra and qubit coherence subject to dielectric loss and 1/f flux noise in order to highlight the advantages and limitations of each qubit type.
In circuit quantum electrodynamics systems, the quasiparticle-related losses in Josephson junctions are suppressed due to the gap in the superconducting density of states which is muchhigher than the typical energy of a microwave photon. In this work, we show that a strong drive even at frequency lower than the double superconducting gap enables dissipation in the junctions due to photon-assisted breaking of the Cooper pairs. Both the decay rate and noise strength associated with the losses are sensitive to the dc phase bias of the junction and can be tuned in a broad range by the amplitude and the frequency of the external driving field, making the suggested mechanism potentially attractive for designing tunable dissipative elements. Furthermore, pronounced memory effects in the driven Josephson junctions render them perspective for both theoretical and experimental study of non-Markovian physics in superconducting quantum circuits. We illustrate our theoretical findings by studying the spectral properties and the steady state population of a low impedance resonator coupled to the driven Josephson junction.
Quantum heat engines (QHEs) have attracted long-standing scientific interest, especially inspired by considerations of the interplay between heat and work with the quantization of energylevels, quantum superposition, and entanglement. Operating QHEs calls for effective control of the thermal reservoirs and the eigenenergies of the quantum working medium of the engine. Although superconducting circuits enable accurate engineering of controlled quantum systems, beneficial in quantum computing, this framework has not yet been employed to experimentally realize a cyclic QHE. Here, we experimentally demonstrate a quantum heat engine based on superconducting circuits, using a single-junction quantum-circuit refrigerator (QCR) as a two-way tunable heat reservoir coupled to a flux-tunable transmon qubit acting as the working medium of the engine. We implement a quantum Otto cycle by a tailored drive on the QCR to sequentially induce cooling and heating, interleaved with flux ramps that control the qubit frequency. Utilizing single-shot qubit readout, we monitor the evolution of the qubit state during several cycles of the heat engine and measure positive output powers and efficiencies that agree with our simulations of the quantum evolution. Our results verify theoretical models on the thermodynamics of quantum heat engines and advance the control of dissipation-engineered thermal environments. These proof-of-concept results pave the way for explorations on possible advantages of QHEs with respect to classical heat engines.
We propose and theoretically analyze a realistic superconducting electric circuit that can be used to realize an autonomous quantum heat engine in circuit quantum electrodynamics. Usinga quasiclassical, non-Markovian theoretical model, we demonstrate that coherent microwave photon generation can emerge solely from heat flow through the circuit and its nonlinear internal dynamics. The predicted generation rate is sufficiently high for experimental observation in circuit quantum electrodynamics, making this work a significant step toward the first experimental realization of an autonomous quantum heat engine in superconducting circuits.
Recently, ultrasensitive calorimeters have been proposed as a resource-efficient solution for multiplexed qubit readout in superconducting large-scale quantum processors. However, experimentsdemonstrating frequency multiplexing of these superconductor-normal conductor-superconductor (SNS) sensors are coarse. To this end, we present the design, fabrication, and operation of three SNS sensors with frequency-multiplexed input and probe circuits, all on a single chip. These devices have their probe frequencies in the range \SI{150}{\mega\hertz} — \SI{200}{\mega\hertz}, which is well detuned from the heater frequencies of \SI{4.4}{\giga\hertz} — \SI{7.6}{\giga\hertz} compatible with typical readout frequencies of superconducting qubits. Importantly, we show on-demand triggering of both individual and multiple low-noise SNS bolometers with very low cross talk. These experiments pave the way for multiplexed bolometric characterization and calorimetric readout of multiple qubits, a promising step in minimizing related resources such as the number of readout lines and microwave isolators in large-scale superconducting quantum computers.
Superconducting qubits are one of the most promising physical systems for implementing a quantum computer. However, executing quantum algorithms of practical computational advantagerequires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.890 GHz, an energy relaxation time with a median of 502 us and a maximum of (765 +/- 82.6) us, and an echo dephasing time with a median of 541 us and a maximum of (1057 +/- 138) us. We report details of our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.
Achieving fast and precise initialization of qubits is a critical requirement for the successful operation of quantum computers. The combination of engineered environments with all-microwavetechniques has recently emerged as a promising approach for the reset of superconducting quantum devices. In this work, we experimentally demonstrate the utilization of a single-junction quantum-circuit refrigerator (QCR) for an expeditious removal of several excitations from a transmon qubit. The QCR is indirectly coupled to the transmon through a resonator in the dispersive regime, constituting a carefully engineered environmental spectrum for the transmon. Using single-shot readout, we observe excitation stabilization times down to roughly 500 ns, a 20-fold speedup with QCR and a simultaneous two-tone drive addressing the e-f and f0-g1 transitions of the system. Our results are obtained at a 48-mK fridge temperature and without postselection, fully capturing the advantage of the protocol for the short-time dynamics and the drive-induced detrimental asymptotic behavior in the presence of relatively hot other baths of the transmon. We validate our results with a detailed Liouvillian model truncated up to the three-excitation subspace, from which we estimate the performance of the protocol in optimized scenarios, such as cold transmon baths and fine-tuned driving frequencies. These results pave the way for optimized reset of quantum-electric devices using engineered environments and for dissipation-engineered state preparation.