While dephasing noise often hinders quantum devices, it can become an asset for quantum thermal machines. Here we demonstrate a three-level thermal machine that leverages noise-assisted quantum transport to enable steady-state cooling of microwave modes. The device exploits symmetry-selective couplings between a superconducting artificial molecule and two physical heat baths. Each bath consists of a microwave waveguide populated with synthesized quasithermal radiation. Energy transport is enabled by injecting dephasing noise through a third channel longitudinally coupled to one artificial atom of the molecule. By varying the effective temperatures of the reservoirs and measuring photonic heat currents with sub-attowatt resolution, we demonstrate energy flow dynamics characteristic of a quantum heat engine, thermal accelerator, and refrigerator. Our work constitutes an experimental demonstration of the key operating principles of a noise-assisted three-level quantum refrigerator and opens new avenues for experiments in quantum thermodynamics using superconducting circuits coupled to physical heat baths.
While dephasing noise often hinders quantum devices, it can become an asset for quantum thermal machines. Here we demonstrate a three-level thermal machine that leverages noise-assisted quantum transport to enable steady-state cooling of microwave modes. The device exploits symmetry-selective couplings between a superconducting artificial molecule and two physical heat baths. Each bath consists of a microwave waveguide populated with synthesized quasithermal radiation. Energy transport is enabled by injecting dephasing noise through a third channel longitudinally coupled to one artificial atom of the molecule. By varying the effective temperatures of the reservoirs and measuring photonic heat currents with sub-attowatt resolution, we demonstrate energy flow dynamics characteristic of a quantum heat engine, thermal accelerator, and refrigerator. Our work constitutes an experimental demonstration of the key operating principles of a noise-assisted three-level quantum refrigerator and opens new avenues for experiments in quantum thermodynamics using superconducting circuits coupled to physical heat baths.
Abstract Although classical thermal machines power industries and modern living, quantum thermal engines have yet to prove their utility. Here, we demonstrate a useful quantum absorption refrigerator formed from superconducting circuits. We use it to cool a transmon qubit to a temperature lower than that achievable with any one available bath, thereby resetting the qubit to an initial state suitable for quantum computing. The process is driven by a thermal gradient and is autonomous, requiring no external feedback. The refrigerator exploits an engineered three-body interaction between the target qubit and two auxiliary qudits. Each auxiliary qudit is coupled to a physical heat bath, realized with a microwave waveguide populated with synthesized quasithermal radiation. If the target qubit is initially fully excited, its effective temperature reaches a steady-state level of approximately 22 mK, lower than what can be achieved by existing state-of-the-art reset protocols. Our results demonstrate that superconducting circuits with propagating thermal fields can be used to experimentally explore quantum thermodynamics and apply it to quantum information-processing tasks.
We use a transmon qubit and its dispersively coupled readout resonator to measure the Fock state populations of another microwave resonator, to which we have attached a quantum-circuit refrigerator (QCR). First, we apply noise generated at room temperature to the resonator and show that such noise drive leads to a thermal distribution of the resonator Fock states. Subsequently, we detune the noise frequency band far away from the resonance condition and vary the power of the noise applied on the QCR. We observe that such artificial thermal noise may lead to major damping of a coherent state of the resonator. Importantly, we also demonstrate that the effective temperature of a thermal resonator state can be reduced from roughly 300 to 130 mK by the introduction of the artificial thermal noise. These observations pave the way for a purely thermally powered quantum-circuit refrigerator that may unlock the use of waste heat in resetting superconducting qubits in a quantum processor and in building autonomous quantum heat engines.
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.
We observe the continuous emission of photons into a waveguide from a superconducting qubit without the application of an external drive. To explain this counterintuitive observation, we build a two-bath model where the qubit couples simultaneously to a cold bath (the waveguide) and a hot bath (a secondary environment). Our results show that the thermal-photon occupation of the hot bath is up to 0.14 photons, 35 times larger than the cold waveguide, leading to nonequilibrium heat transport with a power of up to 132 zW, as estimated from the qubit emission spectrum. By adding more isolation between the sample output and the first cold amplifier in the output line, the heat transport is strongly suppressed. Our interpretation is that the hot bath may arise from active two-level systems being excited by noise from the output line, and that the qubit coherence can be improved significantly by suppressing this noise. We also apply a coherent drive, and use the waveguide to measure thermodynamic work and heat, suggesting waveguide spectroscopy is a useful means to study quantum heat engines and refrigerators. Finally, based on the theoretical model, we propose how a similar setup can be used as a noise spectrometer which provides a solution for calibrating the background noise of hybrid quantum systems. Published by the American Physical Society 2022
Superconducting microwave circuits are a ubiquitous and important tool for devices that exploit the phenomena of superconductivity and cryogenic temperatures as an environment for achieving the generation, manipulation, and detection of quantum states - required for the ongoing development of quantum technologies. In particular, superconducting circuits are a promising platform for the universal quantum computer, which will require an unprecedented density of quantum-coherent elements to perform large-scale quantum-enhanced calculations and simulations, using the framework of cavity quantum lectrodynamics.Dissipation and heat in these superconducting circuits is a key source of error and inefficiency, however the thermodynamics in this regime is poorly understood despite its increasing relevancy.This thesis describes the integration of superconducting resonators and artificial atoms derived from superconducting quantum circuits with ultra-sensitive bolometry, for looking at heat transport through superconducting circuits. We will describe the physics and operation of each of these elements, before combining and utilising them to perform heat transport measurements through a superconducting artificial atom coupled to two resonators, each terminated by a normal-metal mesoscopic resistor, with the resistor temperature measured and temperature gradients across the circuit induced by superconducting tunnel-probes. We will present tunable heat transport through this system, firstly when the resonators are symmetric, allowing us to observe the role of dissipation-limited coupling of the resonators to the artificial atom on the locality of the heat transport, then on an asymmetric system, demonstrating a directional rectification of the heat transport. Additionally, we will discuss how each element of the system can be individually characterised, in particular the quality factor of superconducting resonators in the highly-dissipative limit, by exploiting the superconducting transition to perform a background reference. It is suggested that this hybrid quantum system, and these initial experiments provide a promising platform in the emergent field of circuit quantum thermodynamics. We believe that the techniques and tools developed during this thesis present key steps towards the understanding of the thermodynamics of quantum circuits, towards the realisation of devices that can explore heat transport in the quantum limit, such as a quantum heat engine.
Quantum heat engines require precise control over thermal reservoirs and the energies of the quantum working medium. Although superconducting circuits enable accurate engineering of controlled quantum systems, they have not yet been employed to experimentally realize a cyclic quantum heat engine. Here, we demonstrate a quantum heat engine with superconducting circuits, using a quantum-circuit refrigerator as a tunable heat reservoir and a flux-tunable transmon qubit as the working medium. Starting from a thermal state, we implement a few quantum Otto cycles with a tailored reservoir drive inducing sequential cooling and heating, interleaved with flux ramps controlling qubit frequency. Utilizing single-shot qubit readout, we monitor the qubit state evolution during the cycles and measure positive output powers and efficiencies, agreeing with corresponding simulations. Our results verify thermodynamic models of quantum heat engines, advance control of thermal environments, and open avenues for exploring possible quantum advantages.