Here we present an architecture for the implementation of cyclic quantum thermal engines using a superconducting circuit. The quantum engine consists of a gated Cooper-pair box, capacitively coupled to two superconducting coplanar waveguide resonators with different frequencies, acting as thermal baths. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at GHz frequencies. By terminating the resonators of the measured structure with normal-metal resistors whose temperature can be controlled and monitored, a quantum heat engine or refrigerator could be realized. Furthermore, we numerically evaluate the performance of our setup acting as a quantum Otto-refrigerator in the presence of realistic environmental decoherence.
Here we present an architecture for the implementation of cyclic quantum thermal engines using a superconducting circuit. The quantum engine consists of a gated Cooper-pair box, capacitively coupled to two superconducting coplanar waveguide resonators with different frequencies, acting as thermal baths. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at GHz frequencies. By terminating the resonators of the measured structure with normal-metal resistors whose temperature can be controlled and monitored, a quantum heat engine or refrigerator could be realized. Furthermore, we numerically evaluate the performance of our setup acting as a quantum Otto-refrigerator in the presence of realistic environmental decoherence.
Superconducting circuits present a promising platform with which to realize a quantum refrigerator. Motivated by this, we fabricate and perform spectroscopy of a gated Cooper-pair box, capacitively coupled to two superconducting coplanar-waveguide resonators with different frequencies. We experimentally demonstrate the strong coupling of a charge qubit to two superconducting resonators, with the ability to perform voltage driving of the qubit at gigahertz frequencies. We go on to discuss how the measured device could be modified to operate as a cyclic quantum refrigerator by terminating the resonators with normal-metal resistors acting as heat baths.
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.
Superconducting circuits provide a controllable and versatile platform for studying quantum phenomena and advancing quantum technologies. Josephson junctions and resonators are the central circuit elements, as they enable the realization of macroscopic quantum systems—such as qubits—that can be conveniently controlled, probed, and integrated with non-superconducting components. The implementation of a thermal reservoir through the integration of a dissipative element, such as a normal-metal resistor, offers an experimental platform for investigating quantum thermodynamics, heat management in circuits, on-chip thermal detectors, and the realization of quantum thermal machines. In this thesis, we integrate a mesoscopic normal metal and its associated thermometers as an on-chip bolometer within circuit quantum electrodynamics devices for thermal detection and quantum thermodynamics experiments. The on-chip bolometer is utilized as a sensitive and broadband thermal spectrometer to characterize the properties of a superconducting resonator and to investigate its dominant loss channels. We demonstrate the capability of our thermal detector to resolve the resonator modes over a wide frequency range extending above 20 GHz. When two thermal baths are connected through a qubit–resonator system, a quantum heat valve can be realized. This device has been demonstrated using both transmon and flux qubits. Photonic heat transport between the two baths is controlled by tuning the qubit energy via an applied static magnetic field. To model these experiments, we propose an approach based on electromagnetic simulation and microwave circuit theory, restricted to the linear-response regime. Despite this limitation, the method successfully captures the experimental observations and provides a practical tool for analysing and designing circuits for future experiments. A periodically driven qubit coupled to two unequal resonators, each terminated by a heat reservoir, enables the realization of a quantum Otto refrigerator. The use of a transmon qubit for this purpose is limited by its weak anharmonicity. We therefore explore alternative platforms for the experimental implementation of a quantum refrigerator based on charge and flux qubits, which exhibit strong anharmonicity. As an important step, we perform experiments on a driven flux qubit galvanically coupled to a resonator that is, in turn, weakly capacitively coupled to a heat reservoir. We observe interference patterns in the heat current arising from driving-induced coherence. In particular, resonance peaks in the heat transferred to the bath appear at driving frequencies that are integer fractions of the resonator frequency. These quantum features are governed by the relative phase accumulated by the qubit states under driving.
Quantum thermodynamics is emerging, both as a topic of fundamental research, but also as means to understand and potentially improve the performance of quantum devices. Superconducting circuit QED (Quantum Electro-Dynamics) presents a prominent platform for the necessary manipulation of quantum states. Thermalization of a quantum system, a subject of intense current interest, can be achieved by interfacing the circuit QED subsystem with a thermal reservoir of appropriate Hilbert dimensionality. In this context, we consider an assembly consisting of a superconducting qubit capacitively coupled between two nominally identical coplanar waveguide (CPW) resonators, each terminated by a normal-metal mesoscopic resistor. We report observation of tunable photonic heat transport through the resonator-qubit-resonator assembly. With the aid of a theoretical model presented here, we are able to reproduce experimental data. Importantly, the reservoir-to-reservoir heat flux depends on the interplay between the qubit-resonator and the resonator-reservoir couplings, yielding qualitatively dissimilar results in different coupling regimes. Our Quantum Heat Valve (QHV) is a technological prerequisite for the realisation of quantum heat engines and refrigerators, that can be obtained, e.g., by exploiting the time-domain dynamics and coherence of driven superconducting qubits. This effort would ultimately bridge the gap between the fields of quantum information and thermodynamics of mesoscopic systems.
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 engines operating 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.
Experimental demonstration of a truly quantum mechanical solid-state heat engine or refrigerator remains elusive up to now. Here, a four-stroke quantum refrigerator that can be realized using a superconducting qubit coupled alternately to two resonators with embedded reservoirs is analyzed theoretically. Besides being a practically feasible thermal machine, this concept presents a number of fundamentally interesting features. At high frequencies, the system exhibits nearly coherent dynamics, leading to quantum oscillations in cooling power versus operation frequency. At intermediate frequencies, an ideal Otto cycle can be realized. In the nearly adiabatic regime, the cooling power is quadratic in frequency and can be separated to classical and quantum contributions. Interestingly, it is shown here that quantum coherence leads always to performance that is inferior as compared to that arising from purely classical dynamics. Furthermore, in the nearly adiabatic regime the efficiency of the refrigerator exceeds clearly that of the standard Otto cycle.