Abstract Pistons are elementary components of a wide variety of thermal engines, allowing to convert input fuel into rotational motion. Here, we propose a single-piston engine where the rotational degree of freedom is effectively realized by the flux of a Josephson loop—a quantum rotor—while the working volume corresponds to the effective length of a superconducting resonator. Our autonomous design implements a Carnot cycle, relies solely on standard thermal baths and can be implemented with circuit quantum electrodynamics. We demonstrate how the engine is able to extract a net positive work via its built-in synchronicity using a filter cavity as an effective valve, eliminating the need for external control.
Abstract Pistons are elementary components of a wide variety of thermal engines, allowing to convert input fuel into rotational motion. Here, we propose a single-piston engine where the rotational degree of freedom is effectively realized by the flux of a Josephson loop—a quantum rotor—while the working volume corresponds to the effective length of a superconducting resonator. Our autonomous design implements a Carnot cycle, relies solely on standard thermal baths and can be implemented with circuit quantum electrodynamics. We demonstrate how the engine is able to extract a net positive work via its built-in synchronicity using a filter cavity as an effective valve, eliminating the need for external control.
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.
Quantum thermodynamics sits at the intersection of quantum mechanics and thermodynamics, aiming to understand how key thermodynamic concepts such as heat, work, and entropy behave in quantum systems. A primary interest in this field is the study and development of quantum heat engines, devices that convert the flow of thermal energy into work using a quantum system as the working medium. Conventional quantum heat engines require consistent external control to drive their operating cycle. In contrast, autonomous quantum heat engines operate without such external control, instead, their cyclic behavior emerges entirely from internal dynamics and the coupling to thermal reservoirs. In this thesis, we study an autonomous quantum heat engine circuit based on superconducting quantum circuits, in which thermal noise from heat reservoirs drives cyclic energy exchange between two superconducting resonators coupled with a superconducting quantum interference device (SQUID). In this thesis, classical circuit simulations of the autonomous quantum heat engine are implemented in AWR Design Environment using lumped-element resonators, coupling capacitors, and Josephson junction models. By mapping quantum components to their classical analogues and varying the external magnetic flux threading the superconducting quantum interference device, the resonance frequencies, quality factors, and coupling dynamics of the circuit are analyzed across two parameter sets. The simulated quality factor of the second resonator exceeded the threshold required for negative dissipation in both parameter sets, consistent with conditions favorable for coherent microwave generation. Transient simulations further revealed that above a threshold drive power, energy builds up at the second resonator’s resonance frequency through SQUID-mediated coupling. This provides qualitative evidence that the classical model captures the essential inter-resonator coupling mechanism. However, systematic quantitative discrepancies in resonance frequencies and dissipation rates are identified, attributed primarily to the linearized SQUID representation used in the classical model. These results demonstrate that classical circuit simulations can qualitatively reproduce key dynamical features of the studied autonomous quantum heat engine, while also revealing the limitations of classical approximations in capturing its full quantum behavior.
We study the mechanical performance of quantum rotor heat engines in terms of common notions of work using two prototypical models: a mill driven by the heat flow from a hot to a cold mode, and a piston driven by the alternate heating and cooling of a single working mode. We evaluate the extractable work in terms of ergotropy, the kinetic energy associated to net directed rotation, as well as the intrinsic work based on the exerted torque under autonomous operation, and we compare them to the energy output for the case of an external dissipative load and for externally driven engine cycles. Our results connect work definitions from both physical and information-theoretical perspectives. In particular, we find that apart from signatures of angular momentum quantization, the ergotropy is consistent with the intuitive notion of work in the form of net directed motion. It also agrees with the energy output to an external load or agent under optimal conditions. This sets forth a consistent thermodynamical description of rotating quantum motors, flywheels, and clocks.
A single-piston quantum engine based on a harmonic oscillator acting as the working fluid is proposed. Using the fact that the interaction between the piston and the oscillator depends on the extent of the oscillator wave function, one can control this interaction by modifying the oscillator temperature. By retracting the piston when the interaction is weak (hot oscillator) and returning it to the original position when the coupling is strong (cold oscillator), useful work can be performed assuming the interaction is attractive. The cycle of the engine is simulated numerically using two different powering protocols: bath and measurement. Using the collision model for the baths, the engine is shown to reach a steady state with positive work output. Published by the American Physical Society 2024
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.
Quantum heat engines are novel technologies that leverage quantum mechanical principles alongside thermodynamic theory to generate useful work. While experimental realizations of such devices have existed since the 1950s, they require continuous drive by an external control field. Autonomous quantum heat engines offer an alternate approach where the heat engine cycle is driven from within the device itself. Without being superimposed onto an external control field, the generated work from an autonomous quantum heat engine can more easily be observed and extracted. This thesis studies the low-frequency, energy-storing resonator in a superconducting circuit sample proposed as one such autonomous quantum heat engine. Transmission line measurements are performed to characterize the resonator while also analyzing its behavior in the time domain. When thermal-like noise is introduced to the system, the resonance dip is enhanced by up to 4.5 dB. However, the resonator rapidly decays, exhibiting bare resonance behavior after 6 µs without noise. Furthermore, this thesis finds that the intrinsic dissipation rate of the resonator is greater than the photon generation rate of the system. The internal quality factor peaks at 2.1×10^4, higher than the expected values for the type of resonator utilized and four times higher than the bare noiseless resonance. The total quality factor reaches 1.8×10^4 at ideal noise temperatures.