Near Absolute Zero and 0.55% Efficiency: World's First Superconducting Quantum Heat Engine Demonstrated by Aalto University Researchers

Researchers at Aalto University have experimentally realized the world’s first cyclic superconducting quantum heat engine that converts heat into useful work inside a quantum circuit cooled to near absolute zero. The tiny device marks the first time a complete, repeatable thermodynamic cycle has been demonstrated in a superconducting platform, opening new possibilities for managing heat and control signals in future quantum processors.

Unlike earlier quantum heat machines, the Finnish team achieved a full experimental cycle using a transmon qubit—one of the most common types of superconducting qubits—as the working substance. The circuit was placed inside a cryostat and cooled to temperatures approaching absolute zero. A resonator and a voltage-controlled quantum refrigerator were coupled to the transmon, allowing researchers to switch between heating and cooling phases with a single component instead of two separate thermal reservoirs.

The engine follows the Otto cycle, the same thermodynamic cycle used in conventional internal-combustion engines. In the experiment, the team first lowered the qubit’s transition frequency to let the system perform work, then used the quantum refrigerator to extract heat. After raising the frequency again (compression), the refrigerator reheated the qubit to return the system to its initial state. This sequence was repeated multiple times while the qubit state was read out after each step.

To obtain reliable statistics, the researchers performed 10,000 individual measurements for each data point and calculated average energy flows. The results matched numerical simulations and demonstrated genuine positive output power rather than a mere simulation of thermodynamic behavior. Measured performance remained modest: average power reached only 0.039 electronvolts per second, while efficiency stood at roughly 0.55 percent—about 27 percent of the theoretical Otto-cycle limit for the chosen parameters. Modeling indicates that efficiency could rise to approximately 2.2 percent once the device operates in a steady regime.

The primary value of the technology lies not in large-scale power generation but in on-chip thermal management and autonomous control of quantum processors. Modern superconducting quantum computers rely on numerous microwave lines that connect millikelvin qubits to room-temperature electronics; these cables increase complexity, cost, and noise. A quantum heat engine could eventually perform local operations such as state initialization or readout without requiring every control signal to travel outside the cryostat, thereby reducing wiring density and enabling processors with hundreds of thousands of physical qubits.

In addition to practical applications, the superconducting platform offers physicists a powerful testbed for quantum thermodynamics. Precise control over temperature, heat flow, and energy levels allows direct comparison between experimental observations and theoretical models, potentially revealing whether quantum effects such as superposition or interference can enhance the performance of microscopic heat machines.