In recent years, people’s understanding of how the principles of thermodynamics apply to the quantum field has been continuously improving, promising to drive the development of quantum technology. Conversely, a clearer understanding of quantum thermodynamics can also help deepen our understanding of classical thermodynamics. Today, researchers at Aalto University in Finland have successfully demonstrated the world’s first cyclic quantum heat engine in a superconducting circuit.
Physicists are increasingly fascinated by the idea of combining classical thermodynamics with quantum mechanics. Quantum mechanics describes the behavior of particles on a tiny scale – smaller than atoms, while thermodynamics concerns large-scale systems, from molecules to the entire universe. How can peculiar quantum phenomena like tunneling, quantum entanglement, and superposition be interwoven with the steady and familiar qualities exhibited by the heat engines that sparked the industrial revolution?
Heat engines, such as the famous James Watt steam engine, can convert heat into useful energy, known as “work.” Heat engines power our cars, ships, and airplanes, and most power plants also generate electricity using heat engines.
Now, the world’s first superconducting quantum heat engine has been developed: a micro device composed of “transmon qubits” (a mainstream design of superconducting quantum bits commonly used as foundational units in superconducting quantum computers by companies like IBM and Google), a resonator (used to store and transmit microwave energy and couple with quantum bits), and a quantum refrigerator (which acts similarly to the hot and cold sources of a typical heat engine).
This superconducting heat engine utilizes extremely small amounts of heat energy in an ultra-low-temperature quantum environment to continuously output positive work in a cyclic manner – a goal pursued by quantum engineers for a long time. The device provides solid conceptual verification for superconducting heat engines and could be used in the development of more advanced quantum computing technologies in the future.
The research, led by academician Professor Mikko Möttönen, was published in the journal “Nature Communications” on July 13, 2026.
The research team built an Otto cycle in the superconducting circuit – a thermodynamic cycle process that powers devices like car engines.
“In our experiment, we built a nanoscale heat engine with a superconducting circuit and operated it in a near-absolute-zero temperature bath. At its core is a ‘transmon qubit,’ which is one of the fundamental components of modern quantum technology,” said Tuomas Uusnäkki, the first author of the study.
By connecting the “transmon qubit” to the quantum circuit refrigerator, the team was able to control heat flow at the quantum scale and demonstrate that this heat could be converted into measurable work. Unlike typical heat engines that use independent hot and cold sources, this quantum heat engine relies on a quantum refrigerator to simultaneously provide heat and cooling.
“Our quantum circuit refrigerator can adjust as needed to heat or cool the quantum bits. Through precise timing of control pulses, we drive the heat engine to perform the Otto cycle and monitor the state of the quantum bits during the operation of the heat engine,” explained Uusnäkki.
The researchers found that heat flowing through the quantum bits during the cycle produced positive work.
“This is the first experimental demonstration of a cyclic quantum heat engine in a superconducting circuit. By using a single controllable quantum refrigerator as both the heat source and cooling environment for the engine, the entire system becomes simpler and more flexible,” Uusnäkki said.
The research team is working to further improve their design with the goal of creating a fully autonomous heat engine that can perform tasks like reading quantum bits without the need to transfer microwave signals from the ultra-low temperature environment of milli-kelvin to room-temperature equipment – a current challenge in superconducting quantum computers where quantum chips need to have their microwave readout signals at milli-kelvin temperatures sent through numerous cables to room-temperature electronics for amplification and analysis.
Such autonomous heat engines operated on superconducting circuits hold the promise of reducing the cost and system complexity of “high-qubit-count” quantum computers in the future.
“The quantum technology strategy in Finland aims to achieve a quantum computer with a thousand logical qubits by 2035, which may require hundreds of thousands of physical qubits. Using existing technology to achieve this goal would require millions of microwave cables, each costing thousands of euros, and these cables would introduce noise into the system. On the other hand, switching to autonomous devices could significantly reduce the need for these cables,” Möttönen said. ◇