World’s first superconducting quantum heat engine looks to transform quantum computing A qubit-based heat engine completes an Otto cycle near absolute zero and points toward simpler large-scale quantum computers. Edited By: Joshua Shavit A heat engine small enough to fit inside a superconducting circuit has converted heat into measurable work near absolute zero, offering a new test bed for quantum thermodynamics and a possible route toward simpler, larger quantum computers. The device uses a transmon qubit, a resonator and a quantum-circuit refrigerator. Together, they form a microscopic version of an Otto engine, the same broad thermodynamic cycle used in many car engines. Aalto University researchers operated the system inside a cryostat, where temperatures sit close to absolute zero. Even in those conditions, tiny amounts of heat remain. The team showed that this heat could be directed through the circuit and converted into positive work. The study, led by Academy Professor Mikko Möttönen, was published in Nature Communications. An engine built around a qubit Classical heat engines use temperature differences to produce useful energy. Steam engines helped launch the Industrial Revolution, while modern heat engines still power vehicles and many electricity-generating plants. The new device applies the same basic idea to a quantum system. “In our experiment, we built a nanofabricated heat engine using superconducting circuits and operated it in a cryostat near absolute zero. At its heart is a transmon qubit, one of the basic building blocks of modern quantum technologies,” said first author Tuomas Uusnäkki. The transmon served as the engine’s working material. Researchers changed its energy levels with magnetic-flux pulses and controlled its temperature with a quantum-circuit refrigerator, or QCR. That refrigerator played an unusual double role. Traditional engines rely on separate hot and cold environments. Here, one tunable device supplied both. “Our quantum-circuit refrigerator can be tuned