First Cyclic Quantum Heat Engine Developed in Superconducting Circuit

Researchers at Aalto University have successfully built the first cyclic quantum heat engine within a superconducting circuit, converting heat into useful work at temperatures close to absolute zero. This breakthrough could pave the way for more advanced quantum computing technologies.

The device operates by using a qubit—the fundamental unit of quantum information—as its working substance, repeatedly undergoing processes of heating, cooling, and energy conversion. The findings of this study, led by Academy Professor Mikko Möttönen, were published in Nature Communications.

How the Quantum Heat Engine Functions

The core component of the engine is a flux-tunable transmon qubit linked to a resonator and a quantum circuit refrigerator. Transmon qubits are commonly used in superconducting quantum computers due to their ability to store and manipulate quantum information with microwave control signals.

The quantum circuit refrigerator serves dual purposes by acting as both the heating and cooling source. Instead of employing separate hot and cold reservoirs, the researchers modulated this single device to alternately heat or cool the qubit. The qubit’s energy level was adjusted at precise intervals, completing the four stages of a quantum Otto cycle.

The Otto cycle, known from traditional gasoline engines, was applied here with a qubit exchanging minuscule amounts of energy within a cryogenic circuit rather than compressed gas inside a cylinder. Tuomas Uusnäkki, the study’s first author, explained, “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.”

Tracking Heat, Work, and Efficiency

The team initiated the engine with the qubit in a thermal state and ran it for up to three consecutive cycles. Using single-shot measurements, they monitored changes in the qubit’s state to quantify the heat absorbed, work produced, and operational efficiency.

Uusnäkki added, “Our quantum-circuit refrigerator can be tuned to both heat and cool the qubit on demand. Using carefully timed control pulses, we drove the engine in an Otto cycle and monitored the qubit state as the engine ran.” The results confirmed that the engine generated positive work rather than merely transferring heat within the circuit. The measured power and efficiency aligned with simulations, validating the device’s behavior as a genuine cyclic heat engine.

Significance of Superconducting Circuits

Quantum heat engines have previously been demonstrated using systems such as trapped ions, atomic gases, nuclear spins, and diamond defects. However, superconducting circuits are particularly important because they are a leading platform for quantum computing, communication, and sensing. This experiment marks the first cyclic quantum heat engine completed with this technology.

While the work output is exceptionally small, the key achievement lies in demonstrating controlled heat conversion within the same circuitry used for quantum processors.

Implications for Quantum Computing Expansion

This capability could be crucial as quantum computers scale up. Current superconducting quantum machines rely on numerous microwave cables connecting room-temperature electronics to processors maintained just above absolute zero. Each cable adds expense, occupies space, and can introduce heat or noise.

The researchers aim to develop a fully autonomous version of the engine. One potential application is reading a qubit’s state without sending microwave signals from the cold processor to room temperature, thereby reducing the need for extensive external wiring.

Professor Möttönen stated, “Finland’s Quantum Technology Strategy envisions a quantum computer with one thousand logical qubits by 2035, which probably means hundreds of thousands of physical qubits. Doing that with current technology requires millions of microwave cables costing thousands of euros each. The cables also introduce noise into the system. Using autonomous devices instead would mostly eliminate the need for those cables.”

The study titled “Initial demonstration of a quantum heat engine based on dissipation-engineered superconducting circuits” was authored by Tuomas Uusnäkki, Timm Mörstedt, Wallace Teixeira, Miika Rasola, and Mikko Möttönen, and published on 5 May 2026 in Nature Communications (DOI: 10.1038/s41467-026-72651-x).

The researchers conducted their work at OtaNano, Finland’s national research infrastructure for nano-, micro-, and quantum technology. Funding was provided by the Research Council of Finland and the Finnish Cultural Foundation.