World’s First Superconducting Quantum Heat Engine Could Transform Quantum Computing

A tiny machine, smaller than a grain of sand, has successfully turned heat into useful work. This happened near absolute zero, a temperature close to the coldest possible. This breakthrough could lead to much more powerful quantum computers.

Researchers at Aalto University built the first cyclic quantum heat engine inside a superconducting circuit. This device uses a qubit, which is the basic unit of quantum information. It repeatedly heats, cools, and converts energy using this qubit.

Professor Mikko Möttönen led this study, and the findings were published in Nature Communications.

How a Qubit Becomes an Engine

This new device brings a familiar idea into a new realm. Regular heat engines power cars and power plants by moving energy between hot and cold places. The Aalto device does the same thing, but on a tiny scale. At this size, quantum mechanics dictates how energy behaves.

At the heart of the engine is a special qubit called a flux-tunable transmon. It's connected to a resonator and a quantum circuit refrigerator. Transmons are common in superconducting quantum computers. They store and process quantum information and can be controlled with microwave signals.

The refrigerator had a dual role. Instead of needing separate hot and cold parts, the researchers adjusted one device to either heat or cool the qubit. They then changed the qubit’s energy level at specific times. This completed the four steps of a quantum Otto cycle.

The Otto cycle is a process used in many gasoline engines. But in this experiment, the working part wasn't gas. It was a qubit exchanging tiny amounts of energy inside a super-cold circuit.

Tuomas Uusnäkki, the study's lead author, explained that they built a nanofabricated heat engine using superconducting circuits. They operated it in a cryostat, a very cold environment, near absolute zero. The core of it is a transmon qubit, a key part of modern quantum tech.

Measuring Performance

The team started with the qubit in a thermal state. They ran the engine for up to three cycles in a row. Special measurements allowed them to track changes in the qubit’s state. This helped them figure out how much heat it took in, how much work it did, and how efficient it was.

Uusnäkki noted that their quantum-circuit refrigerator could both heat and cool the qubit as needed. By using carefully timed control pulses, they ran the engine in an Otto cycle and watched the qubit's state.

The measurements showed the engine actually produced work, not just moved heat around. Its power and efficiency matched the researchers' computer models. This proved the device was a real cyclic heat engine.

Uusnäkki highlighted that this is the first time a cyclic quantum heat engine has been shown in superconducting circuits. Using one controllable quantum refrigerator as both the hot and cold parts makes it simpler and more flexible.

Why Superconducting Circuits Matter

Other quantum heat engines have been made using things like trapped ions or atomic gases. But superconducting circuits are especially important. They are already a leading technology for quantum computing, communication, and sensing. Until now, no one had made a cyclic quantum heat engine using this specific technology.

The amount of work produced is very small, so that's not the main point. The real importance is that heat can be controlled and converted within the same type of circuits used for quantum processors.

A Path to Larger Quantum Computers

This ability could be very useful as quantum computers get bigger. Today's superconducting machines need many microwave cables. These cables run from room-temperature electronics to processors kept extremely cold, just above absolute zero. Each cable adds cost, takes up space, and can bring unwanted heat or noise into the system.

The researchers are now working on an engine that can run on its own. One idea is to read a qubit's state without sending a microwave signal from the cold processor to room temperature. Putting more control functions directly into the cold circuit could reduce the need for external wiring.

Möttönen explained that Finland's Quantum Technology Strategy aims for a quantum computer with one thousand logical qubits by 2035. This would likely mean hundreds of thousands of physical qubits. Doing this with current technology would require millions of microwave cables, each costing thousands of euros. These cables also create noise. Using autonomous devices instead would mostly remove the need for these cables.

Deep Dive & References

Initial demonstration of a quantum heat engine based on dissipation-engineered superconducting circuits - Nature Communications, 2026

The researchers used facilities at OtaNano, Finland’s national research center for nano-, micro-, and quantum technology. The Research Council of Finland and the Finnish Cultural Foundation funded the work.