Quantum heat engine advances scalable computing

Researchers from Aalto University have developed a cyclic quantum heat engine inside a superconducting circuit, to advance our understanding of thermodynamics while facilitating the technologies needed for high-qubit quantum computers.

Physicists have become increasingly fascinated with the idea that classical thermodynamics could be combined with quantum mechanics. Quantum mechanics captures the behaviour of particles on tiny scales — smaller than atoms — while thermodynamics is about large systems, from molecules up to the entire universe. How do strange quantum phenomena like tunnelling, entanglement and superposition mix with the stolid familiarity of the heat engines that kickstarted the industrial revolution?

Heat engines, like James Watt’s famous steam engine, convert heat into useful energy, into work. They power cars, ships and planes, and heat engines are how most power plants generate electricity. Now, researchers have built a superconducting quantum heat engine: a tiny device consisting of a transmon qubit, a resonator and a quantum refrigerator.

The superconducting engine harnessed the miniscule amount of heat found in ultracold quantum conditions to cyclically output positive work, a long-sought goal for quantum engineers. The device provides a solid proof of concept for superconducting heat engines, which could be used to develop improved technology for quantum computers.

The study, led by Academy Professor Mikko Möttönen, was published in Nature Communications.

The team created an Otto cycle — the thermodynamic process that powers car engines, among other things — inside a superconducting circuit.

“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 Tuomas Uusnäkki, the study’s first author.

By connecting the transmon qubit to a quantum-circuit refrigerator, the team could control the flow of heat at a quantum scale and show that it can be converted into measurable work. Unlike a typical heat engine, which uses separate hot and cold sources, the quantum heat engine relies on a quantum refrigerator to provide both heat and cold.

“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,” Uusnäkki said.

The researchers saw that the heat flowing through the qubit in the cycle was generating positive work.

“Using a single controllable quantum refrigerator as both the hot and cold environment of the engine makes it simpler and more versatile,” Uusnäkki said.

Autonomous heat engines for future quantum computers

The team is working to improve their design, aiming to create an entirely autonomous heat engine that could do things like read out qubits without the need to bring the microwave pulse from millikelvin to room temperature. An autonomous engine on a superconducting circuit could also reduce the cost and complexity of high qubit-count computers in the future.

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