Scientists are getting a clearer picture of how thermodynamics behaves in the quantum world, and that progress could benefit both quantum technology and our understanding of familiar thermodynamic principles. Researchers at Aalto University have now taken an important step by demonstrating the first cyclic quantum heat engine built inside a superconducting circuit.

The experiment connects two areas of physics that normally describe very different scales. Quantum mechanics explains the behavior of matter at extremely small scales, even below the size of atoms, while thermodynamics describes how heat and energy behave in much larger systems, from collections of molecules to the universe itself. Bringing the two together raises a fundamental question: what happens to familiar thermodynamic processes when quantum effects such as tunneling, entanglement, and superposition enter the picture?

A Heat Engine Built for the Quantum World

Conventional heat engines turn heat into useful work. James Watt's steam engine is one famous example, but the same basic concept remains central to modern transportation and electricity production, powering cars, ships, planes and many power plants.

The researchers have now created the world's first superconducting quantum heat engine. The extremely small device combines a transmon qubit, a resonator and a quantum refrigerator.

Operating under ultracold quantum conditions, the engine was able to use the tiny amount of available heat to repeatedly produce positive work. Achieving this kind of cyclic operation has been an important objective for researchers working on quantum heat engines. The result provides a proof of concept for superconducting heat engines that could eventually contribute to improved quantum computing technology.

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

Recreating an Otto Cycle Near Absolute Zero

To make the engine operate, the researchers reproduced an Otto cycle inside a superconducting circuit. The Otto cycle is a thermodynamic process also used in car engines and other conventional machines.

"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," says Tuomas Uusnäkki, the study's first author.

The researchers connected the transmon qubit to a quantum circuit refrigerator, allowing them to control heat flow on the quantum scale and demonstrate that this heat could be transformed into measurable work. A conventional heat engine normally relies on separate hot and cold environments. In this system, however, the same quantum refrigerator can supply both heating and cooling.

"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," explains Uusnäkki.

Measurements showed that heat passing through the qubit during the cycle was producing positive work.

"This is the first experimental demonstration of a cyclic quantum heat engine in superconducting circuits. Using a single controllable quantum refrigerator as both the hot and cold environment of the engine makes it simpler and more versatile," says Uusnäkki.

Toward Autonomous Quantum Computer Hardware

The researchers are now trying to improve the design and eventually develop a fully autonomous heat engine. One possible use would be reading out qubits without having to carry a microwave pulse from millikelvin temperatures all the way to room temperature.

That capability could become especially valuable as quantum computers grow. Autonomous devices integrated directly into superconducting circuits could reduce both the cost and complexity of machines containing very large numbers of qubits.

"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 thousand euros each. The cables also introduce noise into the system. Using autonomous devices instead would mostly eliminate the need for those cables," Möttönen says.

Reducing the need for those microwave connections could therefore address two challenges at once: the enormous hardware requirements of large quantum computers and the unwanted noise that cables can introduce into quantum systems.

The pioneering experiment was carried out using OtaNano, Finland's national research infrastructure for nano, micro and quantum technology. Funding came from the Research Council of Finland and the Finnish Cultural Foundation.