Researchers have revealed a microscopic engine that can convert heat into useful work while operating at temperatures only fractions of a degree above absolute zero.
Tinier than a single grain of sand, the machine marks the first demonstration of what researchers call a cyclic quantum heat engine composed of superconducting circuits, the technology behind modern quantum computers.
The findings, led by Academy Professor Mikko Möttönen, were published in Nature Communications.
A Sand-Grain-Sized Quantum Heat Engine
The tiny device, developed by researchers at Aalto University in Finland, relies on a single superconducting qubit, which represents the quantum world equivalent of a classical computer bit.
By heating, cooling, and extracting energy through quantum variations of the cycles that provide power to conventional engines by thermodynamic means, researchers say the machine’s demonstration of producing energy, while extremely small, marks a breakthrough that could one day allow for the simplification of much larger-scale quantum computers.
A flux-tunable transmon qubit—one of the most common building blocks of superconducting quantum processors—lies at the heart of the innovation. But as opposed to relying on separate hot and cold reservoirs, the Möttönen and the Aalto University research team used a single quantum circuit refrigerator capable of performing both roles.
The team guided the system through a sort of quantum version of what is known as an Otto cycle (the same essential process most gasoline engines employ) by leveraging precise control of the refrigerator and the qubit’s energy levels.
“In our experiment, we built a nanofabricated heat engine using superconducting circuits and operated it in a cryostat near absolute zero,” said lead author Tuomas Uusnäkki. “At its heart is a transmon qubit, one of the basic building blocks of modern quantum technologies.”
Single Shot Measurements on Repeat
The team also used repeated single-shot measurements to track the qubit, observing as it completed several engine cycles. This helped them calculate precisely how much heat was absorbed, as well as the amount of work produced, and the machine’s overall efficiency.
The results, they say, closely matched theoretical predictions, and now help to confirm that the device functioned as a genuine cyclic quantum heat engine.
Although this is not the first quantum heat engine, and previous demonstrations have been achieved using trapped ions, ultracold atoms, and other quantum systems, the Aalto team says this is the first that operates using superconducting circuits, which could present new possibilities for the future of quantum computing.
As quantum computers grow, such technologies could be especially helpful, since today’s superconducting processors rely on extensive networks of microwave cables connecting room-temperature electronics with processors that operate near absolute zero—the lowest temperature theoretically attainable.
These networks of cabling not only take up space, but also add to costs, while adding to the overall heat and electrical noise in systems that can also introduce interference affecting fragile quantum states.
A Look at the Future of Quantum Computing
With future refinements of their technology, the Aalto team hopes versions of their tiny engine may be able to perform control functions directly within cryogenic quantum processors, thereby reducing the need for external wiring altogether.
“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,” Möttönen said in a statement, describing future large-scale quantum computers.
“Doing that with current technology requires millions of microwave cables costing thousands of euros each,” he added.
“The cables also introduce noise into the system,” Möttönen said. “Using autonomous devices instead would mostly eliminate the need for those cables.”
The team’s recent study, “Initial demonstration of a quantum heat engine based on dissipation-engineered superconducting circuits,” appeared in Nature Communications.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.