At Aalto University, researchers have built the world’s first cyclic quantum heat engine in a superconducting circuit.
“In our experiment, we built a nanofabricated heat engine using superconducting circuits and operated it in a cryostat near absolute zero.
The result: positive work output, confirming long-standing theoretical models of quantum heat engines.
“This is the first experimental demonstration of a cyclic quantum heat engine in superconducting circuits.
Quantum heat engines are more than just scientific curiosities.
James Watt may very well have paved the way to an age of heat-powered humanity when he perfected steam engines in the 18th century. But today, physicists are taking that legacy and reinventing it at the smallest scales imaginable in superconducting circuits cooled nearly to absolute zero.
At Aalto University, researchers have built the world’s first cyclic quantum heat engine in a superconducting circuit. It combines the strange rules of quantum mechanics with more familiar principles in thermodynamics.
Their device, a transmon qubit coupled to a resonator and a quantum-circuit refrigerator, demonstrates that even under ultracold conditions, heat can be harnessed to produce measurable work.
“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,” explained Tuomas Uusnäkki, the study’s first author.
The team implemented an Otto cycle, the same thermodynamic process that powers car engines. But unlike classical engines, which rely on separate hot and cold reservoirs, this quantum engine uses a single quantum refrigerator that can be tuned to both heat and cool the qubit on demand.
“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,” said Uusnäkki.
The result: positive work output, confirming long-standing theoretical models of quantum heat engines. “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,” Uusnäkki added.
Quantum heat engines are more than just scientific curiosities. These more sophisticated networks could assist in the development of quantum computers on a larger scale. Modern designs still need millions of microwave cables to wire qubits together, with each cable costing thousands of euros and causing noise. A quantum engine run entirely by itself could remove a lot of this complexity.
“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, each costing thousands of euros. The cables also introduce noise into the system. Using autonomous devices instead would mostly eliminate the need for those cables,” said Mikko Möttönen, Academy Professor and senior author of the study.
This experiment not only advances quantum technology but also deepens our understanding of thermodynamics itself. Researchers are bringing together the physics of steam engines and that of qubits by showing how a heat engine can work even in a quantum regime.
It’s a reminder that the principles driving the Industrial Revolution are far from over. This revolution, perhaps the next one to come down the road from here; And it may be quantum, and its engines will hum not with pistons and steam but with qubits, sound heard only in refrigerators pushed close enough to almost nothingness.
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