The world’s first superconducting quantum heat engine could help unlock massive quantum computers

Scientists are getting a clearer picture of how thermodynamics behaves in the quantum world, and this 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 periodic quantum heat engine built inside a superconducting circuit.

The experiment links two areas of physics that typically describe very different scales. Quantum mechanics explains the behavior of matter on very small scales, even smaller than the size of atoms, while thermodynamics describes how heat and energy behave in much larger systems, from groups of molecules to the universe itself. Combining the two 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 designed for the quantum world

Traditional heat engines convert heat into useful work. James Watt’s steam engine is one famous example, but the same basic concept remains fundamental to modern transportation and the production of electricity, powering cars, ships, airplanes, and many power plants.

Researchers have now created the world’s first superconducting quantum heat engine. The extremely small device combines a transmitting qubit, a resonator, and a quantum refrigerator.

By operating under extremely cold quantum conditions, the engine was able to use the small amount of heat available to repeatedly produce positive work. Achieving this type of periodic process has been an important goal for researchers working on quantum heat engines. The result provides proof of concept for superconducting heat engines that could ultimately contribute to improved quantum computing technology.

The study conducted by Professor Mikko Mottonen was published at the Academy Nature Communications.

Recreating the Otto cycle near absolute zero

To power the engine, the researchers reproduced the Otto cycle within a superconducting circuit. The Otto cycle is a thermodynamic process also used in automobile engines and other conventional machines.

“In our experiment, we built a heat engine made of nanomaterials using superconducting circuits and ran it in a cryostat near absolute zero,” says Thomas Osnacki, first author of the study. “At its heart is the transmitting qubit, one of the building blocks of modern quantum technologies.”

The researchers connected the transmitting qubit to a quantum circuit refrigerator, allowing them to control the flow of heat on the quantum scale and demonstrate that this heat can be converted into measurable work. A traditional heat engine typically relies on separate hot and cold environments. However, in this system, the same quantum refrigerator can provide both heating and cooling.

“Our quantum circuit refrigerator can be set to heat and cool the qubit on demand,” Osnacki explains. “Using carefully timed control pulses, we drove the motor in an Otto cycle and monitored the state of the qubit while the motor was running.”

Measurements have shown that the heat passing through the qubit during the cycle produces positive work.

“This is the first demonstration of a periodic quantum heat engine in superconducting circuits,” Osnacki says. “Using a single quantum refrigerator that can control the hot and cold environment of the engine makes it simpler and more versatile.”

Towards autonomous quantum computers

Researchers are now trying to improve the design and eventually develop a fully autonomous heat engine. One possible use is to read qubits without having to carry a microwave pulse from millikelvin temperatures up to room temperature.

This ability could become especially valuable as quantum computers grow. Standalone devices integrated directly into superconducting circuits could reduce the cost and complexity of machines containing very large numbers of qubits.

“The Finnish quantum technology strategy envisions a quantum computer containing 1,000 logical qubits by 2035, which would likely mean hundreds of thousands of physical qubits,” says Mottonen. “Doing this with current technology would require millions of microwave cables costing thousands of euros each. Cables also introduce noise into the system. Using standalone devices instead would mostly eliminate the need for those cables.”

Hence, reducing the need for these microwave communications could address two challenges at once: the enormous hardware requirements of large quantum computers, and the unwanted noise that cables can cause in quantum systems.

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