For the first time, researchers have built a quantum heat engine that completes a full, repeating cycle entirely inside a superconducting circuit.
Instead of pistons and cylinders, the tiny engine runs on a single quantum bit, or qubit, operating near absolute zero.
Making a quantum engine runA research team led by Professor Mikko Möttönen at Aalto University fitted three components onto a single silicon chip and chilled them to near absolute zero.
Theorists had proposed similar superconducting heat engines for years, but a full, repeating cycle that measurably produced work had remained out of reach.
Across three cycles, the team tracked the qubit’s energy and found that it produced positive power output, the defining mark of a heat engine.
Car engines turn heat into motion through a repeating cycle of heating, cooling, expansion, and compression. Physicists have now shrunk that basic idea down to the quantum scale.
For the first time, researchers have built a quantum heat engine that completes a full, repeating cycle entirely inside a superconducting circuit.
Instead of pistons and cylinders, the tiny engine runs on a single quantum bit, or qubit, operating near absolute zero.
Despite its microscopic size, the device successfully converts heat into measurable work.
The engine will not be powering anything anytime soon, but its design could still prove useful.
The technology at its core can already help reset qubits, and similar autonomous devices could eventually reduce the massive amount of wiring needed to build larger quantum computers.
Making a quantum engine run
A research team led by Professor Mikko Möttönen at Aalto University fitted three components onto a single silicon chip and chilled them to near absolute zero.
At the center sits a transmon qubit, a tiny superconducting circuit that serves as the engine’s working substance.
Beside it are a resonator that reads the qubit’s state and a quantum refrigerator that can warm or cool it on command.
Theorists had proposed similar superconducting heat engines for years, but a full, repeating cycle that measurably produced work had remained out of reach.
Across three cycles, the team tracked the qubit’s energy and found that it produced positive power output, the defining mark of a heat engine.
No one had demonstrated this before in a looping superconducting device.
How the four-stroke cycle works
The Otto cycle unfolds in four steps, and the researchers drive each one with carefully shaped voltage pulses.
In the first step, the qubit’s two energy levels move closer together. As they do, the qubit transfers energy as work to the magnetic field controlling it.
Next, the refrigerator pulls heat from the qubit, cooling it down. The researchers then push the energy levels back apart, with the control field doing work on the qubit.
Finally, the refrigerator feeds heat back into the qubit, and the cycle starts over.
One device does two jobs
What makes the design unusual is that one component handles both heating and cooling. Earlier engines needed two separate baths, one hot and one cold, each with its own control wiring.
Here, a single tunable source stands in for both. By adjusting a bias voltage, the researchers turn the refrigerator into a warm or cold bath, switching it on and off in time with the cycle.
A nanoscale junction handles the cooling. Electrons tunnel across its thin barrier, with each jump absorbing or releasing a single packet of the qubit’s energy.
Möttönen’s group first demonstrated this on-chip cooling device in 2017. Here, it serves as the engine’s only source of heat.
The first cycles start small
The engine’s output is tiny. Its power came to a few hundredths of an electronvolt per second, and it turned only about 0.5 percent of the absorbed heat into work. That falls far below the efficiency of any everyday motor.
That figure reflects the opening cycles, when the qubit is still warming toward a steady state, rather than a flaw in the design.
As the engine settles, its efficiency rises toward a ceiling of about two percent, close to what the group’s quantum thermodynamics model predicts.
During the three cycles, the qubit’s effective temperature rose from about 200 to 600 millikelvin.
Researchers measured its quantum state tens of thousands of times and fitted the results to determine the temperature.
At such low temperatures, the heating and cooling strokes had not yet reached a balance. The researchers attribute this imbalance to the short run.
What makes this engine different
Heat engines built in other systems, from trapped ions to defects inside diamonds, have reached efficiencies near 45 percent, well above this one.
However, those machines relied on very different hardware and much larger frequency swings.
None of them ran as a repeating cycle inside a superconducting circuit.
The closest earlier superconducting experiment worked only as a one-way thermal machine rather than a looping engine. That’s why the team calls this result the first of its kind.
Cooling already has practical value
The engine’s low power leaves it without any immediate practical use, yet the cooling device inside it is already valuable.
Refrigerators like this reset qubits to a clean starting state, a routine step in every quantum computation. A separate study used a similar cooler to reset a qubit more effectively than standard methods.
For now, the work provides clear evidence that a superconducting circuit can run a full engine cycle and return useful work, something long predicted but never measured.
It also gives theorists a real system for testing their models and provides a starting point for engines that might one day earn a place inside a quantum computer.
Fewer cables could mean more qubits
Möttönen sees a route by which such engines could help quantum computers grow. The wiring is the bottleneck.
Finland’s national quantum plan aims for a machine with 1,000 logical qubits by 2035.
That target could demand hundreds of thousands of physical qubits and millions of costly microwave cables, each one adding noise.
An autonomous engine sitting on the chip could take over jobs like reading out and resetting nearby qubits. It could run on its own internal heat cycles instead of signals piped in from room-temperature equipment.
“Using autonomous devices instead would mostly eliminate the need for those cables,” said Möttönen.
The study is published in the journal Nature Communications.
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