Researchers led by Professor Mikko Möttönen at Aalto University have built the first quantum heat engine that completes a full, repeating cycle within a superconducting circuit. According to a study published in Nature Communications, the device uses a single qubit to convert heat into measurable work at temperatures near absolute zero.
How the Superconducting Quantum Engine Functions
The engine operates on a quantum version of the Otto cycle, the same four-stroke process used in internal combustion engines. Instead of pistons, the Aalto University team used a transmon qubit—a superconducting circuit—on a silicon chip. The process is driven by carefully shaped voltage pulses that manipulate the qubit’s energy levels.
The cycle follows four distinct steps:
- Work Output: The qubit’s energy levels move closer, transferring energy to the controlling magnetic field.
- Cooling: A quantum refrigerator pulls heat from the qubit.
- Work Input: The control field pushes the energy levels apart again.
- Heating: The refrigerator feeds heat back into the qubit to restart the loop.
Did you know? This engine is unique because one component handles both heating and cooling. Traditional quantum engines usually require two separate thermal baths with individual wiring.
Efficiency and Power Output Measurements
The device’s initial output is modest. According to the researchers, the engine produced a few hundredths of an electronvolt per second. In its opening cycles, the machine converted only about 0.5% of absorbed heat into work. This is significantly lower than the 45% efficiency seen in quantum engines using trapped ions or diamond defects.
However, Professor Möttönen’s team notes that this low figure is not a design flaw but a result of the qubit warming toward a steady state. As the engine settles, efficiency is predicted to rise toward a ceiling of approximately 2%. During the three tracked cycles, the qubit’s effective temperature rose from 200 to 600 millikelvin.
Performance Comparison: Superconducting vs. Other Quantum Engines
| Engine Type | Efficiency | Key Characteristic |
|---|---|---|
| Superconducting (Aalto) | ~0.5% to 2% | Repeating cycle on-chip |
| Trapped Ions/Diamond Defects | Up to 45% | Larger frequency swings |
Reducing the ‘Wiring Bottleneck’ in Quantum Computing
While the engine cannot power external devices, its architecture addresses a critical hardware problem: cabling. Finland’s national quantum plan targets a machine with 1,000 logical qubits by 2035. Achieving this could require millions of microwave cables, which introduce noise and physical bulk.
Professor Möttönen suggests that autonomous on-chip engines could replace these cables by handling tasks like reading out and resetting nearby qubits using internal heat cycles. “Using autonomous devices instead would mostly eliminate the need for those cables,” Möttönen stated.
The cooling technology used in the engine—a nanoscale junction where electrons tunnel across a thin barrier—already has practical applications. A separate study demonstrated that similar coolers can reset qubits to a clean starting state more effectively than standard methods.
Frequently Asked Questions
What is a quantum heat engine?
It is a device that operates on the laws of quantum thermodynamics to convert heat into work, mimicking the cycle of a traditional car engine but at a microscopic scale.
Can this engine power a computer?
No. The power output is currently measured in tiny fractions of an electronvolt per second, making it unsuitable for powering hardware. Its value lies in qubit management and reducing wiring.
Why is the Aalto University experiment significant?
Prior superconducting experiments worked as one-way thermal machines. This is the first time a full, repeating cycle has been demonstrated to produce positive power output in a superconducting circuit.
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