Splitting qubits inside a quantum computer into high and low-energy groups can charge a batteryShutterstock / Pavel Chukhov
A 19th-century thought experiment, considered for decades to break the laws of thermodynamics, has been brought to life inside a quantum computer and used to charge a quantum battery.
Physicist James Clerk Maxwell imagined his demon in 1867 while thinking about how to cheat the laws of thermodynamics. He considered two boxes of gas separated by a weightless door and a tiny demon that controls which particles can go through it. The demon uses this control to make one box hotter and the other cooler, contradicting the thermodynamic edict that heat must flow from the hotter to the colder box until they eventually even out.

Later, physicists realised that the demon could not break thermodynamic laws “for free” because it would spend energy during its particle selection process, but the idea remained of interest because it can naturally occur in biology and has uses in chemistry.
“The exploration of Maxwell’s demon in a quantum setting forces us to think deeply about what’s behind the fundamental laws of quantum information, thermodynamics and especially their combination – quantum thermodynamics,” says Bill Munro at the Okinawa Institute of Science and Technology in Japan.
He and his colleagues used a quantum computer comprising 62 quantum bits, or qubits, made from superconducting circuits to explore such “demonic effects” – more qubits than have ever been used to implement Maxwell’s demon before.
Munro and his colleagues divided the qubits into two groups within a quantum computer, with each group representing one of Maxwell’s boxes. Then they implemented a demon-like procedure that used pulses of microwaves to force one group to contain more energetic qubits, and the other to contain far less energetic ones. In this way, the researchers effectively built a quantum battery, or a device that uses quantum processes to fill up with energy.
Quantum batteries are thought to be a promising, fast-charging energy technology of the future, but have so far only been explored in theory and modest proof-of-concept experiments. Here, the researchers could evaluate the effect of the demon on their actual battery. They found that the demon was much faster at changing the temperature – which points to a change in energy – of the two subsystems than a more conventional battery charging protocol.

They also verified that their experiment followed a modified version of the second law of quantum thermodynamics that explicitly accounts for the qubits’ quantum nature. This quantumness is the key novelty of the experiment, says Mauro Paternostro at Queen’s University Belfast in the UK. The experiment included enough qubits to exhibit so-called quantum many-body effects, which are thought to fundamentally affect how qubits can, or cannot, reach a state of equilibrium temperature.
The other exciting feature, he says, is that this version of Maxwell’s demon performs quantum measurements in order to sort qubits, and “the act of measuring something quantum mechanically is so violent, so strong, that you really fundamentally affect its state”. In other words, the new demon does not just measure qubits to sort them, but changes their states in the process, which improves its ability to charge a quantum battery.
“This was not anticipated by James Maxwell back in the 19th century,” says Munro.

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