Splitting the qubits in a quantum computer into high-energy and low-energy groups could recharge a battery
Shutterstock / Pavel Chukhov
A 19th century thought experiment that for decades was thought to defy the laws of thermodynamics has now been realised in a quantum computer and used to charge a quantum battery.
Physicist James Clerk Maxwell came up with the demon in 1867 while thinking of ways to trick the laws of thermodynamics. He came up with two boxes of gas separated by a gravity-free door, and a little demon that controlled which particles could pass through the door. The demon used this control to make one box hotter and the other colder, which contradicts the law of thermodynamics that states that heat flows from the hotter box to the colder box, eventually equalizing the heat.
Physicists have since realized that demons cannot break the laws of thermodynamics “for free,” because the process of particle selection consumes energy, but the idea remains intriguing because it occurs naturally in biology and can be used in chemistry.
“The exploration of Maxwell’s demon in a quantum setting forces us to think deeply about what lies behind the fundamental laws of quantum information, thermodynamics, and especially their combination, quantum thermodynamics,” he said. Bill Munro At Okinawa Institute of Science and Technology, Japan.
He and his colleagues investigated such “demonic effects” using a quantum computer containing 62 quantum bits (qubits) made from superconducting circuits — more qubits than ever before used to implement Maxwell’s demon.
Munro and his colleagues split the qubits in their quantum computer into two groups, assigning each group to one of Maxwell’s boxes. They then performed a devilish step: using microwave pulses to force one group to contain high-energy qubits and the other to contain low-energy qubits. In this way, the researchers effectively built a quantum battery, a device that uses quantum processes to charge itself with energy.
Quantum batteries are considered a promising fast-charging energy technology, but so far they have only been studied in theory and modest proof-of-concept experiments. Here, the researchers were able to evaluate the effect of the demon on a real battery. They found that the demon changes the temperature (indicative of the change in energy) of two subsystems much faster than conventional battery charging protocols.
They also verified that the experiment follows a modified version of the second law of quantum thermodynamics and explicitly takes into account the quantum nature of the qubits, which he says is the experiment’s key novelty. Mauro Paternostro The experiments, carried out at Queen’s University in Belfast, UK, used enough qubits to demonstrate so-called quantum many-body effects that are thought to have a fundamental impact on whether qubits can reach an equilibrium temperature.
Another interesting feature, he says, is that this version of Maxwell’s demon performs quantum measurements to classify qubits, and “the act of measuring something quantum-mechanically is so intense, so powerful, that it really has a fundamental effect on its state.” In other words, the new demon not only measures and classifies qubits, but in the process it changes the qubits’ state, improving their ability to charge their quantum battery.
“This is something that James Maxwell would not have anticipated in the 19th century,” Munro says.
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