CoL49 ch5 - did someone already share this link?

[Michael Bailey]

>
> if you’re hitting a paywall -

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.

 Journal reference: Physical Review A DOI: 10.1103/PhysRevA.109.062614


>