Quantum batteries can be charged in reverse time

A method that can reverse the flow of time in quantum systems could one day help charge quantum batteries.

For every process we observe in the universe, events appear to happen in only one direction, following an apparent arrow of time. But most of the laws and equations of physics work whether time is moving forward or backward.

Physicists have various explanations for why there is a discrepancy between the observed forward arrow of time and the allowed two-way flow. For example, the second law of thermodynamics states that systems are more likely to become disordered over time, creating a preferred direction of time.

In the quantum world, the arrow of time is defined differently. Quantum processes, like the classical laws of physics, can run in both directions, but we can define the arrow of time by comparing our measurements of the quantum system with our calculations of how the quantum system should change over time. When these conform to a particular statistical pattern, we can say that the system is moving forward in time.

Now, Luis Pedro García-Pintos at Los Alamos National Laboratory in New Mexico and his colleagues have found a way to mimic this statistical signature by reverse-engineering the changes that measurements make to the quantum system, effectively disrupting them so that to an observer it appears as if the quantum system is running backwards in time.

“We apply fields and control tools to the system that can reverse what is happening as a result of the measurement,” says García-Pintos. “If a measurement was to move my system up, I can bring it back down. Because we are able to counter effective measurements, we can create trajectories that are more consistent with the process being backwards than forwards.”

For example, the team suggests that you could manipulate the arrow of time in a qubit, the computational component of a quantum computer, by measuring one of its properties, such as its spin, but in an indirect way to avoid disturbing the qubit’s delicate quantum state, allowing it to be continuously measured as it changes over time. This signal can then be used to calculate how to change what the arrow of time looks like by applying a pulse of microwave radiation.

The technique could also enable harvesting energy from quantum systems where you have to make measurements, says García-Pintos, which could one day be useful for applications such as quantum batteries or miniature quantum motors. This is because whenever a measurement is made on a quantum system, it injects energy into that system.

But careful adjustments made to mimic the changing quantum arrow of time can effectively redirect this energy and harvest it for other purposes. “As a result, you get energy from it,” says García-Pintos. “You have a mechanism where you use the measurement as a thermodynamic source.”

It’s a smart idea, he says Mauro Paternostro at Queen’s University Belfast in the UK, but the proposed setup is specific and designed in a way that cannot be used for many real quantum systems.

It’s also important to understand that it doesn’t violate the second law of thermodynamics, Paternostro says, because you have to expend energy to reduce the disorder of the system. “When I get to my son’s room, it’s a mess: balls are here and there, clothes are scattered around the room. If I do work, clean it up and order things, it reduces the mess in that room, but I had to spend some energy,” he says. “They show exactly that with their external control mechanism.”

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