Quantum batteries could be charged by reversing time

A method that can reverse the flow of time in quantum systems could one day be used to 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 physical laws and equations work whether time flows forwards or backwards.

Physicists have different explanations for why a discrepancy exists between the observed forward arrow of time and the permitted two-way flow. For example, the second law of thermodynamics says that systems are more likely to become disordered as time goes on, creating a preferred direction of time.

In the quantum world, the arrow of time is defined differently. Quantum processes, like classical physical laws, can run in either direction, but we can define an arrow of time by comparing our measurements of a quantum system with our calculations of how a quantum system should change over time. When these line up with a particular statistical pattern, we can say the system is moving forwards 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 a quantum system and effectively undoing them, making it appear to an observer as if a quantum system were running backwards in time.

“We apply fields and control tools on the system that can undo what is happening due to the measurements,” says García-Pintos. “If the measurement was going to push my system up, I can make it go back down. Because we’re able to counteract the effective measurements, we can produce trajectories that are more consistent with the process having been backwards than forward.”

For example, the team suggests 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 upsetting the qubit’s delicate quantum state, allowing it to be constantly 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 allow energy to be harvested from quantum systems where you have to make measurements, says García-Pintos, which could one day be useful for applications like quantum batteries or miniature quantum engines. This is because whenever a measurement is made on a quantum system, it injects energy into that system.

But the careful adjustments made to mimic the changing quantum arrow of time can effectively redirect this energy, harvesting it for other uses. “As a result, you’re getting energy out of it,” says García-Pintos. “You have a mechanism where you’re using measurements as a thermodynamic resource.”

It is a clever idea, says Mauro Paternostro at Queen’s University Belfast, UK, but the proposed set-up is specific and engineered in a way that won’t apply to many real quantum systems.

It is also important to understand that it doesn’t break the second law of thermodynamics, says Paternostro, because you have to spend energy to reduce the disorder of the system. “When I get into my son’s room, it’s a mess: balls are here and there, clothes are distributed across the room. If I make work, cleaning it up and ordering stuff, that will reduce the amount of disorder in that room, but I had to spend some energy,” he says. “That’s exactly what they are showing with their external control mechanism.”