When we think about charging a battery, we typically imagine that the charge flows one way. For example, when we plug our smartphones in at night, we think of the charge as flowing from the outlet into the phone’s battery.
While this holds true in the classical physics of our day-to-day world, it is typically not the case in the quantum regime. “When you’re dealing with quantum, the flow of energy is actually symmetrical,” explains Dr. Shabir Barzanjeh, associate professor of physics at the University of Calgary's Faculty of Science.
“The energy bounces back and forth between the charger and battery.”
The smaller the scale, the more problematic this symmetry becomes. At micro and nano scales, it can greatly reduce the efficiency of the charging process. These are precisely the scales at which Barzanjeh focuses his research.
“Just like classical machines, many micro and nanodevices can be used to capture and store energy,” explains Barzanjeh. “Dealing with the inefficiencies of symmetric energy flow is a roadblock in building smaller batteries that can store more energy.”
Barzanjeh has been studying this challenge for quite some time. Now, in a groundbreaking paper published in Physical Review Letters, he and his colleagues have made promising steps toward a solution. Selected as an editor highlight, the paper represents significant progress in addressing some of these miniaturization issues.
The novel process — which Barzanjeh and his University of Gdansk colleagues Borhan Ahmadi, Paweł Mazurek, and Paweł Horodecki proposed — relies on breaking time-reversal symmetry through nonreciprocity.
Let’s take a closer look at these quantum concepts.
Time-reversal symmetry: forwards and backwards
In time-reversal symmetry, an object or process is the same if experienced in either direction. For example, a movie played forward and backward is fundamentally the same collection of images and sounds. Similarly, energy transmission undergoes the same process if it’s flowing from an outlet to a battery, or from the battery back to the outlet.
Most systems in nature are subject to time-reversal symmetry, and this tends to be how we think of the world operating: we can walk into a room or out of it, travel to a destination and then return from it, and so on.
Nonreciprocity: a one-way street
Nonreciprocity occurs when time-reversal symmetry is broken. Suddenly, a process can only unfold one way, with no reverse possible. Imagine a movie that would completely disappear if you tried to play it backwards, or a room that you could walk into — but never out of.
The principle of nonreciprocity has long been a fundamental tool in diverse quantum technology applications. By enabling the unidirectional flow of signals and energy, it effectively suppresses the “noise” that often interferes with quantum systems.
This technique has been applied to isolate systems and information to enable quantum computing, ultra-sensitive measurement with atomic clocks, and more. But it has not been effectively applied to quantum batteries — until now.
Bringing nonreciprocity to batteries
Barzanjeh’s work and resulting paper prove the capability for using nonreciprocity in charging and leveraging quantum batteries. “In a non-reciprocal system, all of the energy flows one way, so there is no backflow,” he explains.
In the quest to miniaturize batteries for micro and nanomachines, this process is poised to help in two important ways. First, it can enable more efficient batteries in terms of charging and storage capacity. Second, it can drive further reductions in battery size. Both developments will contribute to the ongoing improvement of quantum computing and nanomachines.
“It’s exciting to do research at a university where we’re leading the way on so many quantum discoveries and innovations,” says Barzanjeh. “We are really seeing UCalgary realizing its potential as a quantum innovator, and it’s great to be a part of that.”
