Quantum speed limit may put brakes on quantum computers

Heisenberg’s uncertainty
Physicists trace the history of quantum theory back to 1927, when German physicist Werner Heisenberg showed that the classical methods did not work for very small objects, those roughly the size of individual atoms. When someone throws a ball, for instance, it’s easy to determine exactly where the ball is, and how fast it’s moving.

But as Heisenberg showed, that’s not true for atoms and subatomic particles. Instead, an observer can see either where it is or how fast it’s moving – but not both at the exact same time. This is an uncomfortable realization: Even from the moment Heisenberg explained his idea, Albert Einstein (among others) was uneasy with it. It is important to realize that this “quantum uncertainty” is not a shortcoming of measurement equipment or engineering, but rather how our brains work. We have evolved to be so used to how the “classical world” works that the actual physical mechanisms of the “quantum world” are simply beyond our ability to fully grasp.

Entering the quantum world
If an object in the quantum world travels from one location to another, researchers can’t measure exactly when it has left nor when it will arrive. The limits of physics impose a tiny delay on detecting it. So no matter how quickly the movement actually happens, it won’t be detected until slightly later. (The lengths of time here are incredibly tiny – quadrillionths of a second – but add up over trillions of computer calculations.)

That delay effectively slows down the potential speed of a quantum computation – it imposes what we call the “quantum speed limit.”

Over the last few years, research, to which my group has contributed significantly, has shown how this quantum speed limit is determined under different conditions, such as using different types of materials in different magnetic and electric fields. For each of these situations, the quantum speed limit is a little higher or a little lower.

To everyone’s big surprise, we even found that sometimes unexpected factors can help speed things up, at times, in counterintuitive ways.

To understand this situation, it might be useful to imagine a particle moving through water: The particle displaces water molecules as it moves. And after the particle has moved on, the water molecules quickly flow back where they were, leaving no trace behind of the particle’s passage.

Now imagine that same particle traveling through honey. Honey has a higher viscosity than water – it’s thicker and flows more slowly – so the honey particles will take longer to move back after the particle moves on. But in the quantum world, the returning flow of honey can build up pressure that propels the quantum particle forward. This extra acceleration can make a quantum particle’s speed limit different from what an observer might otherwise expect.

Designing quantum computers
As researchers understand more about this quantum speed limit, it will affect how quantum computer processors are designed. Just as engineers figured out how to shrink the size of transistors and pack them more closely together on a classical computer chip, they’ll need some clever innovation to build the fastest possible quantum systems, operating as close as possible to the ultimate speed limit.

There’s a lot for researchers like me to explore. It’s not clear whether the quantum speed limit is so high it’s unattainable – like the car that will never even get close to the speed of light. And we don’t fully understand how unexpected elements in the environment – like the honey in the example – can help to speed up quantum processes. As technologies based on quantum physics become more common, we’ll need to find out more about where the limits of quantum physics are, and how to engineer systems that take the best advantage of what we know.

Sebastian Deffner Assistant Professor of Physics, University of Maryland, Baltimore County. This article is published courtesy of The Conversation (under Creative Commons-Attribution / No derivative).

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