# Understanding A Potential “Speed Limit” For Quantum Computing

# Since We Can’t Measure Departure or Arrival Times Without a Delay, There May be a “Quantum Speed Limit”

Stay with me, here. I recently read an article by Sebastian Deffner that I found fascinating. Sebastian is an assistant professor at the University of Maryland Baltimore County where his research focuses on quantum thermodynamics. In his piece, Professor Deffner discusses the discovery of a “quantum speed limit” that will effectively govern how fast quantum computers will actually operate. You’re probably thinking the same thing I did. What does that mean?

Given that I am not a physicist, I’ll share the quote I believe best summarizes the concept:

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.”

## I Bet You Can Guess What I Asked Next

Once I absorbed that idea and reread the text, I noted a critical question the article doesn’t answer. What exactly is the limit? Is it so high that we won’t reach it for a while? Is it low enough to prevent quantum computers from reaching their potential? Deffner says more research is required to answer that question. Alas! I’m not a quantum researcher, either.

I thought at this point it might be best to ask a few of our superposition friends for their opinions. Some didn’t comment; some were more detailed. As usual, we had one smart ass (who I love). I’ve included my two favorites.

## Bo Ewald

In both classical and quantum computing there are limits—practical limits and nature’s limits. About 50 years ago, Gordon Moore described practical limits on how fast we might be able to increase the number of transistors on a chip, and thus the performance of classical systems. It has taken a long time to approach the fundamental limits that nature imposes on classical systems, but we are starting to see that now.

In quantum computing, we are at a similarly early stage of development, and practical limits are today’s challenge. For example, how quickly can we scale up the number of qubits or their interconnection topology? In some ways, today’s quantum computing is at a point like it must have been in the 1950’s for traditional computing—even before Moore’s famous “law.” From my perspective, the biggest challenges in quantum computing today are to design and build practical quantum systems, to provide a software environment to make them easier to use, and to develop useful applications for them.

## Helmut Katzgraber

This is like asking Mauchly and Eckert about Moore’s Law when they were building the ENIAC . . . Ask me again in a decade or so . . .

I do love me some Katzgraber. But seriously, these two opinions highlight one of the current truths in quantum computing: there is so much left to discover. Will a speed limit throttle our quantum ambitions? Will we never run into such a limit? Only time will tell. How exciting!