John Preskill: Quantum computers will drive scientific discovery in the next 5 or 10 years

This week, in a webinar hosted by Caltech's Science Exchange, John Preskill, a professor of theoretical physics and chair of the Institute for Quantum Information and Matter (IQIM) at Caltech, talked about the promise and challenges of building a quantum computer.

As a theoretical physicist, Preskill developed the currently well-known concepts of quantum computing, including quantum supremacy and noisy intermediate-scale quantum (NISQ).

In a conversation with the school's science writer Whitney Clavin, he discusses how quantum computers might help solve difficult problems in various fields of science, and how the late Richard Feynman first proposed quantum computers in the 1980s have a similar understanding.

Highlights of the conversation are as follows, including Preskill's thoughts on scientific creativity.

1. What is a quantum computer and how is it different from an ordinary computer?

I thought you'd ask me a simple question first! This is actually a tough question to answer, but I'll try to answer it.

I should start by saying what a quantum computer isn't: it's not just a better, more powerful, faster version of the conventional computers we use today. It uses the principles of quantum physics to process information in a fundamentally different way.

We have long known that for nearly a century, all matter is governed by quantum theory, which has given rise to new technologies such as lasers and magnetic resonance imaging (MRI), as well as placing billions of transistors on a chip. But these techniques have only scratched the surface of how quantum theory can change our view of the possibilities of the universe.

In particular, they fail to take into account that when we have many subatomic particles that interact strongly with each other quantum mechanically, these particles speak a very exaggerated language that is incompatible with the language we understand and the language our computers understand very different. There is no way to succinctly translate quantum language into the ordinary bits that our computers know. If you want to use bits to describe what hundreds of particles are doing, you need more bits than there are atoms in the visible universe. This is an extremely complex problem, and we want to use it to speed up some very difficult computational problems.

2. There is a lot of hype about quantum computers. What are they going to do?

To some extent, this hype is natural. Everyone knows that computing is important, it affects our daily lives, and it has economic value. In recent years, we have seen a sharp rise in investor interest in the tech industry and quantum computing. In some ways, this is a good thing. It accelerates progress and provides opportunities for people to work in the field. But we should be realistic about the time frame of quantum computing because it has huge practical implications.

We should also be aware that quantum computers may not be able to speed up everything we want to do with computers, but will be suitable for a special class of problems—problems we still only partially understand. We will understand these questions better when we have quantum computers and can do experiments with them.

3. How long do we have to wait? 1 year? 10 years? 100 years?

It depends on what you want. We are in the very early stages of quantum computer development, but even now, from a scientific point of view, the quantum computers we already have are empowering us. They allow us to explore the behavior of complex quantum systems in unprecedented ways, which will drive scientific discoveries over the next 5 or 10 years. But for broad practical impact, I think a reasonable estimate is decades, or 10+ years.

4. Which potential applications excite you the most?

Remember, I'm a scientist, so I like to think of computers as a tool that drives scientific progress and scientific discovery. We use computers to elucidate how nature works in various ways. This applies to many fields of chemistry and materials science. We know the equations; they describe the electromagnetic interactions between electrons and their interactions with atomic nuclei. But for large molecules or complex materials, they are too difficult to solve. Quantum computers will help solve such problems.

This will eventually have practical implications as well. Computational chemistry, for example, could facilitate the discovery of new drugs and chemical catalysts and have broad implications for humanity, but it may be a while before we see that impact.

Another way we use computers is to help us discover new laws of physics, and I think quantum computers will be useful in that regard as well. One thing I've always been interested in is how should we think about the quantum mechanics of spacetime itself? This is important if I want to understand what happens when something falls into a black hole, or what happened very early in the history of the universe. Quantum computers will help us better grasp these things by allowing us to simulate quantum phenomena that would otherwise be difficult to study.

5. Is this what Richard Feynman had in mind when he first proposed quantum computers in the 1980s?

I knew Feynman; we worked at Caltech for about five years between my arrival at Caltech and his death, and we talked a lot about science. We didn't discuss quantum computing, but we did discuss a lot about subnuclear particles and their behavior. This is an example of where we think we know the equation. We have a theory called quantum chromodynamics, which describes the behavior of protons and neutrons and their components. But like chemistry, these equations are too difficult to solve. Although we know the correct equations, we cannot calculate this process from first principles, which is what quantum computers can do.

Part of what piqued Feynman's interest in the possibilities of quantum computing, I think, was his realization that such problems were too hard to solve. At the same time, he realized that if we could simulate the behavior of quantum systems, there would be other implications, including enabling us to perform calculations in chemistry and materials that we would otherwise be unable to achieve.

6. Do you think your work is creative?

hope so. Scientists are problem solvers. And so does everyone. I mean, it's part of the human condition, because we all have problems and we want to fix them. So you have to find a solution to the problem. You also have to figure out what issues you want to focus on and what questions to ask. I think both of these things have to do with creativity and intuition. If you're solving a problem, and it's necessary to know what the solution is, if it's simple, someone may have already found the answer. So sometimes you get stuck.

But you'll find a connection between what you're thinking about now and what you've thought about before. These analogies are actually very useful. Sometimes it just comes out of nowhere, and sometimes you have to force it. You realize that as long as you work hard enough and the path is clear, you can solve the problem, but you have to work out all the details. These creative steps are most interesting when you can't see where the solution will appear; if you look at the problem from a different angle, you'll realize the right way to think about it.

7. Does Caltech have quantum computers?

Caltech has a partnership with Amazon Web Services (AWS), which has a quantum computing center on our campus, and they're looking at very cold circuits-based approaches. I think it's a good partnership because both parties see the need to focus on long-term issues.

In my opinion, quantum computing applications in the next 5 to 10 years are most likely to be a tool for scientific discovery rather than solving problems of interest in business. To get to the stage where quantum computing has a huge practical impact, we have to have better and more qubits. For that, we're going to have to solve some big systems engineering problems.

The full conversation lasts 46 minutes and the video can be viewed at the link below:

https://scienceexchange.caltech.edu/topics/quantum-science-explained/ask-expert-quantum/quantum-computers-john-preskill?utm_source=caltechcarousel&utm_medium=web&utm_campaign=csequantum