## Quantum Entanglement and Quantum Computing

John Preskill, the Richard P. Feynman Professor of Theoretical Physics, is himself deeply entangled in the quantum world. Different rules apply there, and objects that obey them are now being made in our world, as he explains at 8:00 p.m. on Wednesday, April 3, 2013, in Caltech's Beckman Auditorium. Admission is free.

**Q: What do you do?**

A: I'm trying to understand what a quantum computer would be capable of, how we could build one, and whether it would really work. My background is in particle theory, a subject I still love, but in the spring of 1994 a mathematician at Bell Labs named Peter Shor [BS 1981] discovered an algorithm for factoring large numbers with a quantum computer. I got really excited by this, because it moved the boundary separating "easy" problems, which we can eventually expect to solve with advanced technologies, from truly hard problems that we may never be able to solve. There are problems we can solve using quantum physics that we couldn't solve otherwise. The crucial problem is protecting a quantum computer from the various kinds of "noise" that could destroy quantum entanglement, and we've made a lot of progress on that.

**Q: OK, so what's "entanglement?"**

A: It's the correlations between the parts of a system. Suppose you have a 100-page book with print on every page. If you read 10 pages, you'll know 10 percent of the contents. And if you read another 10 pages, you'll learn another 10 percent. But in a highly entangled quantum book, if you read the pages one at a time—or even 10 at a time—you'll learn almost nothing. The information isn't written on the pages. It's stored in the correlations among the pages, so you have to somehow read all of them at once.

There's another important difference: If Alice and Bob both read this morning's *New York Times*, they will have perfectly correlated information. And if Charlie comes along and reads the same paper later on, he will be just as strongly correlated with Alice as Alice is with Bob, and Bob will be just as correlated with Charlie as he is with Alice. But if Alice reads her quantum newspaper and Bob reads his, they will learn almost nothing until they get together and share their information. Now, when Charlie comes along, Alice and Bob have already used up all their ability to be entangled, and he's completely left out. Entanglement is monogamous—if Alice and Bob are as entangled as they can be, neither of them can entangle with Charlie at all. So if Alice wants to be entangled with *both* Bob and Charlie, there's a limit to how entangled she can be with either one. They have to work out some sort of compromise.

**Q: What gets you excited about this?**

A: The technology is emerging to make it possible to do things we've never done before. We were taught in school that classical physics applies to things you can see, and quantum physics applies to the world at the scale of atoms and below. We're rebelling against that by making systems that are big enough to see, yet still exhibit quantum behavior. For example, Professor of Applied Physics Oskar Painter [MS 1995, PhD 2001] has made a tiny silicon bar that's suspended in space, and he's successfully cooled it all the way down to its quantum-mechanical ground state. It vibrates in a mode that corresponds to its lowest quantum state. He hasn't entangled such bars yet, but he knows how to do it.

We're exploring a new frontier of physics. It's not the frontier of short distances, like in particle physics; or of long distances, like in cosmology. It's what you might call the entanglement frontier.

*Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at **Caltech's iTunes U** site.*