John Preskill

In October 2019, John Preskill visited the QST community in Munich. On this occasion, MCQST awarded him as Distinguished Lecturer . The prize acknowledges Preskill’s scientific achievements in the field of quantum information theory as well as his engagement to communicate recent developments in quantum science and technology to a broader public. On this occasion, science journalist Frank Grotelüschen held an interview with John Preskill on behalf of MCQST.

He is one of the most renowned scientists in his field: the US-American theorist John Preskill explores the fundamental principles of quantum information. Among other things, he established important frameworks for a key technology of the future - the quantum computer. After studying physics at Princeton and completing his doctorate at Harvard, the 66-year-old professor is working since 1983 at the renowned California Institute of Technology (Caltech) in Pasadena near Los Angeles.

Already as a doctoral student, Preskill stood out. In 1979, he dealt with a problem that linked particle physics and cosmology. "I investigated whether particles called magnetic monopole could be created, when the universe was extremely young and hot," he says. "So-called Grand Unified Theories predict that they exist." These hypothetical elementary particles show a special property: instead of a north and a south pole, like any ordinary magnet, they should have only one pole. Aged 26, he found that, according to the then current theory, so many monopoles should have arisen during the Big Bang, that they should have been found long ago - which has not happened until today.

"The fact that we don't see them around tells us something really interesting about the early universe," says Preskill. "And that led to the proposal within the year following my paper, that we now call the inflationary universe." Most cosmologists today follow this inflation hypothesis: according to this hypothesis, the universe inflated incredibly quickly immediately after its birth. As a result, the concentration of monopolies must have diluted so much that they can no longer be detected today.

However, in the early 1990s Preskill's research shifted from cosmology and particle physics to quantum information. "At that time there was a lot of excitement among particle physicists in the US, because we were building what was called the Superconducting Super Collider," reports the physicist. "And this was going, we thought, to give us the answers to many of the questions I had worked on earlier in my career." Although the construction work had long since begun, the project was crushed in 1993 for cost reasons.

Frustrated, Preskill asked himself whether he should continue with particle physics at all – and it was then when he learned about an exciting discovery: "That was Peter Shor's discovery that a quantum computer would be able to solve problems that we think are too hard to solve with ordinary digital computers," he remembers. "Initially I just thought: Wow, this is something fun to learn about. But because I was on the lookout for a new direction to go into, I really got drawn into it." Preskill's career took a new direction: from then on he concentrated on illuminating the interface between information science and quantum physics.

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One problem fascinated him in particular: unlike a classical computer, a quantum computer does not work with bits that are either "zero" or "one", but with so-called qubits. These can be zero and one at the same time, and thus allow - at least for some computing operations – a quasi-parallel and thus extremely fast computing capability. The challenge here are the qubits, which are extremely fragile, and it is due to this property that a quantum computer needs sophisticated error correction. Preskill developed basic mechanisms for this error correction - mechanisms that are now transferred to the prototypes of quantum computers.

Moreover, Preskill came to an amazing conclusion in his work: "What came as a bit of a surprise is, that this same idea of quantum error correction turns out to be very helpful for understanding quantum mechanics and gravity and how they fit together," says the Caltech physicist. "That's been an exciting direction in the last five years." For a long time, experts have been trying to reconcile the two cornerstones of physics - quantum mechanics and Einstein's general relativity, the best theory of gravity to date. On the way to this unification called "quantum gravity”, Preskill's error correction approach could offer decisive impulses.

Today his research field seems to be on the leap to a lucrative market, quantum technology. Companies such as Google, Microsoft, or IBM are investing enormous sums in development, which is fuelling hopes of soon earning billions. But, John Preskill warns against too high expectations. „All this excitement and investment I think is partially driven by the probably unrealistic expectation, that we'll find important applications for quantum computers in five years,“ mentions the theorist. „I think quantum computers really will have a remarkable impact on the world, but it could be 20 or 30 years from now that we really see that fulfilled.“

One episode of his life as a researcher should remain in his memory - a bet with physics legend Stephen Hawking. „Stephen and I had many discussions about a topic which was very interesting to both of us – the question of what happens to information that falls inside black holes,“ says John. „Stephen argued that in the case of black holes information could be permanently erased. I and a lot of others didn't believe that.“ In the mid-1990s the two made a bet – it was decided at a conference in Dublin in 2004. „There Stephen gave a talk in which he said he was conceding the bet: He now believes that black holes do not destroy information," says Preskill. "Under the terms of the bet the winner was supposed to receive an encyclopaedia. Stephen knew that I was a baseball fan, and he got me a baseball encyclopaedia." In front of countless journalists, Preskill received the weighty volume. "The story got into the media, so I became briefly famous," he smiles. "And I'm afraid that will be the opening line of my obituary – that I won a bet with Stephen Hawking!"

John Preskill awarded MCQST Distinguished Lecturer at the Deutsches Museum. © Christoph Hohmann (MCQST)

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