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Mindscape 153 | John Preskill on Quantum Computers and What They’re Good For:

Depending on who you listen to, quantum computers are either the biggest technological change coming down the road or just another overhyped bubble. Today we’re talking with a good person to listen to: John Preskill, one of the leaders in modern quantum information science. We talk about what a quantum computer is and promising technologies for actually building them. John emphasizes that quantum computers are tailor-made for simulating the behavior of quantum systems like molecules and materials; whether they will lead to breakthroughs in cryptography or optimization problems is less clear. Then we relate the idea of quantum information back to gravity and the emergence of spacetime. (If you want to build and run your own quantum algorithm, try the IBM Quantum Experience.)

Preskill coined the term "quantum supremacy", and supposedly that is the biggest accomplishment in the field, but oddly the term is never mentioned.

Sean M. Carroll is an advocate of many-worlds theory, and Preskill said he is comfortable with that belief.

Preskill referred to Feynman kickstarting the field by a lecture that said that quantum systems are hard to simulate. This supposedly implied that quantum computers are inevitable.

As I see it, there are three main views underlying this thinking.

**Many-worlds.** If computers are constantly splitting into multiple computers doing different computers, then possibly they can be put to work doing parallel computation, and reporting back a result in one world.

This used to be the eccentric view of David Deutsch, but now Carroll, Preskill, Aaronson, and many others are on board.

**Negative probability.** In this view, quantum entanglement is a mysterious resource that can have negative or imaginary probabilities, so maybe it can do things better than classical computers that are limited by [0,1] probabilities. This is the preferred explanation of Scott Aaronson.

**Pinball.** The game of pinball is also hard to simulate because a ball can hit a bumper, be forced to one side or the other in a way that is hard to predict, and then the subsequent action of that ball is wildly different, depending on the side of the bumper. It is a simple example of chaos.

Quantum systems can be like pinball. Imagine a series of 100 double-slits. You fire an electron thru all the slits. At the first slit, the electron goes thru one slit, or the other, or some superposition. The electron has the same choice at the next slit, and it is influenced by what happened at the first slit. Simulating the 100 slits requires keep track of 2^{100} possibilities.

So pinball is hard to simulate, but no one would try to build a super-computer out of pinball machines.

My theory here is that your faith in quantum computers depends on which of the above three views you hold.

If quantum experiments are glimpses into parallel worlds, then it is reasonable to expect parallel computation to do useful work. If quantum experiments unlock the power of negative probabilities, then it is also plausible there is a quantum magic to be used in a computer. But it quantum experiments are just pinball machines, then nothing special should be expected.

You might say that we know quantum experiments are not classical, as Bell proved that. My answer is that they are not literally parallel universes or negative probabilities either. Maybe the human mind cannot even grasp what is really going on, so we have to use simple metaphors.

The many-worlds and negative probability views do not even make any mathematical sense. Many-worlds is so nutty that Carroll, Aaronson, and Preskill discredit much of what they have to say by expressing these opinions.

Now Aaronson says:

To confine myself to some general comments: since Google’s announcement in Fall 2019, I’ve **consistently said that sampling-based quantum supremacy is ***not* yet a done deal. I’ve said that quantum supremacy seems important enough to want independent replications, and demonstrations in other hardware platforms like ion traps and photonics, and better gate fidelity, and better classical hardness, and better verification protocols. Most of all, I’ve said that **we needed a genuine dialogue between the “quantum supremacists” and the classical skeptics**: the former doing experiments and releasing all their data, the latter trying to design efficient classical simulations for those experiments, and so on in an iterative process. Just like in applied cryptography, we’d only have real confidence in a quantum supremacy claim once it had survived at least a few years of attacks by skeptics. So I’m delighted that this is precisely what’s now happening.

Wow. He consistently said that?

He was the referee who approved Google's claim of quantum supremacy. He has collected awards for papers on that "sampling-based quantum supremacy". Maybe his referee report should have recommended that Google scale back its claims.

Aaronson has also argued that the quantum computing skeptics have been proven wrong. I guess not. It will still take a few years of consensus building.

I am one of those quantum computing skeptics. I thought that maybe someday and experiment would be done that proves me wrong. But apparently that is not how it works.

The experiments are inconclusive, but the experts will eventually settle into the pro or con sides. Only when that happens will I be seen to be right or wrong.

No, I don't accept that. Expert opinion is shifting towards many-worlds, but it is still a crackpot unscientific theory of no value. When Shor's algorithm on a quantum computer factors a number that could not previously be factored, then I will accept that I was wrong. Currently, Dr. Boson Sampling Supremacy himself admits that I have not been proven wrong.

Update: Here is a SciAm explanation:

If you think of a computer solving a problem as a mouse running through a maze, a classical computer finds its way through by trying every path until it reaches the end.
What if, instead of solving the maze through trial and error, you could consider all possible routes simultaneously?

Even now, experts are still trying to get quantum computers to work well enough to best classical supercomputers.