We’re hearing this week from two very different parts of the string theory community that quantum supremacy (quantum computers doing better than classical computers) is the answer to the challenges the subject has faced.
New Scientist has an article Quantum computers could simulate a black hole in the next decade which tells us that “Understanding the interactions between quantum physics and gravity within a black hole is one of the thorniest problems in physics, but quantum computers could soon offer an answer.” The article is about this preprint from Juan Maldacena which discusses numerical simulations in a version of the BFSS matrix model, a 1996 proposal for a definition of M-theory that never worked out. Maldacena points to this recent Monte-Carlo calculation, which claims to get results consistent with expectations from duality with supergravity.
Maldacena’s proposal is basically for a variant of the wormhole publicity stunt: he argues that if you have a large enough quantum computer, you can do a better calculation than the recent Monte-Carlo. In principle you could look for quasi-normal modes in the data, and then you would have created not a wormhole but a black hole and be doing “quantum gravity in the laboratory”
seeing these quasinormal modes from a quantum simulation of the quantum system under discussion, would be a convincing evidence that we have created something that behaves as a black hole in the laboratory.
This isn’t a publicity stunt like the wormhole one, because the only publicity I’ve seen is a New Scientist article, and this is just a proposal, not actually executed. Maldacena estimates that to reproduce the recent Monte-Carlo calculation you’d need 7000 or so logical qubits, which the New Scientist reporter explains would be something like one million physical qubits. So, there’s no danger Quanta magazine will be producing videos about the creation of a black hole in a Google lab any time soon.
Maldacena has been chosen to give the presentation tomorrow at the SLAC P5 Town Hall about a vision for the future of fundamental theory, no idea whether creating black holes in the lab using quantum computers will be part of it.
At the other extreme of respectability and influence in the physics community, Michio Kaku has a new book out, Quantum Supremacy. I took a quick look yesterday at a copy at the bookstore. I’ll leave it to others to discuss the bulk of the book, which seems to be about how “There is not a single problem humanity faces that couldn’t be addressed by quantum computing.” The last few pages are about string theory, beginning with the usual bogus pro-string theory arguments, working up to the ending of the book: “So quantum computers may hold the key to creation itself” (i.e. they will “solve all the equations of string theory”). His argument for the relevance of quantum computers to string theory is that they will calculate paths in the landscape:
One day, it might be possible to put string theory on to a quantum computer to select out the correct path. Perhaps many of the paths found in the landscape are unstable and quickly decay, leaving only the correct solution . Perhaps our universe emerges as the only stable one.
This is justified by a bizarre paragraph about lattice gauge theory, which explains that since we can’t solve QCD analytically, here’s what theorists do:
One solves the equations for one tiny cube, uses that to solve the equations for the next neighboring cube, and repeats the same process for all that follow. In this way, eventually the computer solves for all the neighboring cubes, one after the other.
This pretty conclusively shows that the explanation for the Kaku phenomenon is simply that he has no idea what he is talking about.
Update: The reviews of the book have been pretty uniformly very enthusiastic, with the reviewers evidently incapable of distinguishing sense from nonsense. A depressing example is at Science magazine. Why choose as reviewers of a book on quantum computing two people who know nothing about the subject? Is it because Science couldn’t find anyone who does know about quantum computing willing to read the book and write about it?
Update: The one thing keeping my spirits up while reading the almost uniformly glowing reviews of this piece of junk has been the thought that “at least the New York Times is kind of doing the right thing”: not reviewing the book. Just noticed they do have a review up:
That mind-blowing future is the focus of the final five or so hours of the audiobook, which explores the real-world impacts quantum computing could have: altering our immune systems to avoid cancer and Alzheimer’s, increasing crop yields, ending world hunger. As Kaku puts it, “the familiar laws of common sense are routinely violated at the atomic level”; but his lucid prose and thought process make abundant sense of this technological turning point.
Just shoot me…