The quantum computer that knows all

This is my first post in more than a month that’s totally unrelated the covid crisis. Or rather, it’s related only insofar as it’s about a Hulu miniseries, the sort of thing that many of us have more occasion to watch while holed up at home.

Three weeks ago, a journalist named Ben Lindbergh—who’d previously asked me to comment on the scientific accuracy of Avengers: Endgame—asked me the same question about the miniseries Devs, which I hadn’t previously heard of.

[Warning: Spoilers follow]

‘Devs,’ I learned, is a spooky sci-fi action thriller about a secretive Silicon Valley company that builds a quantum computer that can perfectly reconstruct the past, down to what Jesus looked like on the cross, and can also (at least up to a point) predict the future.

And I was supposed, not only to endure such a show, but to comment on the accuracy of its invocations of quantum computing? This didn’t sound promising.

But, y’know, I was at home quarantined. So I agreed to watch the first episode. Which quickly turned into the second, third, fourth, fifth, sixth, and seventh episodes (the eighth and final one isn’t out yet).

It turns out that ‘Devs’ isn’t too bad, except that it’s not particularly about quantum computers. The latter is simply a buzzword chosen by the writers for a plot concept that would’ve been entirely familiar to the ancient Greeks, who called it the Delphic Oracle. You know, the mysterious entity that prophesies your fate, so then you try to escape the prophecy, but your very evasive maneuvers make the prophecy come true? Picture that, except with qubits—and for some reason, in a gleaming golden laboratory that has components that float in midair.

If you’re never visited a real quantum computing lab: they’re messier and a lot less golden.

At this point, I’ll just link you to Ben Lindbergh’s article about the show: Making Sense of the Science and Philosophy of ‘Devs.’ His long and excellent piece quotes me extensively enough that I see no need also to analyze the show in this blog post. (It also quotes several academic philosophers.)

Instead, I’ll just share a few tidbits that Ben left out, but that might be amusing to quantum computing fans.

• The show opens with a conversation between two characters about how even “elliptical curve” cryptography is insecure against attack by quantum computers. So I immediately knew both that the writers had one or more consultants who actually knew something about QC, and also that those consultants were not as heavily involved as they could’ve been.

• Similarly: in a later scene, some employees at the secretive company hold what appears to be a reading group about Shor’s algorithm. They talk about waves that interfere and cancel each other out, which is great, but beyond that their discussion sounded to me like nonsense. In particularly, their idea seemed to be that the waves would reinforce at the prime factors p and q themselves, rather than at inverse multiples of the period of a periodic function that only indirectly encodes the factoring problem. (What do you say: should we let this one slide?)

• “How many qubits does this thing have?” “A number that there would be no point in describing as a number.” ROFL

• In the show, a crucial break comes when the employees abandon a prediction algorithm based on the deBroglie-Bohm pilot wave interpretation, and substitute one based on Everett’s many-worlds interpretation. Which I could actually almost believe, except that the many-worlds interpretation seems to contradict the entire premise of the rest of the show?

• A new employee, after he sees the code of the superpowerful quantum computer for the first time, is so disoriented and overwhelmed that he runs and vomits into a toilet. I, too, have had that reaction to the claims of certain quantum computing companies, although in some sense for the opposite reason.

Anyway, none of the above addresses the show’s central conceit: namely, that the Laplace demon can be made real, the past and future rendered fully knowable (with at most occasional breaks and exceptions) by a machine that’s feasible to build. This conceit is fascinating to explore, but also false.

In the past, if you’d asked me to justify its falsity, I would’ve talked about chaos, and quantum mechanics, and the unknowability of the fine details of the universe’s state; I might’ve even pointed you to my Ghost in the Quantum Turing Machine essay. I also would’ve mentioned the severe conceptual difficulties in forcing Nature to find fixed-points in a world where you get to see your own future and act on it (these difficulties are just a variant of the famous Grandfather Paradox).

But it occurs to me that, just as the coronavirus has now made plain the nature of exponential growth, even to the world’s least abstract-minded person, so too it’s made plain the universe’s unpredictability. Let’s put it this way: do you find it plausible that the quantum computer from ‘Devs,’ had you booted it up six months ago, would’ve known the exact state of every nucleotide in every virus in every bat in Wuhan? No? Then it wouldn’t have known our future.

And I see now that I’ve violated my promise that this post would have nothing to do with the covid crisis.