Category: rants

When will we have a quantum computer? Never, with that attitude

We are quantum drunks under the lamp post—we are only looking at stuff that we can shine photons on.

In a recently posted paper, M.I. Dyakonov outlines a simplistic argument for why quantum computing is impossible. It’s so far off the mark that it’s hard to believe that he’s even thought about math and physics before. I’ll explain why.

abstract

Find a coin. I know. Where, right? I actually had to steal one from my kid’s piggy bank. Flip it. I got heads. Flip it again. Heads. Again. Tails. Again, again, again… HHTHHTTTHHTHHTHHTTHT. Did you get the same thing? No, of course you didn’t. That feels obvious. But why?

Let’s do some math. Wait! Where are you going? Stay. It will be fun. Actually, it probably won’t. I’ll just tell you the answer then. There are about 1 million different combinations of heads and tails in a sequence of 20 coin flips. The chances that we would get the same string of H’s and T’s is 1 in a million. You might as well play the lottery if you feel that lucky. (You’re not that lucky, by the way, don’t waste your money.)

Now imagine 100 coin flips, or maybe a nice round number like 266. With just 266 coin flips, the number of possible sequences of heads and tails is just larger than the number of atoms in the entire universe. Written in plain English the number is 118 quinvigintillion 571 quattuorvigintillion 99 trevigintillion 379 duovigintillion 11 unvigintillion 784 vigintillion 113 novemdecillion 736 octodecillion 688 septendecillion 648 sexdecillion 896 quindecillion 417 quattuordecillion 641 tredecillion 748 duodecillion 464 undecillion 297 decillion 615 nonillion 937 octillion 576 septillion 404 sextillion 566 quintillion 24 quadrillion 103 trillion 44 billion 751 million 294 thousand 464. Holy fuck!

So obviously we can’t write them all down. What about if we just tried to count them one-by-one, one each second? We couldn’t do it alone, but what if all people on Earth helped us? Let’s round up and say there are 10 billion of us. That wouldn’t do it. What if each of those 10 billion people had a computer that could count 10 billion sequences per second instead? Still no. OK, let’s say, for the sake of argument, that there were 10 billion other planets like Earth in the Milky Way and we got all 10 billion people on each of the 10 billion planets to count 10 billion sequences per second. What? Still no? Alright, fine. What if there were 10 billion galaxies each with these 10 billion planets? Not yet? Oh, fuck off.

Even if there were 10 billion universes, each of which had 10 billion galaxies, which in turn had 10 billion habitable planets, which happened to have 10 billion people, all of which had 10 billion computers, which count count 10 billion sequences per second, it would still take 100 times the age of all those universes to count the number of possible sequences in just 266 coin flips. Mind. Fucking. Blown.

Why I am telling you all this? The point I want to get across is that humanity’s knack for pattern finding has given us the false impression that life, nature, the universe, or whatever, is simple. It’s not. It’s really fucking complicated. But like a drunk looking for their keys under the lamp post, we only see the simple things because that’s all we can process. The simple things, however, are the exception, not the rule.

Suppose I give you a problem: simulate the outcome of 266 coin tosses. Do you think you could solve it? Maybe you are thinking, well you just told me that I couldn’t even hope to write down all the possibilities—how the hell could I hope to choose from one of them. Fair. But, then again, you have the coin and 10 minutes to spare. As you solve the problem, you might realize that you are in fact a computer. You took an input, you are performing the steps in an algorithm, and will soon produce an output. You’ve solved the problem.

A problem you definitely could not solve is to simulate 266 coin tosses if the outcome of each toss depended on the outcome of the previous tosses in an arbitrary way, as if the coin had a memory. Now you have to keep track of the possibilities, which we just decided was impossible. Well, not impossible, just really really really time consuming. But all the ways that one toss could depend on previous tosses is yet even more difficult to count—in fact, it’s uncountable. One situation where it is not difficult is the one most familiar to us—when each coin toss is completely independent of all previous and future tosses. This seems like the only obvious situation because it is the only one we are familiar with. But we are only familiar with it because it is one we know how to solve.

Life’s complicated in general, but not so if we stay on the narrow paths of simplicity. Computers, deep down in their guts, are making sequences that look like those of coin-flips. Computers work by flipping transistors on and off. But your computer will never produce every possible sequence of bits. It stays on the simple path, or crashes. There is nothing innately special about your computer which forces it to do this. We never would have built computers that couldn’t solve problems quickly. So computers only work at solving problems that can we found can be solved because we are at the steering wheel forcing them to the problems which appear effortless.

In quantum computing it is no different. It can be in general very complicated. But we look for problems that are solvable, like flipping quantum coins. We are quantum drunks under the lamp post—we are only looking at stuff that we can shine photons on. A quantum computer will not be an all-powerful device that solves all possible problems by controlling more parameters than there are particles in the universe. It will only solve the problems we design it to solve, because those are the problems that can be solved with limited resources.

We don’t have to track (and “keep under control”) all the possibilities, as Dyakonov suggests, just as your digital computer does not need to track all its possible configurations. So next time someone tells you that quantum computing is complicated because there are so many possibilities involved, remind them that all of nature is complicated—the success of science is finding the patches of simplicity. In quantum computing, we know which path to take. It’s still full of debris and we are smelling flowers and picking the strawberries along the way, so it will take some time—but we’ll get there.

 

The point of physics

Something I lost sight of for a long time is the reason I study physics, or the reason I started studying it anyway. I got into it for no reason other than it was an exciting application of mathematics. I was in awe, not of science, but of the power of mathematics.

Now there are competing pressures. Sometimes I find myself “doing physics” for reasons that can only best be seen as practical. Fine—I’m a pragmatic person after all. But practicality here is often relative to a set of arbitrarily imposed constraints, such as requiring a CV full of publications in journals with the highest rank in order to be a good academic boi.

You may say that’s life. We all start with naive enthusiasm and end up doing monotonous things we don’t enjoy. But then we tell ourselves, and each other, lies about it being in service of some higher purpose. Scientists see it stated so often that they start to repeat it, and even start to believe it. I know I’ve written and repeated thoughtless platitudes about science many times. It’s almost necessary to convince yourself of these myths as you struggle through your school or your job. Why am I doing this, you wonder, because it certainly doesn’t feel rewarding in those moments.

On the other hand, many people are comfortable decoupling their passion from their job. Do the job to earn money which funds your true passions. Not all passions provide the immediate monetary returns one needs to live a comfortable life after all. So you can study science to learn the skills that someone will pay you to employ. There are many purely practical reasons to study physics, for example, which have nothing to do with answering to some higher calling. This certainly seems more honest than having to lie to yourself when expectations fail.

(I should point out that if you are one of those people currently struggling through graduate school, academia is not the only way—maybe not even the best way—to sate your hunger for knowledge, or just solve cool maths problems.)

A lot of scientists, teachers, and university recruiters get this wrong. There is a huge difference between being curious about nature and reality and suggesting it is morally good to devote one’s life to playing a small part in answering specific questions about such.

Einstein did not develop general relativity to usher in a new era of gravitational wave astronomy, as cool as that is. He did it because he was obsessed with answering his own questions driven by his insatiable imagination. Even the roots of the now enormous collaboration of scientists which detected gravitational waves started in a water cooler conversation among a few physicists, which is best summarized by this tweet:

In other words, we don’t actually do things through a consensual agreement about its potential value to a higher power called science. We think about doing certain things because we are curious, because we want to see what will happen, or because we can.

Like all other myths scientists and their adoring followers like to deride, science as a moral imperative is just that—a myth. Might we not get further with honesty, by telling ourselves and others that we are just people—people trying to do cool shit. The great things will come as they always have, emerging from complex interactions—not by everyone collectively following a blinding light at the end of tunnel, but by lighting the tunnel itself with millions of unique candles.

The real magic of quantum computing

By now you have read many articles on quantum computing. Congratulations. You know nothing about quantum computing.

There is a magician on stage. It’s tense. Maybe it’s a primetime TV show and the production value is super high. The celebrity judges look nervous. There is epic build up music as the magician calls their assistant on stage. The assistant climbs into a box that is covered with a velvet blanket. Why a blanket? I mean, isn’t the box good enough? What a pretentious as… forget it, I’m ruining this for myself. OK, so the assistant is in the box with their head and legs sticking out. What the fuck? Who made this box, anyway? Damn it, I’m doing it again. Then—oh shit—is that a saw? What’s going to happen with that? Fuck! No! The assistant’s been cut in half! And then the quantum computer outputs the answer. Wait, what? Where did the quantum computer come from? I don’t know—quantum computing is magic like that.

By now you have read many articles on quantum computing. Congratulations. You know nothing about quantum computing. I know what you are thinking: Whoa, Chris, I wasn’t ready for these truth bombs. Take it easy on us. But I see a problem and I just need to fix it. Or, more likely, call the rental agent to fix it.

You probably think that a qubit can represent a 0 and a 1 at the same time. Or, that quantum computing takes advantage of the strange ability of subatomic particles to exist in more than one state at any time. I can hardly fault you for that. After all, we expect Scientific American and WIRED to be fairly reputable sources. And, I’m not cherry picking here—these were the first two hits after the Wikipedia entry on a Google search of “What is quantum computing?” Nearly every popular account of quantum computing has this “0 and 1 at the same time” metaphor.

I say metaphor because it is certainly not literally true that the things involved in quantum computing—those qubits mentioned above—are 0 and 1 at the same time. Why? Well, for starters, 0 and 1 are defined to be mutually exclusive (that means it’s either one OR the other). Logically, 0 is defined as [NOT 1]. Then 0 AND 1 is equal to [NOT 1] AND 1, which is a false statement. “0 and 1 at the same time” just doesn’t make sense, and it’s false anyway. Next.

OK, so what’s the big deal? We all play fast and loose with words. Surely this little… let me stop you right there, because it gets worse. Much worse.

The Scientific American article linked above then deduces that, “This lets qubits conduct vast numbers of calculations at once, massively increasing computing speed and capacity.” That’s a pretty big logical leap, but I’d say it’s a correct one. Let’s break it down. First, if a qubit can be 0 and 1 at the same time then two qubits can be 00 and 01 and 10 and 11 at the same time. And three qubits can be 000 and 001 and 010 and 011 and 100 and 101 and 110 and 111 at the same time. And… well, you get the picture. Like mold on that organic bread you bought, exponential growth!

The number of possible ways to set some number of bits, say n of them, is 2n—a big number. If n = 300, 2300 is more than the number of atoms in the universe! Think about that. Flip a coin just 300 times and the number of possible ways they could land is unfathomable. And 300 qubits could be all of them at the same time. If you believe that, then it is easy to believe that quantum computers will just calculate every possible solution to your problem at once and pick the right answer. That would be magic. Alas, this is not how quantum computers work.

Lesson 1: don’t take a bad metaphor and draw your own simplistic conclusions from it.

Try this one out from Forbes: “A bit can be at either of the two poles of the sphere, but a qubit can exist at any point on the sphere.” Spot on. This is 100% accurate. But, wait! “So, this means that a computer using qubits can store an enormous amount of information and uses less energy doing so than a classical computer.” The fuck? No. In fact, a qubit cannot be used to store and retrieve more than 1 bit of data. Again, magic, but not how quantum computers work.

Lesson 2: don’t reduce an entire field to one idea and draw your own simplistic conclusions from it.

I can just imagine what you are thinking right now. OK hotshot, how would you explain quantum computing? I’m glad you asked. After bashing a bad analogy, I’m going to use another, better analogy. I like analogies—they are my favorite method of learning. Teaching by analogy is kind of like being in two places at the same time.

Alright, I’m going to tell you the correct analogy between quantum physics and magic. Let’s think about what a magic trick looks like abstractly. The magician, who is highly trained, spends a huge amount of time choreographing a mechanism which is then hidden from the audience. The show begins, the “magic” happens, and we are returned to reality with bafflement. If you are under 20, then you also take a selfie for the Insta #fuckyeahmagic.

Now here is what happens in a quantum computation. A quantum engineer, who is highly trained, spends a huge amount of time choreographing a mechanism which is then hidden from the audience. The show begins, quantum computation happens, and we are returned the answer to our problem. Tada! Quantum computation is magic. Selfie, Insta, #fuckyeahquantum.

Let’s dig into this a bit deeper, though. Why not uncover the quantum computer—open the box—to reveal the mechanism? Well, we can’t. If we “watch” the computation happen, we expose the quantum computer to an environment and this will break the computation. The kind of things a quantum computer needs to do requires complete isolation from the environment. Just like a magician’s trick, if we reveal the mechanism, the magic doesn’t happen.

OK, fine. The “magic” will be lost, but at least I could understand the mechanism, right? Sure, that’s right. But here’s the catch: a magician spends countless hours training and preparing for the trick. Knowing the mechanism doesn’t help you understand how to actually perform the trick. Nor does seeing that the mechanism of quantum computing is some complicated math actually help you understand how it works. And don’t over simplify it—we already know that doesn’t work.

Let’s look at the example of a sword swallowing illusionist. If you don’t know what I’m talking about, it’s exactly how it sounds—a person puts a sword the length of their torso in their mouth down to the handle. How one figures out they have a proclivity for this talent, I don’t want to know. But what’s the explanation? Don’t worry, I already googled it for you, and it’s simple: “the illusionist positions their head up so that his throat and stomach make a straight line.” Oh, is that it? I’m suddenly unimpressed. So now that you too know how to swallow a sword are you going to go and do it? I fucking doubt it. That would be stupid—about as stupid as reading a few sentence description of some “explanation” of quantum computing and then declaring you understand it.

Lesson 3: don’t place your analogy at the level of explanation—place it at the level of the phenomenon. Let your analogy do the work of explanation for you.

If you like figures, I have prepared a lovely summary for you.

Well there you go. Quantum computing isn’t magic, but it can put on a good show. You can learn about how to do the tricks yourself and even perform a few with a little more effort. I suggest starting with the IBM Quantum Experience. Or, start where the real magicians do with Quantum Computing for Babies 😂