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Comment by sundarurfriend | Hacker News Reader

> Note that I’ve not said — as it often is said — that a quantum computer has an advantage because the availability of superpositions hugely increases the number of states it can encode, relative to classical bits. Nor have I said that entanglement permits many calculations to be carried out in parallel.

I'm really glad to find they did not go for these usual pieces of pop-explanations, and even called them out as not really capturing the essence of quantum computing. It takes courage to not hide behind these inaccurate descriptions, and instead just give the unsatisfying truth that there's no simple layman level description beyond "quantum mechanics somehow creates a “resource” for computation that is unavailable to classical devices"; at least not with today's easily available mental models[1] we have from classical computing.

[1] http://lesswrong.com/lw/kg/expecting_short_inferential_dista...

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I'm also glad they didn't fallback on the "easy" explanation. Funnily enough, an SMBC comic [0] is what made quantum computing "click" for me, and how it's not really easily mappable back to classical ideas. The comic was a joint effort with Scott Aaronson and SMBC creator Zach Weinersmith, and as far as I can tell is pretty close to a faithful explanation of the basics without diving into the technical details.

[0] https://www.smbc-comics.com/comic/the-talk-3

If this comic isn't diving into technical details then I must be an idiot

I meant more that it's not trying to act like a whitepaper and "prove" the concepts it's explaining, which I think a lot of explanations of really complex topics end up spending way too much time on and lose the reader.

There are parts of the comic that I don't even begin to understand (I honestly couldn't tell you if hilbert space is even a real thing, and I don't even want to being to think about how or why amplitudes can be complex, and what you would possibly do with a complex probability...), but the whole idea of amplitudes being loosely analogous to probabilities,and how interference is the real "secret sauce" that allows quantum computers to gain a big advantage in some problems is what it finally made click for me. And it kind of got across the point that the real difficulty with quantum computing is going to be in isolating these "qbits" from the world (which made the originally linked article much easier to read).

I've also attempted to read a bunch about it from many different sources before this, so I had some primer on some of the ideas behind it, but couldn't understand how the parts fit together, or why they were such a big deal (often flip-flopping between the 2 feelings depending on the metaphors the last thing I read used).

> I don't even want to being to think about how or why amplitudes can be complex, and what you would possibly do with a complex probability

I don't pretend to completely understand it either, but one crucial point here is that these complex amplitudes and their interactions are the "real" reality - they are "more real" than the normal probabilities we are used to, in that they are the things that actually exist in the universe and give rise to the 'normal' probabilities that we see.

A lot of mental resistance to quantum mechanical ideas comes from thinking quantum mechanics is weird, instead of thinking quantum mechanics as normal and us being weird[1], because of the scale we live in and normally observe things in.

[1] paraphrased from Eliezer Yudkowsky, Harry Potter and the Methods of Rationality, Ch 36

To chime in on the probabilities issue: they aren’t really probabilities.

Probability amplitudes are something we don’t really understand, but it is true that once we have set a basis for measurement, we can treat the amplitudes with respect to that basis as probabilities (written in complex form).

However, this should not be interpreted as “quantum states are probabilistic mixtures of base states” - changing the measurement basis screws up that intuition. For example, if our new basis has one base vector that is equal to the quantum state we are measuring, suddenly our outcome is certain.

Oh I wish I could explain this better.

I feel like that explanation almost gets me there, but understanding it is just out of reach.

Out of my many questions (for which I should probably seek books and videos), I'll ask this one:

> changing the measurement basis screws up that intuition. For example, if our new basis has one base vector that is equal to the quantum state we are measuring, suddenly our outcome is certain.

How does one change the measurement basis - is it a mathematical exercise done on paper, or is it an actual physical act?

Is this what is meant by collapse of a wave function?

Is this what we are doing when we 'measure' Schrodinger's cat at the end of the experiment and suddenly have a certain outcome?

Different measurement instruments have different bases. But we can also model our quantum system and imagine what would happen if we created a new instrument with any basis we choose.

Measurement is indeed wave function collapse - when a quantum state in a superposition (with regards to our measurement basis) is measured and produces a definite answer.

Wow, I don't think I've ever seen someone cite fanfiction on HN before! But considering HPMOR introduced me to Worm [0] and Unsong [1], I really can't argue with doing so. Also a relatively good way to introduce college friends to rationalism, if for some reason recommending fanfiction to friends is easier than recommending lesswrong.

[0] https://parahumans.wordpress.com/ [1] http://unsongbook.com/

Well, if you want a non-fanfiction citation, there's this from Eliezer's regular non-fiction blogging:

Reality has been around since long before you showed up. Don't go calling it nasty names like "bizarre" or "incredible". The universe was propagating complex amplitudes through configuration space for ten billion years before life ever emerged on Earth. Quantum physics is not "weird". You are weird. You have the absolutely bizarre idea that reality ought to consist of little billiard balls bopping around, when in fact reality is a perfectly normal cloud of complex amplitude in configuration space. This is your problem, not reality's, and you are the one who needs to change.

http://lesswrong.com/lw/hs/think_like_reality/

(Really, a lot of insights in HPMOR are just reiterations of points Eliezer made in his Sequences.)

Let's just say it's important to distinguish between what Yudkowsky considers to be quantum mechanical ideas, and what the equations and experiments actually say.

Yeah, the comic is probably quite a bit more accessible. Although I'm still left wondering if there is a proper ELI5 for quantum computing somewhere out there

I really don't think there is, and I don't think there will be one ever.

If quantum computers ever become "mainstream", I think we will need to start teaching the basics behind them in schools, because there isn't an easy way to map the ideas there to classical ones.

I think of it like imagine trying to explain how a CPU works to a person that doesn't know basic math. There is no way to even "ELI5" that, there just isn't enough of a foundation to work from, and any attempt to do so would take so many liberties that it wouldn't even really be useful. Right now quantum computing seems extremely difficult to even understand the basics because 99.99%+ of us are missing those foundations.

But if you're explaining it to a 5 year old, they've hardly been poisoned by the classical ideology taught in schools. You couldn't analogize it to a classical concept anyway because they wouldn't have the background. So maybe there would be an ELI5 that would be effective, if it was intended to be understood by someone without the baggage of traditionalism. :)

Sure, but the true ELI5 would take years to get through the necessary fundamentals.

And end with a diploma for the now young adult.

The sentence in the last box made me chuckle. It reads like a parody of something Deepak Chopra would write.

With SMBC there is always a "hidden" last panel too, you can see it by clicking the big red button at the bottom right of the comic.

Although this one won't make any sense unless you read a lot of these kinds of webcomics!

The general idea I've had is that a quantum computer will be much more similar to a classical analog computer than digital. It requires setting up a quantum system which models the problem physically, and relies on quantum properties (interference i suppose) to descend on the correct solution without doing any sort of brute-forcing. Thanks for the comic link, really helpful.

Thanks a lot for that link. That comic was a ton of fun (as one would expect from SMBC).

LOL. From your link :

The Kid : Wait, you guys put complex numbers in your ontologies?

The Mom: We do and we enjoy it.

The Kid : Ewww

I don't know much about quantum computation, but David Deutsch does say something along those lines. In particular, that the superiority of quantum computation for certain problems is explained by computation taking place in multiple universes.

He also rejects the idea that there's something ineffable about quantum mechanics, or that the reason quantum computation works can't be explained using ordinary logic and reasoning that is accessible in principle to everybody (I don't know for sure that's what is meant here by "somehow creates a “resource”": but I'm triggered a bit by those words because people often do essentially mean that).

Of course he would (correctly) say that one shouldn't treat him as being an infallible source of truth on the matter. But he does believe that the purpose of science is to explain the world, which puts him in a different class from apologists for quantum mysticism, strict empiricism and other believers in explanation-free knowledge.

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