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Comment by mountainboy | Hacker News Reader

From the article:

"A quantum computer, however, can sidestep the brute-force calculation by simulating the quantum process directly — allowing bosons to interfere and sampling the resulting distribution."

Uhhmmmm... it sounds to me like they are performing an experiment and measuring the results.

It's not a simulation if you actually perform the physical experiment.

This seems a bit like saying "A hurricane will simulate a hurricane better and much faster than a supercomputer".

Umm, true enough, I guess. But can you make a hurricane do anything useful besides just being a hurricane?

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Read Scott Aaronson's take at https://www.scottaaronson.com/blog/?p=5122

To answer your question:

> Is BosonSampling at least a step toward universal quantum computing? I think so! In 2000, Knill, Laflamme, and Milburn (KLM) famously showed that pure, non-interacting photons, passing through a network of beamsplitters, are capable of universal QC, provided we assume one extra thing: namely, the ability to measure the photons at intermediate times, and change which beamsplitters to apply to the remaining photons depending on the outcome. In other words, “BosonSampling plus adaptive measurements equals universality.” Basically, KLM is the holy grail that experimental optics groups around the world have been working toward for 20 years, with BosonSampling just a more achievable pit stop along the way.

So this is at least possibly on track to universality.

If I understand correctly, this is more like an experimental milestone towards an photonic approach to QC. The system is not programmable and cannot be consider as a computer. The article sounds more like the team has demonstrated construction of a better quantum computer than Google's.

What if you can reprogram the beam splitters to move into different positions?

Say the beam splitters can be actuated to different positions and angles automatically via code. Then you can build a general purpose rig, that can be reprogrammed to solve different quantum functions.

Then you can build clones of this rig. And stack them up in racks. And have thousands of racks. Now you have a super quantum computer system.

Then you can begin to miniaturizing the system.

The Chinese quantum supremacy solution only solved one problem. But I would gather that the beam splitters can be modified to solve other quantum functions.

everything requires a measurement of the physical world.

A hurricane's initial conditions cannot be absolutely specified and controlled.

A system of qubits can.

Edit: The research cannot currently control the circuit that defines the wave function evolution (not programmable). But a computation is still being performed. This is a philosophical rabbit hole, though.

Well, the interferometer is not programmable by typing in commands into a classical computer, but it is programmable by instructing the relevant grad students. In other words, we do have an algorithm to create any interferometer we choose.

I agree, the rabbit hole goes deep.

Haha I forgot about grad student control. Apologies, this is definitely a quantum computer operating on yearly timescale qubit flipping.

It's funny, but I am also serious. Historically speaking, the idea that computational devices could have internal stored programs that could be used to drive any function, rather than humans putting in effort every time to program, had to invented.

Some of this historical progression is discussed here https://www.youtube.com/watch?v=njwQgz63rIs

Oh don't get me wrong, I'm taking you seriously. But it's also hilarious!

Yeah but our standards for computers are higher nowadays.

I covered analog computers in school, which as I recall consisted of integrating blocks made with linear circuits. You combine them up as needed to simulate systems of linear differential equations. There was still a degree of programmability in that sense.

it doesn't strike me as impossible or impractical that programmable interferometers could be constructed in the near future. Is there any limit blocking this today?

> This seems a bit like saying "A hurricane will simulate a hurricane better and much faster than a supercomputer".

The crucial difference here is that we don't have a good evidence that the size of the supercomputer grows exponentially in the size of the hurricane. While, it seems likely [1] that classical computers take exponential time to simulate a BosonSampling computer.

[1] as far as research in the computational complexity in this area has yielded proofs or failed to yield proofs despite trying.

Exactly. This sounds like pure propaganda. They're measuring experimental results and calling it a computer.

By your logic, all classical computers are measuring voltages and calling them bits.

A computer is just an experiment, right?

All of the awesome things we build with computers are just propaganda.

Perhaps its a bad article, but I can't see the value of solving the boson sampling problem by sampling bosons having any application in general computing.

It depends how general you mean, I reckon. I don't know enough about physics to know how important boson sampling is, but simulating other systems in particle physics is already one of the big-ticket uses of supercomputers today.

It probably does not, though there are some proposals (such as generating certified random bits I think). But that is not relevant to the discussion here. A computational algorithm does not have to be useful generally, for it to prove a result about complexity classes.

I think "propaganda" is too harsh, more like "spin". This is apparently a fine achievement of quantum optics that will hopefully be applicable to quantum computing. But it is being directly billed as a quantum computer itself, which is a stretch.

A classical computer is more than an experiment. The "just" part of that statement is what matters. Computer implies some degree of generalizability in computing things. If it can only compute one thing, it's at the trivial extreme and would be more logically described in terms of that single things it does. For example a beamsplitter that divides power in half. We call it a beamsplitter, not a classical computer that calculates 1/2 input power.

I'd tend to agree the latter but not the propaganda since the Westerns indeed admit this achievement. Yeah, what is the advantage if not turing complete anyway?

What's the advantage of the abacus or the Enigma machine? Or any of the many Turing-incomplete machines that were built before fully general computers in the 1940s-50s.

This talk might enlighten you https://www.youtube.com/watch?v=njwQgz63rIs

A computation model doesn't have to be Turing complete to be useful in practice. Consider a language where all programs must halt - all loops must be bounded, and recursion is not allowed - you can still solve a lot of problems with a language like this, but you cannot say, run Brainfuck, as such a language is not Turing complete.

Given a set of inputs, it computed the result.

Also, did you read to the end? They directly address the issue.

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