What You Shouldn't Know About Quantum Computers • Hacker News Board

Re: What You Shouldn't Know About Quantum Computers

#101

Post by cwillu » Thu, May 30, 2024, 2:15 PM UTC

NohatCoder wrote: post #100

jessriedel wrote: post #88

Earlier quoted context omitted.

Not really accurate. There are tons of tenured profs who are well-positioned to reveal reasons why QCs are fundamentally infeasible (and those kind of stories play well in the media). You can read about Gil Kalai's arguments here: https://www.quantamagazine.org/the-argument-against-quantum-... In any case, I'm happy to bet on this.

Yes, there are people who don't agree with your consensus, that is part of my point, the consensus is not all that universal. And the other part of my point is that if you only count people who have a full time job developing quantum computers as experts then you get a massive bias in opinion, as the sceptics are more likely to pick different jobs. As for my own belief, I don't know how it will pan out, but I'm inher…

Boson sampling _works_. It's now on the error-correction skeptics to provide a framework where that remains true, but stops being true when a particular combination of gates is applied, because the “naive” math says it should work fine. It would require a new fundamental physical principle to be true, for a quantum system to be sensitive to the nature of the computation involved in generating the distribution being sampled from.

Re: What You Shouldn't Know About Quantum Computers

#102

Post by NohatCoder » Thu, May 30, 2024, 5:50 PM UTC

cwillu wrote: post #101

NohatCoder wrote: post #100

Earlier quoted context omitted.

Yes, there are people who don't agree with your consensus, that is part of my point, the consensus is not all that universal. And the other part of my point is that if you only count people who have a full time job developing quantum computers as experts then you get a massive bias in opinion, as the sceptics are more likely to pick different jobs. As for my own belief, I don't know how it will pan out, but I'm inher…

Boson sampling _works_. It's now on the error-correction skeptics to provide a framework where that remains true, but stops being true when a particular combination of gates is applied, because the “naive” math says it should work fine. It would require a new fundamental physical principle to be true, for a quantum system to be sensitive to the nature of the computation involved in generating the distribution being s…

At toy scale, yes, just like quantum computers. And since we haven't seen any news of working boson sampling with thousands of photons I assume that it stops working when you try to scale up, just like quantum computers.

New fundamental physical principles are at this stage required in any case, quantum mechanics and general relativity don't fit together, meaning that at least one of them need to be overhauled into something that explains the same observations but doesn't have quite the same maths. The lacklustre performance of quantum computers could be a hint to what needs overhauling.

Re: What You Shouldn't Know About Quantum Computers

#103

Post by neolefty » Fri, May 31, 2024, 8:35 PM UTC

sebzim4500 wrote: post #45

neolefty wrote: post #31

Earlier quoted context omitted.

Not the size, but the temperature. If you have to cool to a microkelvin for a certain number of qbits to retain coherence, how low do you need to go to add one more qbit, and how much energy will that require? My thermodynamic instinct says that the cooling effort required rises with the resolving power — which is exponential with the number of qbits. But it's just instinct, not grounded very well in science or engin…

It's plausible to me that cooling becomes exponentially harder as you aim for lower temperatures but I don't understand why you think you need lower temperatures for more qbits? The whole point of quantum error correction is that ones you reach a constant threshold you can use more iterations of error correction to decrease you logical error rate without decreasing your physical error rate.

That's also plausible. My guess is the same logic would apply — you would need exponentially more error correction for a linear increase in qbits.

Re: What You Shouldn't Know About Quantum Computers

#104

Post by neolefty » Fri, May 31, 2024, 8:44 PM UTC

BobaFloutist wrote: post #57

sebzim4500 wrote: post #45

Earlier quoted context omitted.

It's plausible to me that cooling becomes exponentially harder as you aim for lower temperatures but I don't understand why you think you need lower temperatures for more qbits? The whole point of quantum error correction is that ones you reach a constant threshold you can use more iterations of error correction to decrease you logical error rate without decreasing your physical error rate.

They're not talking about lower temperatures, but greater volume.

I had assumed both — greater volume and also lower temperatures. The more qbits you are trying to get to cooperate, the less thermal noise you can tolerate, so the cooler you have to make it.

Yes, more qbits also take up more space, but I hadn't thought of that as a major factor — but it certainly could be if they are physically large! Overall I think the bigger issue is cooling.

The article mentions error correction as an alternative to increased coherence among the qbits. Perhaps what that really means is "to increase meaningful interaction among qbits, make it as cold as you can, and then error correct until you get a meaningful answer." My intuition is that they are both a battle against entropy — the need for error correction will also increase exponentially with the number of qbits, simply because the number of possible combinations increases exponentially.

And the even larger outcome of all this is that if this intuition bears out, quantum computing will have no fundamental advantage over conventional computing — which also has an exponential cost for a linear increase in bits, for computations such as factoring large numbers.