61–70 of 89 posts
If nothing else, that seems to me to be the clearest description of quantum computing I've seen.
" While a conventional computer with N bits at any given moment must be in one of its 2N possible states, the state of a quantum computer with N qubits is described by the values of the 2N quantum amplitudes, which are continuous parameters (ones that can take on any value, not just a 0 or a 1). " If nothing else, that seems to me to be the clearest description of quantum computing I've seen.
A clear description but potentially very misleading and one that will certainly screw up your intuition about what to expect from quantum computers.
As pointed out below, a classical probability distribution of N bits is a 2^N dimensional vector of real numbers. There are many very good reasons to think of the quantum state as "more like" this classical vector than the physical state of the "N classical bits" themselves.
Here is one:
- If I send you the physical systems which encode N classical bits then sure enough, I can communicate to you only N classical bits of information despite it taking exponentially many real parameters to specify the distribution/state I prepared them in.
- If I send you the physical systems which encode N quantum bits then sure enough, I can communicate to you only N classical bits of information despite it taking exponentially many real parameters to specify the distribution/state I prepared them in.
There are many other reasons to think of quantum states as not "inherently real" and more like the classical probability distribution. A key one is that both "instantaneously collapse" when you get information about the outcome of an observation.
The issue of course is that while we know the "real states of the world are" of a conventional computer, nobody agrees on what (if any) they are for quantum systems (though many constraints on such purported real states are known, I write about some of them in _Q is for Quantum_)
Earlier quoted context omitted.
The argument here is nonsensical. For example, "With what exactitude must, say, the square root of 2 (an irrational number that enters into many of the relevant quantum operations) be experimentally realized? Should it be approximated as 1.41 or as 1.41421356237? Or is even more precision needed? Amazingly, not only are there no clear answers to these crucial questions, but they were never even discussed!" This is co…
Doable in principle? My impression was that the author doesn't argue against this. He claims it's unlikely to be doable in practice. That's the whole point of the article.
Earlier quoted context omitted.
Well I don't want to get into a full-on argument in the comments here, but I 100% disagree with your definition of a quantum computer. If a computer uses quantum logic gates then it's a quantum computer.
It’s not particularly important how you define the term “quantum computer”, nor have I presented any definition of a "quantum computer" that you can disagree with. The point is that, as I wrote before, regardless of your definition - currently there is no discernible advantage of quantum computers over classical computers in the only metric that matters: computational power. This is what people refer to when they tal…
Here is a way to see the fallacy of the OMG, its 10^300 variables, thats crazy style of “argument”. Consider a probabilistic classical algorithm on 500 bits. Perhaps a Monte Carlo simulation of an Ising model for example. Note first that the most general probability distribution over the 500 classical bits takes 2^500 real numbers to specify. (You have to specify P(000…0) and P(000…1) and… P(111…1)). [You should comp…
Let me present it a different way. Someone comes to you with two formal models of computing. Both models involve representing the state of the computer as a vector of real numbers, they both involve finite dimensional subsystems combined with a tensor product, both involve gates defined over the reals also combined via the tensor product and so on. That is, both models are just about evolution of a vector in some (ve…
Earlier quoted context omitted.
> the gist of it being that quantum computing is some form of analog computing It absolutely is - at least, in the only practical, real-today, working instantiation of it which is in the form of quantum annealing.
This is an extremely misleading statement. Quantum annealing is most certainly not what quantum computing is about (it is not particularly "quantum" either). At most, you can argue that it is an important first step (which is also doubtful). Most of the interesting hardware currently being developed has nothing to do with quantum annealing and most researchers are fairly annoyed at D-wave for originally pushing this…
Near as I can tell, it's the only form of practical quantum computation that's available to end-users in any meaningful way. Everything else so far is vaporware with ridiculously high error rates.
> (it is not particularly "quantum" either)
The qubits - josephson junctions and superconducting loops - are indeed in quantum superposition with each other if their claims are true. This is a macroscopic quantum-mechanical effect; to call it "not particularly quantum" belies a lack of understanding as to what they're doing.
> most researchers are fairly annoyed at D-wave for originally pushing this nonsense
D-Wave is making their system publicly available for free. Have you signed up and taken a look at their demos? Admittedly it's a bit beyond my mathematical ability but it's clear the basic seed is there for something that works; they just need a bigger graph and more connections between qubits. There's no reason to assume they won't have their own equivalent of Moore's Law at some point where these things grow year over year.
> transmon qubits at Google/IBM/Yale for some well known prototypes of quantum computing hardware (which are admittedly quite far from being practical or usable).
Do they have _any_ results worth caring about yet? I don't think they're anywhere near something working on a practical scale. You can buy a D-Wave machine, or time on it - it's not vaporware, even if most people don't find it useful. For the first few years of gate model machines people aren't really going to find them all that useful for day to day things either - just like computers in the 1940s and 50s, it was very specific, high-dollar applications first, and things for the common man decades later.
Earlier quoted context omitted.
This is an extremely misleading statement. Quantum annealing is most certainly not what quantum computing is about (it is not particularly "quantum" either). At most, you can argue that it is an important first step (which is also doubtful). Most of the interesting hardware currently being developed has nothing to do with quantum annealing and most researchers are fairly annoyed at D-wave for originally pushing this…
> Quantum annealing is most certainly not what quantum computing is about Near as I can tell, it's the only form of practical quantum computation that's available to end-users in any meaningful way. Everything else so far is vaporware with ridiculously high error rates. > (it is not particularly "quantum" either) The qubits - josephson junctions and superconducting loops - are indeed in quantum superposition with eac…
You can't just call all research "vaporware" because it happens to not exist today at this moment...
> ridiculously high error rates. You know that D-Wave's qubits have very high error rates, right? They have always espoused the "more qubits, more error" strategy.
> but it's clear the basic seed is there for something that works; they just need a bigger graph and more connections between qubits.
D-Wave has been trying for quite some time to increase the connectivity of the Chimera graphs with "pegasus" and so forth, it's a Hard Problem.
> There's no reason to assume they won't have their own equivalent of Moore's Law at some point where these things grow year over year.
Yes there is, adding more qubits is not a simple task like adding more transistors. Plus, more qubits doesn't necessarily help you unless you have the necessary connectivity as well, which is even superficially speaking at least quadratically difficult.
> Do they have _any_ results worth caring about yet? You... you do realize that nothing D-Wave has shown so far is even faster than a laptop, right?
And btw, I don't mean to be hating on D-Wave either, I think they get a lot of unnecessary hate.
Earlier quoted context omitted.
This is an extremely misleading statement. Quantum annealing is most certainly not what quantum computing is about (it is not particularly "quantum" either). At most, you can argue that it is an important first step (which is also doubtful). Most of the interesting hardware currently being developed has nothing to do with quantum annealing and most researchers are fairly annoyed at D-wave for originally pushing this…
> Quantum annealing is most certainly not what quantum computing is about Near as I can tell, it's the only form of practical quantum computation that's available to end-users in any meaningful way. Everything else so far is vaporware with ridiculously high error rates. > (it is not particularly "quantum" either) The qubits - josephson junctions and superconducting loops - are indeed in quantum superposition with eac…
The "qubits" Dwave has most certainly are not entangled and even they have never claimed it. Dwave themselves have stopped claiming they have a quantum computer, and have introduced this term "quantum annealing" which is something that you can more efficiently do on a classical computer.
Yes, you can buy something from Dwave, so it is not vaporware, it is just snake oil. This is why it is frustrating for me to respond to your comments: you are dismissing the very measurable progress my colleagues have done over the last two decades (just look at qubit lifetimes, "break even" error correction procedures, generation of entangled pairs, etc) while claiming that a device that is incapable of sustaining any particularly interesting quantum state is to be held in high regard.
And while Dwave's engineering efforts should be praised, their misleading PR has led to false misleading claims like yours and for that at least their PR department deserves to be shunned.