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Quantum computers theoretically can prove more powerful than any supercomputer, and now scientists calculate just what quantum computers need to attain such "quantum supremacy," and whether or not Google achieved it with their claims last year.
Superposition lets one qubit perform two calculations at once, and if two qubits are linked through a quantum effect known as entanglement, they can help perform 2^2 or four calculations simultaneously; three qubits, 2^3 or eight calculations; and so on. In principle, a quantum computer with 300 qubits could perform more calculations in an instant than there are atoms in the visible universe.
It remains controversial how many qubits are needed to achieve quantum supremacy over standard computers. Last year, Google claimed to achieve quantum supremacy with just 53 qubits, performing a calculation in 200 seconds that the company estimated would take the world's most powerful supercomputer 10,000 years, but IBM researchers argued in a blog post "that an ideal simulation of the same task can be performed on a classical system in 2.5 days and with far greater fidelity."
To see what quantum supremacy might actually demand, researchers analyzed three different ways quantum circuits that might solve problems conventional computers theoretically find intractable. Instantaneous Quantum Polynomial-Time (IQP) circuits are an especially simple way to connect qubits into quantum circuits. Quantum Approximate Optimization Algorithm (QAOA) circuits are more advanced, using qubits to find good solutions to optimization problems. Finally, boson sampling circuits use photons instead of qubits, analyzing the paths such photons take after interacting with one another.
Assuming these quantum circuits were competing against supercomputers capable of up to a quintillion (1018) floating-point operations per second (FLOPS), the researchers calculated that quantum supremacy could be reached with 208 qubits with IQP circuits, 420 qubits with QAOA circuits and 98 photons with boson sampling circuits.
How Many Qubits Are Needed For Quantum Supremacy?
[Journal Reference]: Quantum Journal
(Score: 4, Interesting) by stormwyrm on Friday May 22 2020, @02:45AM (4 children)
Numquam ponenda est pluralitas sine necessitate.
(Score: 4, Interesting) by Anonymous Coward on Friday May 22 2020, @06:50AM (2 children)
This is not quantum supremacy. Quantum supremacy is well defined. It is when a quantum computer is capable of solving any well defined problem that a classical machine can not [practically] solve. This is an important distinction because you tackle problems differently with quantum vs classical machines. Classical machines obviously don't factor numbers using a simulated Shor's Algorithm [wikipedia.org]. They use e.g. a quadratic sieve. So you need to actually be able to put those qubits to use in a productive way, and that's a lot harder!
Which hits on the second point, and one you hit on though indirectly. It's only for certain esoteric tasks quantum computers can even *possibly* end up being more powerful than any normal computer. The domain of these problem sets is highly restricted into problems that can be translated into a 'quantum domain'. And that is a *very* tiny set of all problems. For let's say 99.9999999999% of purposes, classical computers will reign supreme. So why are governments and companies pouring billions of dollars into quantum machines? It's just the Enigma Project 2.0. The aforementioned Shor's Algorithm could potentially render all contemporary (and past) encryption completely moot. And the NSA has exabytes (if not zettabytes by now) of encrypted data just sitting around gathering dust.
Whoever cracks encryption first stands to gain a major tactical advantage against the rest of the world, especially as we, for whatever reason, do not seem to be in any rush to move to algorithms more resistant to quantum attacks. You will never have something like personal quantum computers because they'd be useless for nearly all practical purposes.
(Score: 4, Insightful) by DrkShadow on Friday May 22 2020, @10:02AM
Anonymous Coward, of Soylentnews, circa 2020.
Thomas J. of I.B.M., circa 1943, The Yale Book of Quotations quotes.
(Score: 3, Insightful) by sjames on Friday May 22 2020, @11:57PM
That's what I found so annoying about the flurry of press releases this year. The quantum computers didn't do any meaningful computation (open question, CAN they?) and the comparison was assuming that the conventional supercomputer would be emulating the quantum computer.
Of course, they also conveniently ignored the considerable setup time for the quantum run.
(Score: 2) by PiMuNu on Friday May 22 2020, @12:55PM
Sounds like there are two problems:
1. When can a "quantum computer" become impossibly complex to model using a "classical computer"
2. When can a "quantum computer" reproduce all the calculations that can be done with a "classical computer".
Sounds like you are answering the first problem, but this is a very restricted problem. TFS (and quantum computer in popular press) implies the second problem.
E.g. From TFS "Superposition lets one qubit perform two calculations at once" makes me think a "calculation" is a floating point operation (FLOP), as in a classical computer. Comments suggest this is an incorrect interpretation?