Is Quantum Computing Real or Not?

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Is Quantum Computing Real or Not?

#1  Postby kennyc » Jun 20, 2014 1:10 pm

Recent tests on the D-wave computer. I've been and continue to be very skeptical about quantum computing.


Is D-Wave's quantum computer actually a quantum computer?
Jun 20, 2014 1 comment

A team of quantum-computing experts in the US and Switzerland has published a paper in Science that casts doubt over the ability of the D-Wave Two quantum processor to perform certain computational tasks. The paper, which first appeared as a preprint earlier this year, concludes that the processor – built by the controversial Canadian firm D-Wave Systems – offers no advantage over a conventional computer when it is used to solve a benchmark computing problem.
While the researchers say that their results do not rule out the possibility that the processor can outperform conventional computers when solving other classes of problems, their work does suggest that evaluating the performance of a quantum computer could be a much trickier task than previously thought. D-Wave has responded by saying that the wrong benchmark problem was used to evaluate its processor, while the US–Swiss team now intends to do more experiments using different benchmarks.
Quantum insights
D-Wave Two is the second generation of quantum processors sold by D-Wave Systems and one of the devices is owned by NASA, Google and the Universities Space Research Association. The company has also sold a system – claimed to be the world's first commercially available quantum computer – to Lockheed Martin. The tests on D-Wave Two were carried out by Matthias Troyer and colleagues at ETH Zurich, the University of Southern California (USC), the University of California Santa Barbara, Google and Microsoft.
Containing 512 quantum bits (qubits), the D-Wave Two processor was designed specifically to perform a process called "quantum annealing", which is a technique for finding the global minimum of a complicated mathematical function. Unlike "conventional" quantum computers – which are kept in a fragile quantum state throughout the calculation – quantum annealing involves making a transition from a quantum to classical system. As a result, D-Wave's approach might be more immune to noise, which can destroy conventional quantum calculations. However, a quantum annealing processor is not a universal computer like a PC and cannot be programmed to perform a range of tasks.
Fiendishly difficult calculation
Troyer's team tested the processor by using it to solve a particularly difficult task from condensed-matter physics involving "Ising spin glasses". A spin glass is a magnetic material in which the individual magnetic moments – or spins – interact with each other and are also located randomly throughout the material. This is unlike conventional models of magnetic materials, in which the spins are arranged on a regular lattice and tend to all point in specific directions. Instead, the spin glass has an extremely complicated spin configuration that is fiendishly difficult to calculate for large numbers of spins. "Ising spin-glass problems are the 'native' problem that the [D-Wave Two] is designed for," Troyer explained to physicsworld.com.
To evaluate the performance of D-Wave Two, the team measured how long it took the processor to solve an Ising spin-glass problem and compared this with the time it takes with a conventional, classical computer. This ratio, known as the "quantum speed-up", is expected to be around one for small problems – meaning classical devices can do the job just as well – but it should grow in size as the problem becomes larger. In its test, the team carried out lots of quantum and classical simulations on different spin glasses, in which the number of spins and the interaction strengths were varied systematically.
No speed-up found
The results, however, reveal no clear evidence of speed-up. While the D-Wave Two processor was sometimes 10 times faster than the classical computer, it was also sometimes more than 100 times slower.
.....


http://physicsworld.com/cws/article/new ... m-computer
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Re: Is Quantum Computing Real or Not?

#2  Postby DavidMcC » Jun 20, 2014 6:49 pm

kennyc wrote:Recent tests on the D-wave computer. I've been and continue to be very skeptical about quantum computing.


Is D-Wave's quantum computer actually a quantum computer?
...

My skepticism is more along the lines of doubting that the D-wave machine is a genuine quantum computer than that there is such a thing as quantum computing. Big difference. Maybe D-wave sacrifices too much to make there machine's computing robust against noise.
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Re: Is Quantum Computing Real or Not?

#3  Postby newolder » Jun 24, 2018 6:54 pm

Image
LUCY READING-IKKANDA/QUANTA MAGAZINE

Finally, a Problem Only Quantum Computers Will Ever Be Able to Solve

EARLY ON IN the study of quantum computers, computer scientists posed a question whose answer, they knew, would reveal something deep about the power of these futuristic machines. Twenty-five years later, it’s been all but solved. In a paper posted online at the end of May, computer scientists Ran Raz and Avishay Tal provide strong evidence that quantum computers possess a computing capacity beyond anything classical computers could ever achieve.
...
They prove, with a certain caveat, that quantum computers could handle the problem efficiently while traditional computers would bog down forever trying to solve it. Computer scientists have been looking for such a problem since 1993, when they first defined a class of problems known as “BQP,” which encompasses all problems that quantum computers can solve.

Since then, computer scientists have hoped to contrast BQP with a class of problems known as “PH,” which encompasses all the problems workable by any possible classical computer...
...
Here’s the problem. Imagine you have two random number generators, each producing a sequence of digits. The question for your computer is this: Are the two sequences completely independent from each other, or are they related in a hidden way (where one sequence is the “Fourier transform” of the other)? Aaronson introduced this “forrelation” problem in 2009 and proved that it belongs to BQP. That left the harder, second step—to prove that forrelation is not in PH.
...
The new paper by Raz and Tal proves that a quantum computer needs far fewer hints than a classical computer to solve the forrelation problem. In fact, a quantum computer needs just one hint, while even with unlimited hints, there’s no algorithm in PH that can solve the problem. “This means there is a very efficient quantum algorithm that solves that problem,” said Raz. “But if you only consider classical algorithms, even if you go to very high classes of classical algorithms, they cannot.” This establishes that with an oracle, forrelation is a problem that is in BQP but not in PH.
...
The work provides an ironclad assurance that quantum computers exist in a different computational realm than classical computers (at least relative to an oracle). Even in a world where P equals NP—one where the traveling salesman problem is as simple as finding a best-fit line on a spreadsheet—Raz and Tal’s proof demonstrates that there would still be problems only quantum computers could solve.

“Even if P were equal to NP, even making that strong assumption,” said Fortnow, “that’s not going to be enough to capture quantum computing.”

{Phew!}
...(more at link)...

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Re: Is Quantum Computing Real or Not?

#4  Postby newolder » Nov 06, 2018 4:22 pm

This is what we want to see...
Google has enlisted NASA to help it prove quantum supremacy within months

The firm will pit its Bristlecone quantum processor against a classical supercomputer early next year and see which comes out on top.

by Mark Harris November 5, 2018

Quantum supremacy is the idea, so far undemonstrated, that a sufficiently powerful quantum computer will be able to complete certain mathematical calculations that classical supercomputers cannot. Proving it would be a big deal because it could kick-start a market for devices that might one day crack previously unbreakable codes, boost AI, improve weather forecasts, or model molecular interactions and financial systems in exquisite detail.

The agreement, signed in July, calls on NASA to “analyze results from quantum circuits run on Google quantum processors, and ... provide comparisons with classical simulation to both support Google in validating its hardware and establish a baseline for quantum supremacy.”

Google confirmed to MIT Technology Review that the agreement covered its latest 72-qubit quantum chip, called Bristlecone. Where classical computers store information in binary bits that definitely represent either 1 or 0, quantum computers use qubits that exist in an undefined state between 1 and 0. For some problems, using qubits should quickly provide solutions that could take classical computers much longer to compute.

...

More @ technonlogyreview link
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Re: Is Quantum Computing Real or Not?

#5  Postby Zwaarddijk » Nov 06, 2018 10:36 pm

newolder wrote:This is what we want to see...
Google has enlisted NASA to help it prove quantum supremacy within months

The firm will pit its Bristlecone quantum processor against a classical supercomputer early next year and see which comes out on top.

by Mark Harris November 5, 2018

Quantum supremacy is the idea, so far undemonstrated, that a sufficiently powerful quantum computer will be able to complete certain mathematical calculations that classical supercomputers cannot. Proving it would be a big deal because it could kick-start a market for devices that might one day crack previously unbreakable codes, boost AI, improve weather forecasts, or model molecular interactions and financial systems in exquisite detail.

The agreement, signed in July, calls on NASA to “analyze results from quantum circuits run on Google quantum processors, and ... provide comparisons with classical simulation to both support Google in validating its hardware and establish a baseline for quantum supremacy.”

Google confirmed to MIT Technology Review that the agreement covered its latest 72-qubit quantum chip, called Bristlecone. Where classical computers store information in binary bits that definitely represent either 1 or 0, quantum computers use qubits that exist in an undefined state between 1 and 0. For some problems, using qubits should quickly provide solutions that could take classical computers much longer to compute.

...

More @ technonlogyreview link

I don't see that that makes any sense. We will only know that quantum machines are faster than classical machines for some classes of problems once the complexity theorists have proven that BQP > P, where > means 'strictly includes'.

Doing it by just running 'the fastest known' algorithms proves nothing except predictable things about the fastest known algorithms.
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Re: Is Quantum Computing Real or Not?

#6  Postby newolder » Nov 06, 2018 11:10 pm

...
Together, the two organizations will work out how to map “a diverse array of optimization and sampling problems” to Bristlecone’s gate-model quantum computing system. Early next year, when they have agreed on the problems and initial targets for simulation, NASA will code the software necessary to run those simulations on its petaflop-scale Pleiades supercomputer, also located at Ames. Pleiades is NASA’s most powerful supercomputer, currently ranked in the top 25 worldwide.
...

No mention of "fastest known algorithms". :scratch:

Google are trying to make a buck out of their new toys. As yet unknown tests, agreed as "fair" by computer bods, can't harm the marketing and development cycles.

Still, if it doesn't make any sense, this too will become apparent soon enough.
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Re: Is Quantum Computing Real or Not?

#7  Postby Zwaarddijk » Nov 07, 2018 3:23 am

newolder wrote:
...
Together, the two organizations will work out how to map “a diverse array of optimization and sampling problems” to Bristlecone’s gate-model quantum computing system. Early next year, when they have agreed on the problems and initial targets for simulation, NASA will code the software necessary to run those simulations on its petaflop-scale Pleiades supercomputer, also located at Ames. Pleiades is NASA’s most powerful supercomputer, currently ranked in the top 25 worldwide.
...

No mention of "fastest known algorithms". :scratch:

Google are trying to make a buck out of their new toys. As yet unknown tests, agreed as "fair" by computer bods, can't harm the marketing and development cycles.

Still, if it doesn't make any sense, this too will become apparent soon enough.

Well, why would they use slower algorithms than the fastest known ones? (Ok, I can see some circumstances where that might make sense, i.e. if the hardware used has certain restrictions on it with regards to memory or something, that a slower algorithm will not run into).

Thing is, it's notoriously hard to prove that there's no faster algorithm for a class of problems. We don't necessarily need the algorithm itself though, sometimes one can come up with indirect proofs regarding the time needed - there's some very clever proofs involved in showing that L⊆NL⊆P⊆ NP⊆PSPACE∧L⊂PSPACE, so we know that somewhere along that chain of ⊆, at least one has to be a ⊂, but we don't know which one and people aren't even sure there's just one ⊂ in there. One could imagine a similar piece of evidence for P and BQP, in which case we'd know that the fastest algorithm for BQP indeed were not polynomial in time on a classical machine.
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