Friday, April 13, 2018

Quantum random number generator is bogus

A new Nature article (paywalled) claims to have invented a random number generator:
Researchers have come up with a way to generate truly random numbers using quantum mechanics. The method uses photons to generate a string of random ones and zeros, and leans on the laws of physics to prove that these strings are truly random, rather than merely posing as random. The researchers say their work could improve digital security and cryptography.

The challenge for existing random number generators is not only creating truly random numbers, but proving that those numbers are random. "It's hard to guarantee that a given classical source is really unpredictable," says Peter Bierhorst, a mathematician at the National Institute of Standards and Technology (NIST), where this research took place. "Our quantum source and protocol is like a fail-safe. We're sure that no one can predict our numbers."
He is "sure", because it is "like a fail-safe"?!

No, they are making a fundamental conceptual error here is talking about "truly random numbers", and thinking that quantum mechanics is some sort of magic ingredient.

There are lots of commercially available random number generators, whose output no one can predict. There is probably one on whatever computer you are using to read this message. See RdRand for details.
"Something like a coin flip may seem random, but its outcome could be predicted if one could see the exact path of the coin as it tumbles," adds Bierhorst, "Quantum randomness, on the other hand, is real randomness. We're very sure we're seeing quantum randomness because only a quantum system could produce these statistical correlations between our measurement choices and outcomes."

When it comes to the source of the randomness, we're back to good old quantum superpositions, where a quantum particle can be a one, zero, or both at once. The measurement of these superpositions has fundamentally unpredictable results. The method doesn't so much use the superpositions themselves to generate the data, but the correlations between these superpositions when photons are looked at in pairs.
No, this is nonsense. Quantum randomness, if there is such a thing, probably affects coin tosses also, and they are not predictable.

Quantum randomness is often emphasized in the Copenhagen interpretation, but those who follow other interpretations, such as pilot wave, many worlds, and super-determinism, deny that there is any such thing. If we could prove quantum randomness, then we could eradicate these other silly interpretations. Unfortunately, we cannot.
However, the team goes a step further to improve the quality of its data. By analyzing the data produced, the researchers can home in on shorter strings where the occurrence of ones and zeros is nearer to fifty-fifty. The team has written a computer program to select the strings, which ironically uses a conventional random number generator to provide seed data, effectively telling the program what to look for.

The researchers call this proximity to fifty-fifty perfection "uniformity." From the more than 100 million bits generated, the researchers found 1,024 certified to be uniform to a trillionth of a percent. "A perfect coin toss would be uniform, and we made 1,024 bits almost perfectly uniform, each extremely close to equally likely to be 0 or 1," Bierhorst explains.
Okay, this is so stupid that it appears that we are being trolled.

They make this perfect quantum truly random number generator, but then they scanned the output to pick out the more random-looking numbers?!!

It is well-known that people are lousy at detecting random sequences, because they expect too much uniformity. See How to tell whether a sequence of heads and tails is random for a simple explanation. But these authors have rigged their system to get greater uniformity.

A advantage of the mathematical pseudo-random number generators is that they automatically get the uniformity correct. It is inherent in the generating formulas. A particular output may look non-uniform, but that is supposed to possible in a random number generator.

This paper was published in one of the world's top two scientific journals, as a great advance in quantum randomness or whatever the subject is. It is not. As a random number generator, it is not as good as what is currently in use in billions of computers. As a paper in quantum mechanics, it is hopelessly confused.

1 comment:

  1. There are fairly simple quantum sources of randomness but there are many classical sources that are much easier to implement, such as with clock drift. However, all of these sources present bias. You can take a fairly simple approach as refresh a PRNG with a hardware source of randomness.

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