Physicists go totally random

Calculations suggest way to boost the independence of information flow

Casino bosses, take note: Physicists have described a way to guarantee complete randomness in a flow of information, such as the numbers generated by a roulette wheel.

The work could allow a person to transform a stream of not-quite-random information into a totally random one. “Even if you don’t have perfect randomness, you can make perfect randomness,” says Roger Colbeck, a theoretical physicist at ETH Zurich in Switzerland. “This is sort of a guarantee given to you by the laws of physics.”

Colbeck and ETH Zurich colleague Renato Renner describe the finding online May 6 in Nature Physics.

True randomness — in which a coin flip has a 50 percent chance of coming up heads — is hugely important in fields such as cryptography. Patterns could be detected in a code based on less than random numbers, allowing it to be broken. And if a casino’s random number generators aren’t absolutely random, a gambler might be able to exploit that weakness.  

Colbeck and Renner wanted to better understand the role of randomness in quantum theory, which describes a world operating under apparently different rules than those that govern the everyday world. For instance, in quantum theory two particles can be mysteriously linked, or entangled, with one another such that measuring a property of one instantly determines the same property in the other, even if the particles are separated by vast distances. In studying such systems, physicists often assume that the measurements on the particles are chosen randomly to start with.

In the new work, the scientists have calculated what might happen to a stream of partially random information, in which some of the bits are correlated with other variables, thus making the bits not random. By using these bits to choose measurements on pairs of entangled particles sent down two separate paths, a final outcome can be produced that is independent of any other variables, Colbeck and Renner showed. In other words, perfectly random bits are produced.

“Imagine an adversary who can guess at what choice you’re going to make at a given time,” says Colbeck. “Now we’ve shown there’s a procedure where you can make it so the adversary cannot guess at all, within certain limits. In principle, one could use this to make better quality random numbers.”

Nicolas Gisin, a physicist at the University of Geneva, says “the fact that the quality of randomness can be improved is new and surprising.” As a next step, he says, researchers might explore exactly how not-random the information flow can be and still be transformed into a purely random stream.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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