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Quantum computers should theoretically outpace ordinary ones, but attempts to build a speedy quantum machine have so far come up short. Now an approach based on a Victorian counting device seems to be getting close.

This proto-quantum computer can only solve one problem. But that problem, called boson sampling, seems to be very difficult for an ordinary computer to solve, so physicists hope that such a device will conclusively demonstrate the promise of computing based on exotic physics. "The goal is to show quantum supremacy with the simplest approach," says Fabio Sciarrino of Sapienza University in Rome, Italy, who helped develop the new machine.

Boson samplers are based on a device created by the 19th-century polymath Francis Galton to study statistical distributions. It consists of a wooden board studded with offset rows of pegs. Balls are dropped one by one from the top of the board and ping their way down, bouncing left or right at each peg, before collecting in bins at the bottom. Since balls are more likely to end up in a central bin than one at the edges, you end up with a bell curve distribution across the width of the board. In the pre-computer age, it was one of the best ways to compute this distribution, which often crops up in statistics.

The quantum version swaps balls for photons, which travel along a network of intersecting channels in an optical chip. When two photons collide, their ensuing paths are determined by the laws of quantum mechanics, producing a unique distribution. With enough photons, calculating this distribution becomes very difficult on an ordinary computer, so doing it with real photons in a quantum device is the only practical option.

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