A quantum computer that measures light achieves quantum supremacy

By Leah Crane

A new type of quantum computing called boson sampling is capable of calculating that no classical computer can accomplish any reasonable time. This is the second time the quantum algorithm has claimed to achieve this principle, known as quantum supremacy, after Google claimed quantum supremacy using its Psychomore device in 2019.

The wonder of a boson sampling photon depends on its quantum properties – particles of light – displayed as it travels through a beam splitter, splitting one beam of light into two beams scattering in different directions. If two identical photons hit the splitter at exactly the same time, they will not run away from each other, instead they stick together and both travel in the same direction.

If you shoot multiple photons through a sequence of beam splitters several times in a row, patterns begin to emerge in the path of the photons that are extraordinarily difficult to simulate or predict with classical computers. Finding possible sets of photon paths in this setup is called boson sampling, and a boson sampling device is a kind of quantum computer, albeit with a very narrow purpose.

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A team led by Jian-Wei Pan at China’s University of Science and Technology created a boson sampling device called Jizhang using 300 beam splitters and laser pulses sent through 75 mirrors. A complete boson sampler has a fidelity of 1 in many tests, meaning that it matches the theoretical predictions perfectly. Jijung’s loyalty is 0.99.

Researchers calculated that it would be impossible to simulate boson samples with such high fidelity to a classical computer – the world’s most powerful classical computer Japanese Fugaku supercomputer would take 600 million years to accomplish what Jiujung could do. Only 200 seconds. The fourth most powerful supercomputer, the Sunway Tyhulite, will take 2.5 billion years.

“It shows that it is possible to achieve quantum supremacy using photonic boson sampling, which many have suspected, and which represents a completely different hardware path than the superconducting quits used by Google,” says Scott Aronson at Stein’s University of Texas .

While this is an impressive achievement, quantum supremacy means that this device is better than classical computers on a very specific task. “That doesn’t mean creating a scalable quantum computer, or a universal quantum computer, or a useful quantum computer.”

Changing the boson sampling method to pause the experiment, to measure, and to redirect some photons may allow researchers to perform more varied calculations, but it would be unusual to take the next step. Until then, the boson may not be of much practical use for sampling.

“It’s not clear if the boson has any of the sample applications and demonstrates quantum supremacy,” says Aronson. However, he says, it is possible that it could be useful in quantum chemistry or random number generation for encryption.

Journal Reference: Science, DOI: 10.1126 / Science