#quantum computation

A leaked paper suggests that Google has achieved a milestone known as quantum supremacy, using a quantum computer to perform a calculation that couldn’t be achieved even with the world’s most powerful supercomputers.

It’s a hotly anticipated goal, and one intended to mark the beginning of a new era of quantum computation (SN: 6/29/17). But it’s also largely symbolic: The calculation in question serves no practical purpose and is designed to be difficult for classical computers, standard computers that are not rooted in quantum physics.

On September 20, the Financial Times reported that a scientific paper, briefly published on a NASA website before being removed, claims that Google has built a quantum computer that achieved quantum supremacy. It’s a benchmark that the company’s quantum researchers, led by physicist John Martinis of the University of California, Santa Barbara, have set their sights on for years (SN: 3/5/18). An apparent plain-text version of the paper, posted anonymously on the site Pastebin, has since been circulating among scientists and on Twitter. A spokesperson for Google declined to comment to Science News.

According to the Pastebin version of the paper, Google created a quantum computer named Sycamore with 54 quantum bits called qubits, 53 of which were functional. The researchers used it to perform a series of operations in 200 seconds that would take a supercomputer about 10,000 years to complete.

The calculation consists of performing random operations on the qubits and reading out the result. After doing this many times, the researchers are left with a nearly random assortment of numbers, one that is extremely difficult to reproduce with a classical computer.

Despite its lack of applications, quantum supremacy has been billed as a major breakthrough in the quest for a quantum computer that could eventually perform useful calculations that are not possible with classical computers. “This dramatic speedup relative to all known classical algorithms provides an experimental realization of quantum supremacy on a computational task and heralds the advent of a much-anticipated computing paradigm,” the text of the Pastebin paper reads.

The machines might eventually be capable of defeating encryption techniques used to secure certain transmissions, such as financial transactions made by computers. But that advance will require many more qubits and a method to correct the errors that inevitably creep into quantum calculations. “While this is a milestone, it is *very* far from being a quantum computer that can compute anything useful,” physicist Jonathan Oppenheim of University College London wrote on Twitter.

Not everyone agrees that quantum supremacy is a useful benchmark. “Quantum computers are not ‘supreme’ against classical computers because of a laboratory experiment designed to essentially (and almost certainly exclusively) implement one very specific quantum sampling procedure with no practical applications,” IBM’s director of research Dario Gil wrote in a statement sent to Science News.

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