Researchers claim quantum device performs 9,000-year calculation in microseconds

In Gaussian boson sampling at least, quantum supremecy is here

Researchers in Canada have conducted a quantum computing experiment that they claim completes a calculation in just a fraction of a second that would take a conventional computer 9,000 years.

Jonathan Lavoie, an experimental physicist at quantum computing company Xanadu, and colleagues reported the results from a device designed to sample an unknown probability distribution of light passing through a network of optical fibers.

Fully fledged quantum computers are still a distant prospect, but [Lavoie and colleagues] have taken us a step closer to making such devices a reality

Known as Gaussian boson sampling, a task can be calculated by conventional computers, up to certain parameters, while realistic estimates can be produced for calculation times beyond these limits.

Published in Nature this week, the work gave the authors confidence to predict their device, which they christened Borealis, had achieved "quantum advantage" over conventional computing.

"On average, it would take more than 9,000 years for the best available algorithms and supercomputers to produce, using exact methods, a single sample from the programmed distribution, whereas Borealis requires only 36 microseconds," the paper said.

The result also showed an advantage of more than 50 million times that reported from earlier photonic machines, they said.

The researchers implemented transformations on a train of squeezed light, or light pulses which exhibit quantum uncertainty. They also used a sequence of three variable beamsplitters (VBSes) and phase-stabilized fiber loops that act as effective buffer memory for light, which allowed the system to be tuned or "programmed" and store information.

Nature said the experiment was important because programmable photonic processors were closer to the form that a quantum commercial device might take than other previous proof-of-principle quantum computing experiments.

Quantum supremacy or quantum advantage is the idea that a quantum computer, which processes information as quantum bits (qubits), can solve problems conventional computers cannot in a reasonable amount of time.

In December 2019, Google claimed quantum supremacy. The company had configured a quantum computer to produce results that would take conventional computers some 10,000 years to replicate – a landmark event.

But the claim was hotly contested by IBM, which also has a stake in the quantum computing race.

According to an accompanying article by Daniel Jost Brod, assistant professor at the Physics Institute, Federal Fluminense University, Rio de Janeiro, said the Canadian experiments gets researchers closer to quantum advantage for a narrow set of algorithms and is an important step forward — although quantum computers still fall well short of the usefulness of conventional general-purpose digital computers.

A periodic pulse train of single-mode squeezed states from a pulsed OPO enters a sequence of three dynamically programmable loop-based interferometers. Each loop contains a VBS, including a programmable phase shifter, and an optical fibre delay line. At the output of the interferometer, the Gaussian state is sent to a 1-to-16 binary switch tree (demux), which partially demultiplexes the output before readout by PNRs. The resulting detected sequence of 216 photon numbers, in approximately 36 μs, comprises one sample.

A periodic pulse train of single-mode squeezed states from a pulsed OPO enters a sequence of three dynamically programmable loop-based interferometers. image:Lavoie et al

"Fully fledged quantum computers are still a distant prospect, but [Lavoie and colleagues] have taken us a step closer to making such devices a reality," he said.

However, Jost Brod pointed out that quantum advantage is something of a moving target as conventional computers run ever more sophisticated algorithms which can spoof quantum results.

"Quantum advantage is not a well-defined threshold, based on a single figure of merit. And as experiments develop so, too, will techniques to simulate them — we can expect record-setting quantum devices and classical algorithms in the near future to take turns in challenging each other for the top spot," he said. ®

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