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A shortcut to quantum computing

International groups test strategy to perform an operation in a glass chip that conventional computers cannot

Ana Paula CamposFour international teams of researchers have independently developed a calculator that operates using the strange quantum properties of light. The version of this quantum calculator created by the team involving Brazilians, for example, solves a mathematical operation after thousands of photon (light particle) triplets pass through a small chip of glass the size of a microscope slide.

These devices are part of another attempt to prove in practice that quantum computing has the ability to surpass conventional computing—for now, something only predicted in theory. The calculators created by these groups are actually what physicists have been calling restricted quantum computers. Designed to perform a specific type of calculation, they are a simplified version of the long dreamed of universal quantum computers, which in principle could carry out any kind of mathematical operation. While universal quantum computers are expected to take decades to be able to out-perform classical computers, physicists believe that quantum computers will perform restricted calculations, impossible for even the most powerful current supercomputers, in just over 10 years.

The apparatuses designed and developed by the four teams currently take two weeks to complete a complicated mathematical operation involving matrices that, although not trivial, any personal laptop could solve in seconds. Although not impressive due to their speed, these devices are exciting physicists because slightly more refined versions may soon challenge the limits of classical computing.

“These are the first in a series of experiments designed to perform calculations that are increasingly difficult to do on ordinary computers,” says physicist Ernesto Galvão, of the Fluminense Federal University in Niterói, Rio de Janeiro. He and his doctoral student Daniel Brod collaborated on the experiment conducted last year in the physics laboratory of Paolo Mataloni and Fabio Sciarrino, at Sapienza – Università di Roma, Italy.

Quantum computers exploit the laws of quantum mechanics, which govern the behavior of light, atoms and molecules, to perform calculations at an exponentially higher speed. They could, for example, write any integer as the product of a series of prime numbers, an operation known as factoring. While today’s computers take years to factor large numbers with hundreds of digits, a quantum computer with enough memory could do the calculations in seconds. But so far, physicists have only managed to build physical quantum computers with memory sufficient to factor the number 21.

Reduced expectations
Given the difficulty of creating universal quantum computers that can be programmed to perform various tasks, physicists have recently begun to design restricted quantum computers that operate more like calculators than computers.

Ana Paula CamposThe restricted quantum computers that are being developed and refined by the four international groups are based on a strategy proposed in 2010 and named bosonic sampling. Introduced in 2010 by computer scientist Scott Aaronson and mathematician Alex Arkhipov, both from the Massachusetts Institute of Technology, this strategy uses the difficulty in estimating the behavior of photons traversing an optical circuit to perform a difficult computational task.

In a circuit in which there are five parallel paths for photons, what is the probability that three identical photons entering at the same time, each through a different entrance—for example, 1,2 and 3—jump from one path to another and exit in a specific sequence, such as 2, 3 and 5? To obtain the result of this calculation, one must perform a mathematical operation with matrices whose numbers depend on the properties of the circuit and the number of photons (see infographic).

The pair from MIT found that the time it would take for a conventional computer to perform the calculation increases exponentially as the number of photons and circuit paths increases. In an example with 30 photons, supercomputers would probably spend hours to calculate the answer. One hundred photons, then, would take years.

On December 21, 2012, the date on which a prophecy attributed to the Mayans predicted the apocalypse would occur, Aaronson joked in his blog: “If the world ends today, at least it won’t do so without three identical photons having been used to sample from a probability distribution … thereby demonstrating the Aaronson-Arkhipov Boson Sampling protocol.” On that day, the journal Science published the results of two experiments that had implemented his idea, one led by Ian Walmsley, of the University of Oxford, England, and another by Andrew White, of the University of Queensland, Australia. The results achieved by the other two groups—the Italian-Brazilian team and Philip Walther’s team at the Institute of Quantum Optics and Quantum Information in Austria—appeared that same week on the arXiv site, a repository of scientific papers, and were published on May 26 this year in the journal Nature Photonics.

According to Galvão, his team’s work has an advantage that will be important in future experiments. While most of the properties of the circuits used by other groups is fixed, the glass chip made in the laboratory of the physicist Roberto Osellame, of the Institute of Photonics and Nanotechnology in Milan, is flexible. Researchers can arbitrarily adjust the probabilities that the photons will jump from one circuit path to another. “You can control the path of the photons,” he explains. “This could be useful in quantum optics research in general.”

Paulo Souto Ribeiro, an experimental physicist specializing in quantum optics at the Federal University of Rio de Janeiro, thinks 10 years is a reasonable estimate for when the calculation speed of devices using bosonic sampling will exceed that of classical computers. “But this is an estimate involving great uncertainty,” says Ribeiro. This is because it is very difficult to create identical photons in large quantities and control the loss of photons along the path in larger circuits. We also don’t know if the bosonic sampling calculations will be useful in practice. Ribeiro says that similar devices are being developed in the hopes of simulating the behavior of electrons in superconducting materials in the future.

Scientific article
CRESPI, A. et al. Integrated multimode interferometers with arbitrary designs for photonic boson sampling. Nature Photonics. Published on-line. May 26, 2013.