Quantum calculation claims world-first

The first mathematical calculation performed on a primitive quantum computer using single photons passing through a silicon chip has been made by physicists and engineers from the University of Bristol’s Centre for Nanoscience and Quantum Information.

The team coupled four photons into and out of the chip using optical fibres. On the chip the photons traveled through silica waveguides that were brought together to form a sequence of quantum logic gates. The chip then implements a quantum programme (known as Shor’s algorithm) to find the prime factors of 15, and outputs the answer – 3 and 5.

The output was determined by which waveguides the photons exited the chip in. By detecting the photons at the output of the device they confirmed the high-performance operation of the quantum algorithm.

Finding prime factors lies at the heart of contemporary encryption schemes, including those used for secure Internet communication. The ability of quantum computers to simulate quantum systems may also prove to be a powerful tool in the development of new materials or pharmaceuticals.

The Bristol team describes the breakthrough - the result of a three years’ work - as a ‘major step forward in the quest to realise an all-optical quantum computer’. “This task could be done faster by any school kid,” comments PhD student Alberto Politi, who, with fellow PhD student Jonathan Matthews, performed the experiment, “but this is an important proof-of-principle demonstration.”

The team from the University of Bristol’s newly established Centre for Nanoscience and Quantum Information have spent several years developing devices where photons propagate in silica waveguides, similar to optical fibres micro-fabricated on a silicon chip. “The next step is to develop ways to scale-up the circuit count,” says the director of the Centre for Quantum Photonics Professor Jeremy O’Brien, who led the research. “It opens the way for quantum computers that are 10 or even 100 times larger.”

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