Optical quantum computers promise to deliver processing performance exponentially faster and more powerful than today's digital electronic microprocessors. To make this technology a reality, however, photonic circuitry must first become at least as efficient at multi-tasking as the microprocessors they are designed to replace. Towards this end, researchers from the University of Bristol and Nippon Telegraph and Telephone (NTT) claim to have developed a fully-reprogrammable quantum optical chip able to encode and manipulate photons in an infinite number of ways.

Created from glass and silicon using standard semiconductor fabrication techniques, the new device ups the ante on previous photonic chips by incorporating six wave-guides for universal linear optic transformations and 15 integrated interferometers (devices that superimpose one photon beam over another to look for anomalies in intensity or phase), each of which is individually programmable. As a result, a range of different quantum processor operations can be performed at one time.

Even better, the stable and quickly reprogrammable nature of the chip's architecture – changeable by means of software code – means that a vast range of existing and yet-to-be-devised quantum experiments may be conducted rapidly in succession, or simultaneously, to help realize what may well be myriad future protocols.

"Once we wrote the code for each circuit, it took seconds to re-program the chip, and milliseconds for the chip to switch to the new experiment," said University of Bristol PhD student and research team member, Jacques Carolan. "We carried out a year’s worth of experiments in a matter of hours. What we’re really excited about is using these chips to discover new science that we haven’t even thought of yet ... This chip has been fabricated and packaged up, so that we never need to re-align it. It sits there, and we can perform literally 1000s of different experiments in a single day – this was simply unthinkable a few years ago."

The University of Bristol researchers (from left to right): Chris Sparrow, Chris Harrold, Jacques Carolan and Dr Anthony Laing(Credit: University of Bristol)

The number of photon inputs and outputs also means the the new processor can be applied to new areas of research straight away, as its ability to produce what is known as unitaries. These are mathematical operators which can be applied to sets of qubits to perform the equivalent of Boolean algebraic functions found in standard electronic logic processors – also referred to as quantum gates. The fact that it is able to perform a significant number of computational processes at the one time mean that it can emulate the performance of standard logic arrays.

The inherent flexibility of this technology also leads toward a state of "universality," where a universal quantum computer can efficiently simulate an arbitrary digital computer. Although still as yet at a modest scale, if the researcher's work produces even most of what it promises, significant steps towards designing and creating a large scale universal quantum computer will have been made.

The next phases in its development will be to scale-up its function and capacity, then prove the technology for use in the realms of telecommunication through partnership with NTT and other computer and networking companies.

As part of this greater encouragement of quantum computing research and development, the University of Bristol has pioneered the "Quantum in the Cloud" service, which allows public access via the Internet to a working quantum processor, with plans to add even more chips in the near future.

"Over the last decade, we have established an ecosystem for photonic quantum technologies, allowing the best minds in quantum information science to hook up with established research and engineering expertise in the telecommunications industry," said Professor Jeremy O'Brien, Director of the Centre for Quantum Photonics at Bristol University. "It’s a model that we need to encourage if we are to realise our vision for a quantum computer."

The results of this research were recently published in the journal Science.

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