Quantum computing boosted by 'photon walk' chip

More than particle spin

Scientists led by a Bristol University team have pioneered an experimental method for quantum computers to perform calculations using photons travelling inside a silicon chip.

The design is based on getting two photons to travel through the multi-path chip on what is called a 'quantum walk', the quantum equivalent of how in classical physics a particle might get from A to B by via random points in between.

The mathematics around modelling what happens on this journey using one photon without 'decoherence' (interference that pulls the quantum particle back into a classical state) have been well explored, but the team, which included contributions from Japanese, Dutch and Israeli physicists, were able to model what happens for two photons for the first time.

The team hasn't explained in detail how they solved the formidable issues involved, but the implications for quantum computing theory are intriguing. A major line of development in quantum computing is using particle entanglement, an approach that forms the basis of many quantum bit (qubit) designs.

Quantum walks offer another path to create photonic qubits capable of performing useful calculations.

"Each time we add a photon, the complexity of the problem we are able to solve increases exponentially, so if a one-photon quantum walk has 10 outcomes, a two-photon system can give 100 outcomes and a three-photon system 1000 solutions and so on," said Professor Jeremy O'Brien, director of the Centre for Quantum Photonics at Bristol University.

"Using a two-photon system, we can perform calculations that are exponentially more complex than before," says Prof O'Brien. "This is very much the beginning of a new field in quantum information science and will pave the way to quantum computers that will help us understand the most complex scientific problems."

It's not universally accepted that quantum computers could be used to model the same sorts of calculations output by ordinary computers. It could be that their greatest contribution will be to model aspects of physics which are themselves hard to understand because of their 'quantumness' such as superconductivity and important chemical reactions.

Currently, science has to make do with vast numbers of mechanistic generalisations regarding such phenomena, which underlie almost everything that science works with.

The next frontier will be sending three photons on quantum walks trough the specially-designed chip.

"Now that we can directly realise and observe two-photon quantum walks, the move to a three-photon, or multi-photon, device is relatively straightforward, but the results will be just as exciting" said O'Brien.

Quantum computing 'breakthroughs' are claimed several times a year, and it's fair to say that as with any science sensitive to on theoretical advances, these claims are not necessarily hyperbole. Despite this, quantum computing is still a technology stuck on the drawing board where advances in the physical setup, mathematics and experimental models move knowledge forward in lots of tiny but important leaps.

The end result could be a generation of computing devices capable of performing not more calculations in a given time, but radically different ones that tells us different things.

Quantum principles are also used in the tense science of quantum cryptography, the distribution of encryption keys with absolute levels of certainty.

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John E Dunn

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