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Multi-coloured photons in 100 dimensions may make quantum computers stronger

By using manipulating the frequency of entangled photons, researchers have found a way to make more stable tools for quantum computing from off-the-shelf equipment. Andrew Masterson reports.

An illustration showing high-dimensional color-entangled photon states from a photonic chip, manipulated and transmitted via telecommunications systems.
An illustration showing high-dimensional color-entangled photon states from a photonic chip, manipulated and transmitted via telecommunications systems.
Michael Kues

Researchers using off-the-shelf telecommunications equipment have created a 100-dimensional quantum system from the entanglement of two subatomic particles.

The system can be controlled and manipulated to perform high-level gateway functions – a critical component of any viable quantum computer – the scientists report in the journal Nature.

The team, led by Michael Kues of the University of Glasgow, effectively created a quantum photon generator on a chip. The tiny device uses a micro-ring resonator generate entangled pairs of photons from a laser input.

The entanglement is far from simple. Each photon is composed of a superposition of several different colours, all expressed simultaneously, giving the photon several dimensions. The expression of any individual colour – or frequency, if you like – is mirrored across the two entangled photons, regardless of the distance between them.

The complexity of the photon pairs represents a major step forward in manipulating quantum entities.

Almost all research into quantum states, for the purpose of developing quantum computing, has to date focussed on qubits: artificially created subatomic particles that exist in a superposition two possible states. (They are the quantum equivalent of standard computing ‘bits’, basic units that are capable only of being switched between 1 and 0, or yes/no, or on/off.)

Kues and colleagues are instead working with qudits, which are essentially qubits with superpositions comprising three or more states.

In 2016, Russian researchers showed that qudit-based quantum computing systems were inherently more stable than their two dimensional predecessors.

The Russians, however, were working with a subset of qudits called qutrits, which comprise a superposition of three possible states. Kues and his team upped the ante considerably, fashioning qudits comprising 10 possible states – one for each of the colours, or frequencies, of the photon – giving an entangled pair a minimum of 100.

And that’s just the beginning. Team member Roberto Morandotti of the University of Electronic Science and Technology of China, in Chengdu, suggests that further refinement will produce entangled two-qudit systems containing as many as 9000 dimensions, bringing a robustness and complexity to quantum computers that is at present unreachable.

Kues adds that perhaps the most attractive feature of his team’s achievement is that it was done using commercially available components. This means that the strategy can be quickly and easily adapted by other researchers in the field, potentially ushering in a period of very rapid development.

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Andrew Masterson is news editor of Cosmos.
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