New switch could help build ‘quantum internet’

A new switching device could represent a big step towards the creation of quantum networks, say researchers at Northwestern University.

They’ve managed to route quantum bits – or entangled particles of light – at very high speeds along a shared network of fiber-optic cable without losing the entanglement information embedded in the quantum bits.

The switch would enable the fiber-optic infrastructure to be shared among many users of quantum information. It could route a quantum bit to its final destination just as an email is routed across the internet.

“My goal is to make quantum communication devices very practical,” says Prem Kumar, professor of information technology. “We work in fiber optics so that as quantum communication matures it can easily be integrated into the existing telecommunication infrastructure.”

While a standard bit can only exist in one of two states – 0 or 1 – a quantum bit, or qubit, can be 0, 1, or both simultaneously. Two or more qubits at different locations can also be entangled: if one assumes one state, its mate assumes a corresponding state, even if the two photons are hundreds of kilometers apart.

Researchers have been working to build an infrastructure that can transport this ‘superposition and  entanglement’ for ultra-fast communications.

To demonstrate their switch, the researchers first produced pairs of polarization-entangled photons, which were emitted into standard telecom-grade fiber.

One photon of the pair was transmitted through the all-optical switch. Using single-photon detectors, the researchers found that the quantum state of the pair of photons wasn’t disturbed – the encoded entanglement information remained intact.

“Quantum communication can achieve things that are not possible with classical communication,” says Kumar.

“This switch opens new doors for many applications, including distributed quantum processing where nodes of small-scale quantum processors are connected via quantum communication links.”