Caught in a traffic jam with the internet, quantum teleportation persists. An experiment demonstrating that the two can share an optical fibre may point towards an easier way to build an unhackable quantum internet.
“For many years, I’ve had the thought of how can we share quantum information over the same fibres that carry classical communication signals?” says at Northwestern University in Illinois. Now, he and his colleagues have teleported a quantum state over 30.2 kilometres of one such fibre.
Quantum teleportation is a protocol that includes three devices – nicknamed Alice, Bob and Charlie – that can manipulate quantum states of particles of light, or photons. The goal is to transfer the set of a photon’s properties that can encode information, known as its quantum state, from Alice to Bob without that photon travelling through a fibre directly from one to the other. Instead, they rope in Charlie to help and use quantum entanglement, which can link two photons so inextricably that they always have correlated properties.
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In the protocol, Bob entangles a pair of photons, sends one to Charlie and keeps the other. Alice, in the meantime, simply sends Charlie a photon encoded with information. After receiving the two photons, one from Alice and one from Bob, Charlie manipulates them in such a way that the state of Alice’s photon is “teleported” through entanglement to the photon that Bob is still holding.
This set-up has been used in many previous experiments, but they always involved fibres devoid of any other signals. Researchers had not used cables filled with internet signals before because photons’ quantum states are so fragile that any other traffic threatens to destroy the information they carry.
The team prevented this by finding a wavelength of light where photons experienced the lowest level of disturbance from all other signals, says team member , also at Northwestern University. In fact, when they connected their versions of Alice, Bob and Charlie with a spool of fibre in the lab, they could send as much as 400 gigabits per second of classical information through it without disrupting the quantum teleportation.
“We clearly showed that both next-generation quantum and classical networks can share a single fibre infrastructure,” says Thomas. The team leveraged a lot of equipment that is already used in telecommunications, which may make it more plausible for this experiment to be repeated anywhere where similar fibres exist, he says.
at the University of Bristol in the UK says that taking advantage of fibres that are already used for the traditional internet may be the only way to build quantum communication networks in places where laying new fibre would be very expensive, such as cities with high property prices.
“Quantum teleportation is a cornerstone of quantum communication,” says at the quantum communication start-up Q*BIRD. But he says that taking the new experiment out of the lab will raise many engineering challenges, such as dealing with extra disturbances from a less-controlled outside environment and having to synchronise all devices very carefully.
The team is now planning to do exactly that and repeat the experiment with fibres that already exist in Chicago.
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