Can entanglement make communication safer?

Scientists say they are a step closer to developing a secure internet after unveiling a prototype of what they believe is the largest quantum communication network of its kind.

Writing in the journal Science, they say it can simultaneously connect at least eight users across 17 kilometres while avoiding the risks and hardware requirements inherent in previous designs.

It has a range wide enough to serve as a city-wide system, says lead author Siddarth Joshi from the University of Bristol, UK, with simulations showing its architecture can be extended to 32 users.

Secure quantum networks require “keys” to decode transmitted data.  If hackers intervene hacker, signs of interference occur, with errors created in what is received by the legitimate parties.

To scale this technology beyond two users, many networks rely on “trusted nodes”, which have access to the key information. This compromises security, however, and is costly, which has limited its potential. Other networks rely on active switching, allowing only certain pairs of users to exchange a key at a given time.

Joshi and colleagues turned to the principle of entanglement, which exploits the power of two different particles placed in separate locations to simultaneously mimic each other.

“Instead of having to replicate the whole communication system, this latest methodology, called multiplexing, splits the light particles, emitted by a single system, so they can be received by multiple users efficiently,” he says.

Multiplexing increases bandwidth over existing fibre networks by transmitting multiple signals at different wavelengths on the same fibre, reducing the infrastructure needed for a quantum network.

Instead of making a physical connection, such as a glass fibre, between each and every user, the researchers created a scheme where every user only has a single glass fibre connected to a source of quantum entanglement.

They needed only 16 wavelength channels for their network architecture, down from the 56 channels necessary in a previous design on which Joshi collaborated in 2018.

It is, they believe, the largest quantum network yet created without trusted nodes.

To demonstrate its functionality across distance, receiver boxes were connected to optical fibres via different locations across Bristol and the ability to transmit messages via quantum communication was tested using the city’s existing optical fibre network.