Researchers from Twente develop quantum security multimode glass fibres

Researchers at the University of Twente have developed a variant for the distribution of quantum keys that works for multimode glass fibres. There was already this kind of quantum security for single-mode fibres.

The basis of the security is wave front shaping and the University of Twente describes that this in fact involves programming light. The sender and receiver agree on a number of points within the communication path at which the light must be present via the fiber optic at a certain moment. The sender then programs the light so that it ends up exactly at the agreed locations. They also use existing methods to encode a symbol of the alphabet into a single photon. This gives the two people the opportunity to agree on quantum keys for the security of the communication.

A person who intercepts the photon and wants to find out its position will see random dots on the detector, even when sending the same symbol regularly. That person does not have enough information to find out which symbol was sent. Only the intended recipient can locate the photon at the correct, agreed-upon position, corresponding to a symbol of the alphabet.

The no-cloning principle of quantum mechanics also prevents interceptors from making and forwarding identical copies of the keys. “The phase in which the sender and receiver make agreements seems to be the most vulnerable. But even then it remains unclear how the light is programmed by the sender,” said research leader Pepijn Pinkse.

There are already ways for quantum key distribution for single-mode fibre, but not yet for the multimode variant. Multimode fiber has a higher data throughput by allowing more than one signal to pass at a time. The light signals are sent into the optical fiber at different angles. They are mainly used for relatively short distances, such as for interconnecting nodes in data centers and 5G base stations.

The research was conducted in the Complex Photonic Systems group, which is part of the MESA+ Institute of the UT. The paper Quantum key establishment via a multimode fiber has appeared in Optics Express. An earlier version is publicly available.