Crystals could enable long-distance quantum communication

Researchers at the University of Geneva (UNIGE) are extending the reach of quantum communications with a protocol based on a crystal that can emit quantum light, as well as store it for arbitrary long times. Their study has been published in the journal Physical Review Letters and is said to be paving the way for a quantum repeater — a device that can extend the range of quantum communication between sender and receiver.

Quantum physics guarantees that a message cannot be intercepted before reaching its destination. Thanks to the laws of quantum physics, a particle of light can be in two distinct states simultaneously — comparable to a coin thrown in the air, which is showing both heads and tails before reaching the ground. This superposition of states is destroyed as soon as the message is read, like grabbing our metaphorical coin out of the air.

“In order to test the security of a communication link, we can use particles of light, photons, onto which we encode quantum bits (analogous to the bit used in computing),” said study co-author Cyril Laplane. “We then take advantage of the properties of quantum superposition, allowing the photon to be simultaneously in two states, to test the security of a communication link.”

If the photon is intercepted and read, the superposition of states is lost and only one of the two states remains; hence, the recipient can know if the message has been intercepted. However, since this relies on the use of single photons, there is a chance of losing the particles when they propagate in traditional communication links such as optical fibre. This problem becomes more and more critical with distance.

In order to communicate over long distances, one would need repeaters, which amplify and rebroadcast the signal. It is, however, impossible to use such a procedure in quantum communication without destroying the superposition of states. Physicists need to build a quantum repeater not only able to store the dual character of the photon but also to produce such a state.

To build a quantum repeater, scientists have investigated atomic gases, which usually require heavy experimental apparatus. UNIGE is taking a different approach, with study co-author Jean Etesse revealing that the researchers are using “a crystal capable of storing quantum state of light. It possesses the advantage of being relatively simple to use with potential for very long storage times.”

These crystals are able to absorb light and restore it later, without reading the information encoded on it. Furthermore, they can generate single photons and store them on demand. Another major asset is their potential for miniaturisation.

Since the crystal is the source and memory for quantum information, it simplifies the protocol for quantum repeaters and lays the foundation of a quantum internet. Physicists at UNIGE are already working on the creation of an elementary link of quantum communication using a repeater.

Image caption: The quantum repeater: two crystals in operation. ©UNIGE