Continuous and discrete Quantum Key Distribution (QKD)

The importance of the key

This last problem on how the key is exchanged between parties is the problem that Quantum Key Distribution solves.

The concept of Quantum Key Distribution (QKD) allows the exchange of highly secure cryptography keys between two remote parties known in cryptography as Alice and Bob. The security of QKD relies on the laws of quantum mechanics by which quantum states cannot be cloned or copied. This means that an eavesdropper trying to look at the information encoded by Alice will irreversibly change their quantum properties letting Bob and Alice know that someone has tried to intercept the information.

In general, there are two different approaches to QKD to generate the secret key: one was born in 1984 and focuses on particles of light (discrete variable QKD) while the other was conceived later on the early 2000s and looks at the wave nature of light (continuous variable QKD)

In Continuous-Variable QKD (CV-QKD), the quantum signals typically consist of light with information encoded in the quadrature of its electromagnetic fields. Instead of single photon detectors, CV-QKD uses coherent homodyne or heterodyne detection to continuously retrieve the quadrature value of the light to distil the key. This is a much younger technology but it has developed rapidly and it is the core technology of Luxquanta – in fact our vision sole provider.

In addition, CV-QKD can be readily used in coexistence with classical communication in the same fibre by means of wavelength-division multiplexing making integration into existing networks less complicated.

Rather than competing, DV-QKD or CV-QKD are complementary technologies with different application spaces. Which one to use will depend on the fibre link characteristics, such as losses or power, and the number of available optical fibre channels.