Abstract : 

The security of the most common cryptography RSA algorithm is now threatened by the rapid development of quantum computing, and thus a new encryption method is needed. Quantum key distribution (QKD) is one of the most well-known choices. Through the properties of quantum mechanics, it enables two parties to produce a shared random secret key known only to them, while any eavesdropping can be detected.

In this presentation, I will first introduce the principle of a time-energy entanglement based QKD protocol: BBM92, and how it works exactly with Franson interferometer, which consists of two unbalanced Michelson interferometers with a long arm and a short arm respectively. By adjusting the sum of path differences between two unbalanced arms, the probabilities of two photons passing through the same arm in each Michelson interferometer exhibit nonlocal constructive and destructive interference, which can be regarded as the classical bits 0 and 1 respectively.

In the end, the experimental setup of Franson interferometer, experimental problems, and the measured visibility of short-distance and long-distance nonlocal interference are mentioned.