# Photonic Quantum Technologies

Article from NICT NEWS 2021 No. 2（Vol. 486)

Researcher: Yoshiaki TSUJIMOTO

A quantum network is the ultimate network which aims to connect quantum devices such as quantum computers in order to derive the maximum functionality based on the principles of physics. This network uses a correlation specific to quantum mechanics called “quantum entanglement” as a resource to realize a protocol that is not attainable with conventional technology based on classical physics. This article explains the latest results of our research and development on the fast generation and application of entangled photon pairs that have such a quantum correlation, and a proof-of-principle experiment of the quantum protocol made possible using those entangled photon pairs.

### Creating a quantum network

### Entanglement

### Development of a source of ultrafast entangled photon pairs

### Experimental demonstration of quantum protocol

One of the protocols that use such entangled photon pairs as a resource is device-independent quantum key distribution (DIQKD). DIQKD is a next-generation QKD protocol that can obtain the secret key without any knowledge of the QKD device by monitoring the degree of quantum correlation between entangled photon pairs. The quantum correlation can be evaluated with the correlation parameter S, which is obtained from measurement, and if S exceeds 2, the security of the private key is guaranteed. However, when performing DIQKD using entangled photon pairs generated with SPDC, the optimal average photon number that maximizes S has not been clearly known. Using the experimental setup illustrated in Figure 2(a), we experimentally confirmed that S takes the maximum in a region of the average photon number that is significantly higher than conventionally thought (Figure 2(b)).

In addition, we experimentally demonstrated that long-distance DIQKD is possible by using a technique called entanglement swapping. The experimental setup shown in Figure 3(a) forms a quantum entanglement by generating two pairs of entangled photons and concatenating them. We applied entanglement swapping after adding the loss corresponding to a 50 km-long optical fiber to entangled photon pairs. The final state estimated from the result indicated that S > 2 (Figure 3(b)).