Attila Pereszlenyi

Quantum networks are moving rapidly from research laboratories to practical applications. Most quantum networks are based on the trusted node approach because the distance for quantum key distribution (QKD) is limited by photon loss. Shorter distances quantum networks providing any to any connectivity require N(N-1)/2 dark fiber lines, where N is the number of users. Telecom operators, which are the most active players in quantum networks today, can become trusted node owners, which may be an additional barrier to the adoption of quantum networks. An alternative solution is to use entanglement in quantum networks at the city level. In our work we have demonstrated it on a network with three nodes. The center of the network is the PPLN-based source for polarization entangled photon pairs at 1310 and 1316nm. The outputs of the source are connected to a 2x32 optical switch to which any two users can be connected in pairs. To make the receiver suitable for measuring both photons, we have assembled a 2-wavelength Bragg filter that enables the measurement of photons in both wavelengths with a bandwidth of 2 nm. The receivers are designed to be completely passive - the fiber is connected to the BBM92 polarization projection system in free space box, followed by single photon detectors and a time tagger. Polarization distortion is compensated with a fiber-based polarization controller on the source side using the publicly announced QBER. The key is followed by the standard procedures of sifting, cascade error correction and finite key e=10-10 privacy amplification. The derived keys are uploaded to 10G L2/L3 encryption systems, which are able to establish quantum-safe VPN tunnels between any participants. List below describes results of a key rate for 3 node network when the entangled source is connected to a 2x32 optical switch and its outputs are connected to receivers A1(direct) and A2, A3 with 10 km fiber spools each. All secret key tares include finite key size effects A2 (10 km) - A1 (direct). QBER ~2.8%, Secret key rate ~125 b/s A3 (10 km) - A2 (10km). QBER ~4.9%, Secret key rate ~50 b/s A3 (10 km) - A1 (direct). QBER ~3.9%, Secret key rate ~100 b/s