In this paper we prove both theoretically and experimentally the insecurity of the Ekert’s protocol for quantum-key distribution when based on the Wigner inequality when entangled photon pairs are used as source. The security proof of the Eckert’s protocol based on Wigner’s inequality consists in verifying the violation of W ≥ 0 (which corresponds to local-realistic theories). We highlight that a violation of the Wigner’s inequality occurs when Eve controls the two quantum channels. We give the theoretical motivation for this result and we perform an experiment proving this weakness, by realizing the condition of Eve gaining total control of the source of photon pairs. In addition we calculate the modified version of this inequality which guarantees safe quantum-key distribution, thus proving it experimentally.
We present results of experimental demonstration of secure Quantum Key Distribution (QKD) at Elsag spa based on the implementation of BB84 protocol using polarization entangled states produced in the nonlinear process of type-II spontaneous parametric down conversion (SPDC). This enables us to avoid the use of active polarization modulation components and increases the overall key distribution rate. The high quality of polarization-entangled state generated by parametric down conversion and the high efficiency of coupling entangled-photon pairs into a single-mode optical fiber has enabled us to perform QKD with quantum bit-error rate compatible with acceptable security levels. The complete software system architecture includes a QKD protocol implementing all phases of the key distillation process. The system runs in a server and two users configuration on three different PCs connected over a local area network (LAN). Friendly graphical user interfaces (GUI) are available to start and to monitor the whole key generation and distillation process.