Abstract

Entanglement is a fundamental resource in several quantum communication schemes used for quantum-key distribution (QKD), teleportation or quantum repeaters. In the case of QKD, using pairs of entangled photons allows the exchange of random but yet correlated qubits used for the distribution of a cryptographic key. Most sources of entangled photon pairs use spontaneous parametric down-conversion (SPDC) in quasi-phase matched (QPM) periodically poled (PP) crystals or waveguides, which allow collinear emission of the emitted pairs. Long crystals are needed there to increase the yield of emitted pairs and narrow their bandwidth. The latter is a necessary condition to minimize chromatic dispersion in the fiber, which limits transmission distance as a spread of the wave packet requires a longer gate time of the single photon detector and thus increases its noise. Narrowband photons are also required for compatibility of the source with current high speed optical networks, which operate with 0.8 nm spacing between channels in a typical WDM environment. At the same time, WDM provide the means to multiplex in the same optical fiber each qubit-encoded photon with the synchronization pulse needed to gate the single photon detector at the receiver side. Multiplexing would naturally limit the cost of deploying quantum cryptography, as no dedicated fiber would be needed for the quantum channel, which carries an intensity of only −100 dBm average power at 1 Mbps. However, about 100 dB isolation is necessary to avoid cross-talk between the quantum and the synchronization channels.

© 2007 IEEE