Although entangled-photon pairs have been generated in the 1550 nm telecommunications band within optical fiber for research into secure quantum-key distribution (QKD), previous experiments have only estimated the quantum bit-error rate (QBER) and sifted-key generation rate from prestage results. But now, sifted keys-defined as keys created after reconciliation of the measurement basis-have been generated for the first time in an entanglement-based differential-phase QKD experiment by researchers at NTT (Atsugi, Japan) and Osaka University (Osaka, Japan).
Light from an external-cavity semiconductor laser at 1551.11 nm is converted into a pulse stream by a lithium niobate modulator followed by an erbium-doped fiber amplifier and launched into a cooled 500 m length of dispersion-shifted fiber with 1551 nm zero-dispersion wavelength. Spontaneous four-wave mixing produces a series of time-correlated entangled-photon pairs that are then actively phase-modulated by a lithium niobate modulator followed by a planar-lightwave-circuit Mach-Zehnder interferometer. Sifted keys with a length of 133 bits were generated, and the QBER was measured at 8.3% with a key generation rate of 0.3 bit/s, good enough to be distilled to a secure key through error correction and privacy amplification. Contact Toshimori Honjo at [email protected].