Digital coherent PSK Y-00 quantum stream cipher with 2¹⁷ randomized phase levels

We report a 10-Gbaud fiber-optic cipher transmission system by using a phase-shift keying (PSK) Y-00 quantum stream cipher. The PSK Y-00 cipher is a symmetric-key direct data encryption technique based on extremely high-order random phase modulation using a pre-shared short key. Neighboring signal phases following encryption are masked by quantum (shot) noise, which provides security based on shot noise's inherent effects. To implement such a system, we utilize coarse-to-fine phase modulation with two cascaded phase modulators and digital decryption incorporated into digital signal processing (DSP) for intra-dyne coherent detection. We demonstrate 10-Gbaud PSK Y-00 cipher transmission over a 400-km standard single-mode fiber (SSMF). The coarse-to-fine phase modulation achieves 2¹⁷ phase levels for signal masking by shot noise. The DSP with decryption realizes detection of the cipher without penalties. Masking 167 signal phase levels by shot noise is achieved at a bit-error ratio defined by a hard-decision forward-error correction threshold (3.8 × 10^-3) in the transmissions over the 400-km SSMF.