Experimental demonstration of the quantum noise randomized cipher based on cascaded phase shift keying without a high-level digital-to-analog converter.

To break the dependence on a high-speed and high-resolution digital-to-analog converter (DAC) in the traditional quantum noise randomized cipher (QNRC), a practical DAC-free modulation scheme based on cascaded phase-shift keying (PSK) is proposed and demonstrated by a proof-of-concept experiment. By employing seven cascaded phase modulators (PMs) driven by designed electrical voltage signals, a 128 PSK-QNRC system is achieved with a transmission rate of 10 Gbaud/s and a transmission distance more than 50 km, which eliminates the need for a DAC on the transmitter side. The bit error rate (BER) performance of the proposed scheme is compared to that of a traditional scheme based on an arbitrary waveform generator (AWG) with a sampling rate of 25 GSa/s. The results show that compared to a traditional scheme, the power penalties of the proposed scheme are -1.8 , 0.9 dB, and 1 dB, respectively, at the rates of 10, 5, and 2.5 Gbps. In other words, the BER performance of the proposed scheme is close to the traditional scheme at a low transmission rate, but better than that of the traditional scheme at a high transmission rate, where the sampling rate of the DAC is not high enough to generate a complete waveform. This work greatly enhances the security of a QNRC system.