  High-speed physical key distribution is diligently pursued for secure
communication. In this paper, we propose and experimentally demonstrate a
scheme of high-speed key distribution using mode-shift keying chaos
synchronization between two multi-longitudinal-mode Fabry-Perot lasers commonly
driven by a super-luminescent diode. Legitimate users dynamically select one of
the longitudinal modes according to private control codes to achieve mode-shift
keying chaos synchronization. The two remote chaotic light waveforms are
quantized to generate two raw random bit streams, and then those bits
corresponding to chaos synchronization are sifted as shared keys by comparing
the control codes. In this method, the transition time, i.e., the chaos
synchronization recovery time is determined by the rising time of the control
codes rather than the laser transition response time, so the key distribution
rate is improved greatly. Our experiment achieved 0.75-Gbit/s key distribution
rate with a bit error rate of 3.8*10-3 over 160-km fiber transmission with
dispersion compensation. The entropy rate of the laser chaos is evaluated as 16
Gbit/s, which determines the ultimate final key rate together with key
generation ratio. It is therefore believed that the method pays a way for
Gbit/s physical key distribution.
