  A high-speed Gaussian modulated continuous-variable quantum key distribution
(CVQKD) with a local local oscillator (LLO) is experimentally demonstrated
based on pilot-tone-assisted phase compensation. In the proposed scheme, the
frequency-multiplexing and polarization-multiplexing techniques are used for
the separate transmission and heterodyne detection between quantum signal and
pilot tone, guaranteeing no crosstalk from strong pilot tone to weak quantum
signal and different detection requirements of low-noise for quantum signal and
high-saturation limitation for pilot tone. Moreover, compared with the
conventional CVQKD based on homodyne detection, the proposed LLO-CVQKD scheme
can measure X and P quadrature simultaneously using heterodyne detection
without need of extra random basis selection. Besides, the phase noise, which
contains the fast-drift phase noise due to the relative phase of two
independent lasers and the slow-drift phase noise introduced by quantum channel
disturbance, has been compensated experimentally in real time, so that a low
level of excess noise with a 25km optical fiber channel is obtained for the
achievable secure key rate of 7.04 Mbps in the asymptotic regime and 1.85 Mbps
under the finite-size block of 10^7.

Quantum Physics