Experimental study on all-fiber-based unidimensional continuous-variable quantum key distribution

We experimentally demonstrated an all-fiber-based unidimensional continuous-variable quantum key distribution (CV QKD) protocol and analyzed its security under collective attack in realistic conditions. A pulsed balanced homodyne detector, which could not be accessed by eavesdroppers, with phase-insensitive efficiency and electronic noise, was considered. Furthermore, a modulation method and an improved relative phase-locking technique with one amplitude modulator and one phase modulator were designed. The relative phase could be locked precisely with a standard deviation of 0.5° and a mean of almost zero. Secret key bit rates of 5.4 kbps and 700 bps were achieved for transmission fiber lengths of 30 and 50 km, respectively. The protocol, which simplified the CV QKD system and reduced the cost, displayed a performance comparable to that of a symmetrical counterpart under realistic conditions. It is expected that the developed protocol can facilitate the practical application of the CV QKD.

Figure 1

UD protocol schemes. (a) Prepare-and-measure scheme. (b) EB scheme.

Figure 2

Modulated coherent states of UD protocol in phase space (a) without relative phase locking and with relative phase locked to (b) $\theta$, (c) zero, and (d) $\pi /2$.

Figure 3

Experimental setup. AM: amplitude modulator; SM: single mode; PM fiber: polarization-maintaining fiber; DAQ: data acquisition card; QRNG: quantum random number generator; DPC: dynamic polarization controller; PBHD: pulsed balanced homodyne detector.

Figure 4

Intensity and amplitude output of amplitude modulator versus modulated voltage.

Figure 5

Sums of modulated voltages and bias voltages applied to phase modulator on Bob's side.

Figure 6

Locked relative phase over 100 s.

Figure 7

Variances of excess noise and phase quadrature $V_y^B$ over 1 h and transmission lengths of 30 and 50 km.

Figure 8

Secret key rate versus modulation variance curves corresponding to transmission lengths of 30 and 50 km.

Figure 9

Secret key rate versus transmission distance curves corresponding to different amounts of excess noise.