Simultaneous classical communication and quantum key distribution using continuous variables^*

Presently, classical optical communication systems employing strong laser pulses and quantum key distribution (QKD) systems working at single-photon levels are very different communication modalities. Dedicated devices are commonly required to implement QKD. In this paper, we propose a scheme which allows classical communication and QKD to be implemented simultaneously using the same communication infrastructure. More specially, we propose a coherent communication scheme where both the bits for classical communication and the Gaussian distributed random numbers for QKD are encoded on the same weak coherent pulse and decoded by the same coherent receiver. Simulation results based on practical system parameters show that both deterministic classical communication with a bit error rate of ${10}^{-9}$ and secure key distribution could be achieved over tens of kilometers of single-mode fibers. It is conceivable that in the future coherent optical communication network, QKD will be operated in the background of classical communication at a minimal cost.

Figure 1

Phase-space representations of various coherent communication schemes. (a) Classical BPSK scheme. (b) GMCS QKD scheme. (c) The proposed scheme. The figures on the right show the probability distributions of $X$-quadrature measurement.

Figure 2

Security models. HD is homodyne detection. (a) Our protocol. (b) A virtual QKD scheme equivalent to (a). (c) Standard GMCS QKD.

Figure 3

Simulation results of the required displacement $\alpha$ to achieve a BER of ${10}^{-9}$ in the classical channel. Simulation parameters are $V_A=4, \gamma =0.2$ dB/km, $\eta =0.5, {\nu }_{el}=0.1$.

Figure 4

The expected quadrature distribution at the receiver's end. ${\alpha }^{\prime }=\sqrt{T_{ch}\eta }\alpha$; the variance of the Gaussian distribution is $V_B=(T_{ch}\eta V_A+1)N_0$; the linear range of the homodyne detector is $[-x_m,x_m]$.

Figure 5

Simulation results based on security analysis given in [31]. Simulation parameters are $\gamma =0.2$ dB/km, ${\varepsilon }_0=0.01, {\nu }_{el}=0.1, V_A=4, \eta =0.5, f=0.95$, and ${\sigma }_{\varphi }={10}^{-3}$ (solid line), ${10}^{-4}$ (dashed line), ${10}^{-5}$ (dotted line), and ${10}^{-6}$ (dash-dotted line).