Quantum key distribution over $25\mathrm{km}$ with an all-fiber continuous-variable system

We report on the implementation of a reverse-reconciliated coherent-state continuous-variable quantum key distribution system, with which we generated secret keys at a rate of more than $2\mathrm{kb}∕\mathrm{s}$ over $25\mathrm{km}$ of optical fiber. Time multiplexing is used to transmit both the signal and phase reference in the same optical fiber. Our system includes all experimental aspects required for a field implementation of a quantum key distribution setup. Real-time reverse reconciliation is achieved by using fast and efficient low-density parity check error correcting codes.

Figure 1

(Color online) Entanglement-based scheme of CVQKD. The transmittance $T_A$ and $\eta$ characterize the measurements at Alice’s and Bob’s sides, while the channel transmittance $T$ and added noise ${\chi }_{\text{line}}$ are controlled by Eve. The QM box corresponds to Eve’s quantum memory.

Figure 2

(Color online) Experimental setup for CVQKD.

Figure 3

(Color online) Principle of multilevel reconciliation with LDPC codes.

Figure 4

(Color online) Effective key distribution rate as a function of the SNR, or equivalently, as a function of the modulation variance $V_A{N}_0$. We assume a linear dependence of $\epsilon$ on $V_A$, and use the experimental parameters given in Sec. 3. The curves show the key distribution rate for values of $\beta$ ranging from 0.6 to 1.0, and the filled circles show the efficiency we actually achieve for different SNR.

Figure 5

(Color online) Raw and effective key distribution rates for communication secure against individual (Shannon) and collective (Holevo) attacks. The reconciliation efficiency for the effective key distribution rates is assumed to be $\beta =0.898$. The parameters used in the calculations are $V_A{N}_0=18.5N_0$, $\epsilon =0.005$, $\eta =0.606$, $v_{\mathrm{el}}=0.041$, the effective repetition rate is $350\mathrm{kHz}$, and the fiber loss is assumed to be $0.2\mathrm{dB}∕\mathrm{km}$.