Long-Distance Continuous-Variable Quantum Key Distribution over 202.81 km of Fiber

Quantum key distribution provides secure keys resistant to code-breaking quantum computers. The continuous-variable version of quantum key distribution offers the advantages of higher secret key rates in metropolitan areas, as well as the use of standard telecom components that can operate at room temperature. However, the transmission distance of these systems (compared with discrete-variable systems) are currently limited and considered unsuitable for long-distance distribution. Herein, we report the experimental results of long distance continuous-variable quantum key distribution over 202.81 km of ultralow-loss optical fiber by suitably controlling the excess noise and employing highly efficient reconciliation procedures. This record-breaking implementation of the continuous-variable quantum key distribution doubles the previous distance record and shows the road for long-distance and large-scale secure quantum key distribution using room-temperature standard telecom components.

Figure 1

Optical layout of our long-distance CV-QKD system. Alice sends an ensemble of 38 ns weak Gaussian-modulated coherent states to Bob multiplexed with a strong local oscillator (LO) in time and polarization by using a delay line and a polarization beam-combiner, respectively. Then the two optical paths are demultiplexed at Bob’s side by a polarizing beam splitter placed after an active dynamic polarization controller. We perform phase modulation on the LO path to select the signal quadrature randomly. Finally, the quantum signal interferes with the LO on a shot-noise-limited balanced pulsed homodyne detector. Laser, continuous-wave laser; AM, amplitude modulator; PM, phase modulator; BS, beam splitter; VATT, variable attenuator; PBS, polarization beam splitter; DPC, dynamic polarization controller; EDFA, Erbium doped fiber amplifier; PD, photodetector.

Figure 2

Covariance matrix. We depict the covariance matrix of Alice’s and Bob’s variables after 202.81 km of fiber link transmission, which is based on the calibrated parameters in Table 1 using a standard expression. This follows the ordering $\{x_A,p_A,x_B,p_B\}$ and its components are expressed in shot-noise units.

Figure 3

Experimental key rates and numerical simulations. The six five-pointed stars correspond to the experimental results at different fiber lengths of 27.27, 49.30, 69.53, 99.31, 140.52, and 202.81 km. The blue solid curve is a numerical simulation of the key rate which is computed starting from the experimental parameters at 140.52 km. The red solid curve is the corresponding numerical simulation computed from the parameters at 202.81 km. For comparison, we also show previous state-of-the-art CV-QKD experimental results [21, 22, 37, 38, 39], DV-QKD experimental results from 200 to 250 km [40, 41, 42], and we compare the values and the scaling of our rates with the PLOB bound [43], i.e., the fundamental limit of repeaterless quantum communications. In particular, the PLOB bound is plotted with respect to our clock rate (5 MHz, black solid line) and with respect to the clock rate of Ref. [41, 42] (2.5 GHz, black dashed line).