Experimental Demonstration of High-Rate Measurement-Device-Independent Quantum Key Distribution over Asymmetric Channels

Measurement-device-independent quantum key distribution (MDI-QKD) can eliminate all detector side channels and it is practical with current technology. Previous implementations of MDI-QKD all used two symmetric channels with similar losses. However, the secret key rate is severely limited when different channels have different losses. Here we report the results of the first high-rate MDI-QKD experiment over asymmetric channels. By using the recent 7-intensity optimization approach, we $\text{demonstrate}>10\times \text{higher}$ key rate than the previous best-known protocols for MDI-QKD in the situation of large channel asymmetry, and extend the secure transmission distance by more than 20–50 km in standard telecom fiber. The results have moved MDI-QKD towards widespread applications in practical network settings, where the channel losses are asymmetric and user nodes could be dynamically added or deleted.

Figure 1

An illustration of a star-type MDI-QKD network providing six users with access to the untrusted relay, Charlie. Inset: an example of the possible implementation by Charlie.

Figure 2

MDI-QKD setup. Alice’s (Bob’s) signal laser pulses are modulated into signal and decoy intensities by three amplitude modulators (AM1-AM3). Key bits are encoded by a Mach-Zehnder interferometer, AM4, and a phase modulator (PM). In Charlie, the polarization stabilization system in each link includes an electric polarization controller (EPC), a polarization beam splitter (PBS) and a superconducting nanowire single-photon detector (SNSPD); the Bell state measurement (BSM) system includes a $50/50$ beam splitter (BS), SNSPD1 and SNSPD2. Abbreviations of other components: DWDM, dense wavelength division multiplexer; ConSys, control system; ATT, attenuator; PSL, phase-stabilization laser; Circ, circulator; PC, polarization controller; PS, phase shifter; SPAPD, single-photon avalanche photodiode.

Figure 3

Simulation (curve) and experiment results (data points) for secret rate (bit/pulse) vs the total distance $L_{AB}$ in standard telecom fiber. (a) $L_A$ is fixed at 10 km, while $L_B$ is selected at 40, 60, 80, and 90 km. (b) $L_A$ is fixed at 0 km, while $L_B$ is selected at 40, 60, 80, and 100 km. The points (curves) in the figure indicate the experimental (simulation) results for (i) the 4-intensity method shown as blue diamonds (blue dashed line), where the same intensities and proportions for Alice and Bob are selected and optimized in the 4-intensity protocol [21, 28]; (ii) the $4\text{−}\text{intensity}+\text{fiber}$ method [14] shown as black dots (black dot-dash line); (iii) the 7-intensity method [34], shown in red squares (red solid line). As can be seen, for the 4-intensity methods, adding fibers improves the key rate in long distances, but it does not in short distances. In contrast, the 7-intensity method always achieves a substantially higher key rate than any of the other two methods, especially when channel asymmetry is high.