Quantum key distribution over 120 km using ultrahigh purity single-photon source and superconducting single-photon detectors

Abstract

Advances in single-photon sources (SPSs) and single-photon detectors (SPDs) promise unique applications in the field of quantum information technology. In this paper, we report long-distance quantum key distribution (QKD) by using state-of-the-art devices: a quantum-dot SPS (QD SPS) emitting a photon in the telecom band of 1.5 μm and a superconducting nanowire SPD (SNSPD). At the distance of 100 km, we obtained the maximal secure key rate of 27.6 bps without using decoy states, which is at least threefold larger than the rate obtained in the previously reported 50-km-long QKD experiment. We also succeeded in transmitting secure keys at the rate of 0.307 bps over 120 km. This is the longest QKD distance yet reported by using known true SPSs. The ultralow multiphoton emissions of our SPS and ultralow dark count of the SNSPD contributed to this result. The experimental results demonstrate the potential applicability of QD SPSs to practical telecom QKD networks.

Figure 1. Experimental setup of single-photon QKD.

Test-bed system, i.e., a time-bin encoding QKD system based on the standard BB84 protocol. PPG: pulse pattern generator, HJ: hybrid junction. The entire system was operated at the repletion rate of 62.5 MHz.

Figure 2. Photon correlation measurement.

Results of correlation measurements of emitted photons associated with this experiment; g⁽²⁾(τ) is plotted as a function of the time delay τ of the arrival time of the photons. We chose g⁽²⁾(0) = 0.0051 and  = 0.05 at the repetition rate of 62.5 MHz. The lower trace is a magnification of the central part of the upper trace around τ ~ 0, and g⁽²⁾(0) is plotted on a logarithmic scale.

Figure 3. Histograms of detected signal.

Histograms showing the arrival time distributions of photons for the SNSPDs. Only ten bits of data are shown here. Encoded letters are shown in temporal order on the abscissa axis. The predefined temporal positions associated with meaningful events are shown as vertical dashed lines. Successfully decoded events are shown as circles.

Figure 4. QBER.

Measured QBER as a function of transmission distance. Data sent through 50, 100, 110, and 120 km-long SMF spools are shown. The rectangles and circles are, respectively, the best data and the average of the top ten data during the experiment. The solid line is the fit for obtaining some of the experimental parameters.

Figure 5. Key rate as a function of the transmission distance.

Measured raw key rates plotted as a function of transmission distance (red open circles and red dotted line) together with the estimated secure key rates (red closed circles and red solid line). The previous result (50-km-long transmission) is shown for comparison. Orange dotted (solid) and green dotted (solid) lines shows raw (secure) key rates for ideal horn structure and micropillar cavity structure, respectively (see text).