Secure Quantum Key Distribution over 421 km of Optical Fiber

We present a quantum key distribution system with a 2.5 GHz repetition rate using a three-state time-bin protocol combined with a one-decoy approach. Taking advantage of superconducting single-photon detectors optimized for quantum key distribution and ultralow-loss fiber, we can distribute secret keys at a maximum distance of 421 km and obtain secret key rates of 6.5 bps over 405 km.

Figure 1

Schematics of the experimental setup. Laser: 1550 nm distributed feedback laser; filter: 270 pm bandpass filter; piezo: piezoelectric fiber stretcher; FM: Faraday mirror; IM: intensity modulator; DCF: dispersion compensating fiber; VA: variable attenuator; ULL fiber: ultralow-loss single-mode fiber; BS: beam splitter; SNSPDs: superconducting nanowire single-photon detectors. Dashed lines represent temperature stabilized boxes.

Figure 2

Circles denote experimental final SKR versus fiber length. Triangles denote simulation of an idealized BB84 protocol with the same block sizes as the corresponding experimental points. Squares denote results of other long-distance QKD experiments using finite-key analysis: (1) BB84, Frölich et al. [22]; (2) coherent one-way, Korzh et al. [23]; (3) measurement-device-independent QKD, Yin et al. [3]. (Average fiber loss for: (1): $0.185\mathrm{dB}/\mathrm{km}$; (2): $0.169\mathrm{dB}/\mathrm{km}$; (3): $0.168\mathrm{dB}/\mathrm{km}$; this work: $0.171\mathrm{dB}/\mathrm{km}$.) The upper axis indicates the overall attenuation based on a fiber loss of $0.17\mathrm{dB}/\mathrm{km}$.

Figure 3

System stability over more than 24 h for a distance of 302 km of ULL SMF. (a) RKR, SKR, and (b) corresponding QBER in the $\mathsf{Z}$ basis and ${\varphi }_{\mathsf{Z}}$ as a function of time.