Quantum key distribution (QKD)1,2 has the potential to enable secure communication and information transfer3. In the laboratory, the feasibility of point-to-point QKD is evident from the early proof-of-concept demonstration in the laboratory over 32 centimetres4; this distance was later extended to the 100-kilometre scale5,6 with decoy-state QKD and more recently to the 500-kilometre scale7,8,9,10 with measurement-device-independent QKD. Several small-scale QKD networks have also been tested outside the laboratory11,12,13,14. However, a global QKD network requires a practically (not just theoretically) secure and reliable QKD network that can be used by a large number of users distributed over a wide area15. Quantum repeaters16,17 could in principle provide a viable option for such a global network, but they cannot be deployed using current technology18. Here we demonstrate an integrated space-to-ground quantum communication network that combines a large-scale fibre network of more than 700 fibre QKD links and two high-speed satellite-to-ground free-space QKD links. Using a trusted relay structure, the fibre network on the ground covers more than 2,000 kilometres, provides practical security against the imperfections of realistic devices, and maintains long-term reliability and stability. The satellite-to-ground QKD achieves an average secret-key rate of 47.8 kilobits per second for a typical satellite pass—more than 40 times higher than achieved previously. Moreover, its channel loss is comparable to that between a geostationary satellite and the ground, making the construction of more versatile and ultralong quantum links via geosynchronous satellites feasible. Finally, by integrating the fibre and free-space QKD links, the QKD network is extended to a remote node more than 2,600 kilometres away, enabling any user in the network to communicate with any other, up to a total distance of 4,600 kilometres.
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The data presented in the figures and that support the findings of this study are available from the corresponding authors on reasonable request.
The code used for modelling the data is available from the corresponding authors on reasonable request.
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This work was supported by the National Development and Reform Commission, the Department of Science and Technology of Shangdong province, Anhui Development and Reform Commission, the China Banking Regulatory Commission, the CAS, the NNSFC and the National Key R&D Program of China.
Each of an entity controlled by USTC, C.-Z.P. and J.-W.P., holds shares in QuantumCTek Co., Ltd. (“QuantumCTek”), a public company listed on SSE STAR Market (Shanghai Stock Exchange Sci-Tech Innovation Board). C.-Z.P. is also the Chairman of QuantumCTek on behalf of the university without receiving any compensation.
Peer review information Nature thanks the anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, Overview of the typical hardware. b, QKD devices. c, Control and classical communication devices. SPD, single-photon detector; PDU, power distribution unit.
a, The 1.2-m telescope at the Nanshan ground station. b, The 1-m telescope at the Xinglong ground station.
a, The upgraded receiving optics at the Xinglong ground station. b, c, The two major changes were the beam expander (b) and the BB84 module (c). A beam expander with lower magnification is used to increase the field of view. The BB84 module is modified to match the larger size of the incident light beam. A beam splitter with a bias of 10:90 (50:50) for the X–Z basis in the BB84 module is used in Nanshan (Xionglong).
On Alice’s side, four signal lasers (S) and four decoy lasers (D) are combined via polarization-maintaining filter couplers (PMFCs), and then combined again into a single PMFC (with 45° difference for fibre-axis inputs), before outputting through a single-mode fibre coupler (SMFC). One beam is for testing; another goes to the optical circulator associated with a 10G fibre Bragg grating filter (FBG) before monitoring and attenuating with two cascaded MEMS attenuators. The synchronization laser (SYN) goes through a dense wavelength-division multiplexing (DWDM) and then a MEMS attenuator, before splitting for monitoring and outputting to the communication channel. On Bob’s side, the signal light enters first a DWDM and then a SMFC, with one set of electrically driven polarization controllers (EPCs) consisting of three components in every path, before single-mode polarizing beam splitters (SMPBSs) detect the four different signal states. The synchonization laser also propagates via a DWDM and is detected by a PIN photodetector. PMPBS, polarization-maintaining polarizing beam splitter; Com, common port for the coarse wavelength-division multiplexing (CWDM) device; Pass, pass port for the CWDM device; Ref, reflect port for the CWDM device; H, quantum state |H⟩; V, quantum state |V⟩; P and +, quantum state |+⟩; N and −, quantum state |−⟩.
The transmitter and the receiver are combined in the same terminal, with a channel multiplexing module including a DWDM and two circulators (CIR). They are similar to those of the 625-MHz system. The main difference is that the system operates at a frequency of 40 MHz. The transmitter adopts one laser doide with two different driving signals for decoy-state modulation instead of two, and there is no a FBG for wavelength filtering. The InGaAs/InP detector of the receiver is operated in rectangular-wave gated mode at a frequency of 40 MHz, with a detection efficiency of about 15% at a gate width of 1.6 ns and a coincidence width of 400 ps. Its dark count rate is less than 250 counts per second, and the after pulse probability is less than 1% at a dead time of 5 μs. VOA, variable optical attenuator; LH, laser diode for quantum state |H⟩; LV, laser diode for quantum state |V⟩; LP, laser diode for quantum state |+⟩; LN, laser diode for quantum state |−⟩; DH, detector for quantum state |H⟩; DV, detector for quantum state |V⟩; DP, detector for quantum state |+⟩; DN, detector for quantum state |−⟩.
Extended Data Fig. 6 Schematic of 40-MHz transceiver QKD system with a single-laser transmitter and a passive receiver.
The transmitter and the receiver are combined in the same terminal by a channel multiplexing module including two DWDMs and a circulator. In the transmitter, the pulse generated by one laser diode first passes through an intensity modulator (IM) for decoy-state active modulation, and then a custom-made Sagnac interferometer module, which consists of a polarization-sensitive circulator (PCIR) and a phase modulator (PM) for polarization-state modulation. The polarization-sensitive circulator is an integrated micro-optical system that combines a circulator and a polarizing beam splitter (PBS), with their directions offset by 45° (inset). TIA, transimpedance amplifier; SIG, signal.
Supplementary Figures providing further results for the reliability test for the Jinan QKD network (Supplementary Fig. 1), the backbone network in 2018 (Supplementary Fig. 2) and 2019 (Supplementary Fig. 3), in days (Supplementary Fig. 4), in hours (Supplementary Fig. 5), and in minutes (Supplementary Fig. 6).
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Chen, YA., Zhang, Q., Chen, TY. et al. An integrated space-to-ground quantum communication network over 4,600 kilometres. Nature 589, 214–219 (2021). https://doi.org/10.1038/s41586-020-03093-8