Search

Networks of optical clocks find applications in precise navigation1,2, in efforts to redefine the fundamental unit of the ‘second’3–6and in gravitational tests7. As the frequency instability for state-of-the-art optical clocks has reached the 10−19level8,9, the vision of a global-scale optical network that achieves comparable performances requires the dissemination of time and frequency over a long-distance free-space link with a similar instability of 10−19. However, previous attempts at free-space dissemination of time and frequency at high precision did not extend beyond dozens of kilometres10,11. Here we report time–frequency dissemination with an offset of 6.3 × 10−20± 3.4 × 10−19and an instability of less than 4 × 10−19at 10,000 s through a free-space link of 113 km. Key technologies essential to this achievement include the deployment of high-power frequency combs, high-stability and high-efficiency optical transceiver systems and efficient linear optical sampling. We observe that the stability we have reached is retained for channel losses up to 89 dB. The technique we report can not only be directly used in ground-based applications, but could also lay the groundwork for future satellite time–frequency dissemination.

Quantum key distribution (QKD)1,2has 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 scale5,6with decoy-state QKD and more recently to the 500-kilometre scale7–10with measurement-device-independent QKD. Several small-scale QKD networks have also been tested outside the laboratory11–14. 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 repeaters16,17could 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.

High-precision time synchronization for remote clocks plays an important role in fundamental science1–3and real-life applications4,5. However, current time synchronization techniques6,7have been shown to be vulnerable to sophisticated adversaries8. There is a compelling need for fundamentally new methods to distribute high-precision time information securely. Here, we propose a satellite-based quantum-secure time transfer (QSTT) scheme based on two-way quantum key distribution in free space and experimentally verify the key technologies of the scheme via the Micius quantum satellite. In QSTT, a quantum signal (for example, a single photon) is used as the carrier for both the time transfer and the secret-key generation, offering quantum-enhanced security for transferring the time signal and time information. We perform a satellite-to-ground time synchronization using single-photon-level signals and achieve a quantum bit error rate of less than 1%, a time data rate of 9 kHz and a time-transfer precision of 30 ps. These results offer possibilities towards an enhanced infrastructure for a time-transfer network, whose security stems from quantum physics.

Quantum key distribution (QKD)1–3is a theoretically secure way of sharing secret keys between remote users. It has been demonstrated in a laboratory over a coiled optical fibre up to 404 kilometres long4–7. In the field, point-to-point QKD has been achieved from a satellite to a ground station up to 1,200 kilometres away8–10. However, real-world QKD-based cryptography targets physically separated users on the Earth, for which the maximum distance has been about 100 kilometres11,12. The use of trusted relays can extend these distances from across a typical metropolitan area13–16to intercity17and even intercontinental distances18. However, relays pose security risks, which can be avoided by using entanglement-based QKD, which has inherent source-independent security19,20. Long-distance entanglement distribution can be realized using quantum repeaters21, but the related technology is still immature for practical implementations22. The obvious alternative for extending the range of quantum communication without compromising its security is satellite-based QKD, but so far satellite-based entanglement distribution has not been efficient23enough to support QKD. Here we demonstrate entanglement-based QKD between two ground stations separated by 1,120 kilometres at a finite secret-key rate of 0.12 bits per second, without the need for trusted relays. Entangled photon pairs were distributed via two bidirectional downlinks from the Micius satellite to two ground observatories in Delingha and Nanshan in China. The development of a high-efficiency telescope and follow-up optics crucially improved the link efficiency. The generated keys are secure for realistic devices, because our ground receivers were carefully designed to guarantee fair sampling and immunity to all known side channels24,25. Our method not only increases the secure distance on the ground tenfold but also increases the practical security of QKD to an unprecedented level.

An arbitrary unknown quantum state cannot be measured precisely or replicated perfectly. However, quantum teleportation enables unknown quantum states to be transferred reliably from one object to another over long distances, without physical travelling of the object itself. Long-distance teleportation is a fundamental element of protocols such as large-scale quantum networks and distributed quantum computation. But the distances over which transmission was achieved in previous teleportation experiments, which used optical fibres and terrestrial free-space channels, were limited to about 100 kilometres, owing to the photon loss of these channels. To realize a global-scale ‘quantum internet’ the range of quantum teleportation needs to be greatly extended. A promising way of doing so involves using satellite platforms and space-based links, which can connect two remote points on Earth with greatly reduced channel loss because most of the propagation path of the photons is in empty space. Here we report quantum teleportation of independent single-photon qubits from a ground observatory to a low-Earth-orbit satellite, through an uplink channel, over distances of up to 1,400 kilometres. To optimize the efficiency of the link and to counter the atmospheric turbulence in the uplink, we use a compact ultra-bright source of entangled photons, a narrow beam divergence and high-bandwidth and high-accuracy acquiring, pointing and tracking. We demonstrate successful quantum teleportation of six input states in mutually unbiased bases with an average fidelity of 0.80 ± 0.01, well above the optimal state-estimation fidelity on a single copy of a qubit (the classical limit). Our demonstration of a ground-to-satellite uplink for reliable and ultra-long-distance quantum teleportation is an essential step towards a global-scale quantum internet.

Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. However, the distance over which QKD is achievable has been limited to a few hundred kilometres, owing to the channel loss that occurs when using optical fibres or terrestrial free space that exponentially reduces the photon transmission rate. Satellite-based QKD has the potential to help to establish a global-scale quantum network, owing to the negligible photon loss and decoherence experienced in empty space. Here we report the development and launch of a low-Earth-orbit satellite for implementing decoy-state QKD—a form of QKD that uses weak coherent pulses at high channel loss and is secure because photon-number-splitting eavesdropping can be detected. We achieve a kilohertz key rate from the satellite to the ground over a distance of up to 1,200 kilometres. This key rate is around 20 orders of magnitudes greater than that expected using an optical fibre of the same length. The establishment of a reliable and efficient space-to-ground link for quantum-state transmission paves the way to global-scale quantum networks.

In the past, long-distance free-space quantum communication experiments could only be implemented at night. During the daytime, the bright background sunlight prohibits quantum communication in transmission under conditions of high channel loss over long distances. Here, by choosing a working wavelength of 1,550 nm and developing free-space single-mode fibre-coupling technology and ultralow-noise upconversion single-photon detectors, we have overcome the noise due to sunlight and demonstrate free-space quantum key distribution over 53 km during the day. The total channel loss is ∼48 dB, which is greater than the 40 dB channel loss between the satellite and ground and between low-Earth-orbit satellites. Our system thus demonstrates the feasibility of satellite-based quantum communication in daylight. Moreover, given that our working wavelength is located in the optical telecom band, our system is naturally compatible with ground fibre networks and thus represents an essential step towards a satellite-constellation-based global quantum network.

Reconciliation protocol is used in quantum cryptography to correct errors in qubits by exchanging information via classical channel. Winnow is a widely used efficient reconciliation protocol. It separates qubits into blocks, then utilizes Hamming coding to remove bit errors in each block. Performance of Winnow protocol relies deeply on the size of the blocks. In this paper, implementation of winnow protocol is introduced and optimization of block size for the protocol is analyzed. It is shown through experiment results that the optimized value of block size is 0.4/BER, where BER is bit error rate of qubits.

Qubits distillation is used to extract key in quantum cryptography. However, it may transmit much data when two parities distill the keys via the synchronous light time and keys’ pulse position. In order to reduce the transmitted data, we compress the information by arithmetic code. In this paper, an improved qubits distillation has been implemented. It is shown through experiment results that the data compression rate of arithmetic code is about 0.13; the transmitted data with compression is less than that without compression. It is suitable for us to distill the qubits with the narrow bandwidth.

Reconciliation protocol is used in quantum key distribution to remove residual errors in qubits distilled through quantum channel. This paper introduces the implementation of reconciliation protocol in Digital Signal Processor. Memory required by the implementation is only 96K bytes. TI TMS2812 with 128K bytes off-chip memory is used for the implementation and the performance evaluation is carried out.