[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

비만아 부모의 돌봄 경험: q 방법론

The internet-a vast network that enables simultaneous long-range classical communication-has had a revolutionary impact on our world. The vision of a quantum internet is to fundamentally enhance internet technology by enabling quantum communication between any two points on Earth. Such a quantum internet may operate in parallel to the internet that we have today and connect quantum processors in order to achieve capabilities that are provably impossible by using only classical means. Here, we propose stages of development toward a full-blown quantum internet and highlight experimental and theoretical progress needed to attain them.

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Quantum internet: A vision for the road ahead

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.

Satellite-to-ground quantum key distribution

An overview is given of the state-of-the-art research into secure communication based on quantum cryptography. The present security model together with its assumptions, strengths and weaknesses is discussed. Recent experimental progress and remaining challenges are surveyed as are the latest developments in quantum hacking and countermeasures.

Secure quantum key distribution

To improve the data rate in laser communication systems, a new modulation scheme, which uses the photon polarisation property, is proposed. Performance analysis and Monte-Carlo simulation are made, considering both non-turbulent and heavy-turbulent atmospheric conditions. The simulation results show that the data rate is twice as much as that of pulse-position modulation in a broad range of optical link efficiency.

Performance analysis of APD-based photon polarisation modulation system in turbulent atmosphere

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.

Optimization of Block Size for Reconciliation Protocol

Quantum key distribution (QKD) enables secure key exchanges between two remote users. The ultimate goal of secure communication is to establish a global quantum network. The existing field tests suggest that quantum networks are feasible. To achieve a practical quantum network, we need to overcome several challenges, including realising versatile topologies for large scales, simple network maintenance, extendable configuration, and robustness to node failures. To this end, we present a field operation of a quantum metropolitan-area network with 46 nodes and show that all these challenges can be overcome with cutting-edge quantum technologies. In particular, we realise different topological structures and continuously run the network for 31 months, by employing standard equipment for network maintenance with an extendable configuration. We realise QKD pairing and key management with a sophisticated key control center. In this implementation, the final keys have been used for secure communication such as real-time voice telephone, text messaging, and file transmission with one-time pad encryption, which can support 11 pairs of users to make audio calls simultaneously. Combined with inter-city quantum backbone and ground-satellite links, our metropolitan implementation paves the way toward a global quantum network.

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Implementation of a 46-node quantum metropolitan area network

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.

Compression Used in Qubits Distillation

Significant progress has been made in satellite-based quantum key distribution (QKD), and urgent follow-up work is to explore the optimal solution for building practical quantum constellations. Here, we demonstrate successful QKD based on the compact terminal on the Tiangong-2 Space Lab and construct a space–ground quantum network among four ground stations. The medium-inclination orbit of Tiangong-2 Space Lab can obtain multiple available passes for the same ground station in one night, increasing the key generation amount directly. Further analysis results show that the medium-inclination orbit and Sun-synchronous orbit can form good complementarity in future quantum constellations. As a comprehensive demonstration, this work takes a step toward cost-effective quantum satellites and provides a perspective for satellite constellation construction with different orbit types. 

Wei-Yue Liu

Papers

Performance analysis of APD-based photon polarisation modulation system in turbulent atmosphere

Optimization of Block Size for Reconciliation Protocol

Implementation of a 46-node quantum metropolitan area network

Compression Used in Qubits Distillation

Space-ground QKD network based on a compact payload and circular orbit