Contributed Talk 2b

Abstract: Quantum key distribution (QKD) has been proven to be an essential technology to resist the threat of quantum computing. Major technological powers such as the United States and countries in Europe have already begun the development of quantum key distribution networks. After years of technological research, China has further developed the operational-level 10,000-kilometre quantum secure communication network for business applications ‘China Quantum Communication Network’ (CN-QCN), developed based on ‘Shanghai-Beijing Backbone’. This report summarizes the advanced practices of CN-QCN in areas such as Backbone and　Metropolitan networks organization and network management. It also highlights several initiatives designed to enhance the network usability, proposes a method for evaluating the availability of quantum networks.

Abstract: The no-cloning theorem leads to information-theoretic security in various quantum cryptographic protocols. However, this security typically derives from a possibly weaker property that classical information encoded in certain quantum states cannot be broadcast. To formally capture this property, we introduce the study of ``orthogonality broadcasting." When attempting to broadcast the orthogonality of two different qubit bases, we establish that the power of classical and quantum communication is equivalent. However, quantum communication is shown to be strictly more powerful for broadcasting orthogonality in higher dimensions. We then relate orthogonality broadcasting to quantum position verification and provide a new method for establishing error bounds in the no pre-shared entanglement model that can address protocols previous methods could not. Our key technical contribution is an uncertainty relation that uses the geometric relation of the states that undergo broadcasting rather than the non-commutative aspect of the final measurements.

Abstract: In this work, we address the challenge of dynamically rerouting quantum key distribution (QKD) links during live operation without the need for a system restart. Our novel resynchronization method, combined with a qubit-based clock frequency recovery algorithm, enables seamless rerouting of quantum channels in software-defined networks (SDNs). We validate our method with our 625 MHz real-time BB84 QKD system, using free-running cost-effective quartz oscillators and without an optical clock channel. The effectiveness of our method is demonstrated by the reliable system operation covering fiber length changes exceeding 100 km and sustaining channel interruptions of multiple minutes. We believe that our findings will significantly enhance the utility of QKD systems and simplify their flexible integration into existing and future telecom infrastructures, including optically switched SDNs.

Abstract: Quantum networks, which integrate multiple quantum computers and the channels connecting them, are crucial for distributed quantum information processing but remain inherently susceptible to channel noise. The channel purification protocol emerges as a promising technique for directly suppressing noise in quantum channels without complex encoding and decoding operations, making it particularly suitable for remote quantum information transmission in optical systems. In this work, leveraging the spatial and polarization degrees of freedom of photons, we propose a novel experimental configuration that efficiently implements the channel purification protocol, utilizing two Fredkin gates to coherently interfere independent noise channels. Based on this configuration, we experimentally demonstrate that the protocol can suppress the noise with various noise levels and forms. Furthermore, we apply our protocol in a practical application of entanglement distribution, showing that the channel purification can effectively protect the distributed entanglement from channel noise.