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1. Concurrence percolation threshold of large-scale quantum networks

   https://doi.org/10.1038/s42005-022-00958-4  

   Malik, Omar; Meng, Xiangyi; Havlin, Shlomo; Korniss, Gyorgy; Szymanski, Boleslaw Karol; Gao, Jianxi (July 2022, Communications Physics)

   Abstract Quantum networks describe communication networks that are based on quantum entanglement. A concurrence percolation theory has been recently developed to determine the required entanglement to enable communication between two distant stations in an arbitrary quantum network. Unfortunately, concurrence percolation has been calculated only for very small networks or large networks without loops. Here, we develop a set of mathematical tools for approximating the concurrence percolation threshold for unprecedented large-scale quantum networks by estimating the path-length distribution, under the assumption that all paths between a given pair of nodes have no overlap. We show that our approximate method agrees closely with analytical results from concurrence percolation theory. The numerical results we present include 2D square lattices of 200²nodes and complex networks of up to 10⁴nodes. The entanglement percolation threshold of a quantum network is a crucial parameter for constructing a real-world communication network based on entanglement, and our method offers a significant speed-up for the intensive computations involved. 

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2. High-Dimensional Quantum Key Distribution by a Spin-Orbit Microlaser

   https://doi.org/10.1103/PhysRevX.15.011024  

   Zhang, Yichi; Zhao, Haoqi; Wu, Tianwei; Gao, Zihe; Ge, Li; Feng, Liang (February 2025, Physical Review X)

   Quantum key distribution offers a promising avenue for establishing secure communication networks. However, its performance is significantly hampered by the conventional two-level information carriers (i.e., qubits) due to their limited information capacity and noise resilience. A fundamental approach to overcoming these limitations involves the adoption of high-dimensional qudits. Practical qudit platforms require robust propagation, outstanding controllability, and extreme compactness, to which integrated photonics provides a promising solution. Here, we achieved, for the first time, microlaser-enabled high-dimensional quantum communication through leveraging spin-orbit photon qudits, where the dynamical generation and manipulation of these multi-degrees-of-freedom complex quantum state are realized by a non-Hermitian-physics-driven integrated microlaser quantum transmitter. Such a microlaser photon manipulation, as a novel route towards high-dimensional quantum state generation, promises high energy efficiency, along with fast, compact, and precise qudit state reconfigurability. The four spin-orbit eigenstates emitted by the microlaser possess the same spatial-temporal structures, ensuring homogeneity between all qudit states used for key distribution, which effectively eliminates propagation dephasing and walk-off problems, thereby delivering the high-dimensional spin-orbit secret key generation to construct a robust quantum link. The demonstrated long-term system stability showcases the practical potential of the microlaser quantum transmitter, providing a critical step towards compact, high-information-capacity quantum communication networks. Published by the American Physical Society2025 

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3. Photon-mediated interactions between quantum emitters in a diamond nanocavity

   https://doi.org/10.1126/science.aau4691  

   Evans, R. E.; Bhaskar, M. K.; Sukachev, D. D.; Nguyen, C. T.; Sipahigil, A.; Burek, M. J.; Machielse, B.; Zhang, G. H.; Zibrov, A. S.; Bielejec, E.; et al (September 2018, Science)

   Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically mediated interactions. Such controlled interactions will be crucial in developing cavity-mediated quantum gates between spin qubits and for realizing scalable quantum network nodes. 

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4. Distributed quantum sensing with a mode-entangled network of spin-squeezed atomic states

   https://doi.org/10.48550/arXiv.2205.06382  

   Malia, B.K.; Wu, Y.; Martínez-Rincón, J.; Kasevich, M.A. (July 2022, ArXivorg)

   Quantum sensors are used for precision timekeeping, field sensing, and quantum communication. Comparisons among a distributed network of these sensors are capable of, for example, synchronizing clocks at different locations. The performance of a sensor network is limited by technical challenges as well as the inherent noise associated with the quantum states used to realize the network. For networks with only local entanglement at each node, the noise performance of the network improves at best with square root of the number of nodes. Here, we demonstrate that nonlocal entanglement between network nodes offers better scaling with network size. A shared quantum nondemolition measurement entangles a clock network with up to four nodes. This network provides up to 4.5 dB better precision than one without nonlocal entanglement, and 11.6 dB improvement as compared to a network of sensors operating at the quantum projection noise limit. We demonstrate the generality of the approach with atomic clock and atomic interferometer protocols, in scientific and technologically relevant configurations optimized for intrinsically differential comparisons of sensor outputs. 

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5. Cavity piezo-mechanics for superconducting-nanophotonic quantum interface

   https://doi.org/10.1038/s41467-020-17053-3  

   Han, Xu; Fu, Wei; Zhong, Changchun; Zou, Chang-Ling; Xu, Yuntao; Sayem, Ayed Al; Xu, Mingrui; Wang, Sihao; Cheng, Risheng; Jiang, Liang; et al (June 2020, Nature Communications)

   Abstract Hybrid quantum systems are essential for the realization of distributed quantum networks. In particular, piezo-mechanics operating at typical superconducting qubit frequencies features low thermal excitations, and offers an appealing platform to bridge superconducting quantum processors and optical telecommunication channels. However, integrating superconducting and optomechanical elements at cryogenic temperatures with sufficiently strong interactions remains a tremendous challenge. Here, we report an integrated superconducting cavity piezo-optomechanical platform where 10 GHz phonons are resonantly coupled with photons in a superconducting cavity and a nanophotonic cavity at the same time. Taking advantage of the large piezo-mechanical cooperativity (C_em ~7) and the enhanced optomechanical coupling boosted by a pulsed optical pump, we demonstrate coherent interactions at cryogenic temperatures via the observation of efficient microwave-optical photon conversion. This hybrid interface makes a substantial step towards quantum communication at large scale, as well as novel explorations in microwave-optical photon entanglement and quantum sensing mediated by gigahertz phonons. 

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