Nitrogen-vacancy (NV) centers in diamond have been developed into essential hardware units for a wide range of solid-state-based quantum technology applications. While such applications require the long spin coherence times of the NV centers, they are often limited due to decoherence. In this study, we theoretically investigate the decoherence of NV-spin ensembles induced by nitrogen impurities (P1 centers), which are one of the most dominant and inevitable magnetic field noise sources in diamond. We combined cluster correlation expansion and density functional theory to compute the Hahn-echo spin-coherence time of the NV centers for a broad range of P1 concentrations. Results indicate a clear linear dependence of T₂ on P1 concentrations on a log scale with a slope of −1.06, which is in excellent agreement with previous experimental results. The interplay between the Jahn–Teller effect and the hyperfine interaction in the P1 center plays a critical role in determining the bath dynamics and the resulting NV decoherence. Our results provide a theoretical upper bound for the NV-spin T₂ over a wide range of P1 densities, serving as a key reference for materials optimization and spin bath characterization to develop highly coherent NV-based devices for quantum information technology.

Developing quantum key distribution (QKD) has been recently directed toward distance extension and network expansion for real-world secure communications. Considering a recent report on a quantum communication network over 4,600 km, it seems that QKD networks using conventional protocols have been sufficiently studied. However, although the twin-field QKD (TF-QKD) proposed for long-distance QKD has been studied deeply enough to succeed the demonstrations over 428- and 511-km deployed fibers, TF-QKD networks have been verified only for a ring network. In this work, we propose a star topological 2 × N TF-QKD network scheme, where the coherence maintenance issue, being the primary obstacle to implementing TF-QKD, can be minimized by the automatic mode-matching feature of the Sagnac-based plug-and-play architecture. A lower number of active controllers is required for our scheme in comparison with one-way TF-QKD networks. Moreover, our scheme adopts a cost-effective configuration that requires only a single pair of single-photon detectors for the entire network system. We conducted a proof-of-concept experiment over a 50-km fiber successfully, achieving an average secret key rate of 1.31 × 10⁻⁴bit per pulse (1.52 bit per second) with the finite-size effect.

We present a true random number generator (TRNG) using dark noise of a CMOS image sensor. Because the proposed TRNG is based on the dark characteristics of the CMOS image sensor, it does not require any additional hardware, such as light source and optics, for providing true randomness. Therefore, it can be a promising solution for compact and low-cost mobile application. By using NIST SP 800-90B entropy assessment suite, we evaluate the min-entropy for the raw outputs of our original noise source and the final random numbers including post-processing as well. We also adopt NIST SP 800-22 statistical randomness test suite for the evaluation of the random numbers. The test results demonstrate that the generated random numbers pass all the statistical tests and have high entropy.

Metasurface technology is revolutionizing the field of optics and pursuing expanded functions via technical developments, such as the integration of multiple metasurfaces with optical fibers. Despite several attempts to realize metasurface‐on‐fiber platforms, negligible fiber‐facet areas pose a serious obstacle to efficient and precise fabrication. Herein, we demonstrate a novel sequential micro‐punching process that enables rapid and precise stacking of multiple polymer metasurfaces on the end face of a single‐mode optical fiber. Mesh‐type nanohole metasurfaces are fabricated on a 1.8‐μm‐thick polymethyl methacrylate (PMMA) layer via e‐beam lithography, and the PMMA layer is separated from the substrate and prepared in the form of a membrane using the external frame. Furthermore, the PMMA metasurfaces are sequentially punched through the fiber and stacked on top. Employing a micro‐punching process, we demonstrate highly efficient all‐polymer metalenses and orbital angular momentum (OAM) metasurfaces coupled with single‐mode fibers operating in the telecommunication band. A 1550 nm laser beam passing through three metalens layers stacked on the fiber is focused at a distance of 135 μm with 83% efficiency. In addition, the 1550 nm beam passing through three OAM metasurfaces on the fiber is converted into a perfect vortex beam with a topological charge of 3. We believe that our proposed micro‐punching process will cause a breakthrough in the fabrication of metasurface‐integrated optical fibers that will be utilized in a wide range of applications.

We have implemented a practical plug-and-play measurement-device-independent quantum key distribution (MDI QKD) system with polarization division multiplexing (PDM). MDI QKD is known as a secure protocol that can inherently prevent detection loopholes. Significant efforts have been made toward implementing MDI QKD systems. Recently, a plug-and-play architecture has been proposed for implementing the MDI QKD system, and its feasibility has been experimentally verified in both free space and fiber channel. However, in order to apply it to the real world, it is necessary to develop a more practical architecture including multiplexing methods and self-compensation techniques. In this paper, we have proposed and implemented a practical plug-and-play MDI QKD architecture that can be implemented regardless of whether the quantum channel lengths of Alice and Bob are symmetric or asymmetric using PDM and can be operated even under ambient-temperature-changing environments through an optical path-length self-compensation technique. Experimentally, we have achieved $6.25\times 10^{-6}$ bits per pulse as the key rate and the quantum bit error rates under 3% on 25-km quantum channels.