Heonoh Kim

Distributed entanglement is the foundational resource of quantum networks, enabling quantum key distribution, teleportation, and entanglement swapping between distant nodes. However, as transmission distance increases, channel loss and environmental noise inevitably degrade entanglement quality. Entanglement purification protocols (EPPs) are therefore an essential ingredient of practical quantum repeaters. A central challenge in implementing EPPs for long-distance quantum communication is the choice of photonic encoding. Time-bin encoding is well suited to optical-fiber transmission and has demonstrated robust operation at telecommunication wavelengths. Recent experiments have further distributed fully controllable time-bin entangled states over 100-km single-mode fibers with high two-photon visibility and state fidelity. However, the early-late temporal separation that makes time-bin encoding robust for fiber transmission also complicates local entangling operations: logic operations between time-bin qubits generally require coherent manipulation of early and late temporal modes, making their implementation with static linear optics alone highly nontrivial. In this work, we develop an implementation-level feasibility analysis of entanglement purification for time-bin entangled photons. We evaluate candidate purification architectures using time-bin specific implementation criteria, including early-late temporal-mode interference, active switching or temporal demultiplexing, interferometric phase stability, ancillary resource requirements, and post-selection or feed-forward control. This operation-level comparison identifies the main bottleneck that distinguishes time-bin purification from polarization-based linear-optical schemes: the difficulty of implementing temporal-mode parity checks or CNOT-like operations with passive optical elements alone. We examine candidate implementations of time-bin-compatible EPPs, including temporal-to-spatial or temporal-to-polarization mode conversion followed by linear-optical parity checking, active-switch-assisted temporal-mode operations, and nondestructive-measurement-assisted or memory-compatible node-level approaches. We further discuss how key realization issues, such as optical loss, interferometric visibility, detector efficiency, and conversion overhead, affect the feasibility of each approach in terms of fidelity improvement, success probability, and resource requirements. This analysis provides an implementation-oriented perspective on the experimental trade-offs between conversion-assisted linear-optical purification and resource-intensive deterministic or near-deterministic approaches for long-distance fiber quantum communication.

Quantum key distribution (QKD) is a promising technology for secure communication, particularly for satellite-based global networks, but its implementation is limited by atmospheric turbulence. In this study, we develop a 1.3 km campus-scale free-space optical link as a testbed for entanglement-based QKD and implement a Sagnac-based polarization-entangled photon source using a type-II PPKTP crystal. The photon-pair generation rate is characterized as a function of pump power, showing slopes of 244.8 kHz/mW with a 10 nm bandpass filter, achieving up to 1 MHz pair rate. The coincidence-to-accidental ratio (CAR) is also measured, revealing a trade-off between pair rate and multi-pair-induced degradation. These results establish a robust platform for future entanglement distribution over atmospheric channels and provide a foundation for satellite-based QKD systems.

While long-distance fiber QKD suffers from transmission loss, satellite-to-ground quantum key distribution (Sat-QKD) can extend key distribution through free-space links. However, to make such keys usable in terrestrial QKD networks, a satellite link must be connected to ground fiber infrastructure through wavelength and encoding interfaces. This work studies a simulation framework for Sat-QKD as a key-supply layer for fiber-based QKD networks. The framework focuses on downlink decoy-state BB84 and combines LEO pass geometry, candidate OGS atmospheric data, receiver/detector models, QBER estimation, and finite-key analysis. After satellite-to-OGS free-space collection, an OGS-side optical-conversion interface is modeled before ground fiber extension. The simulator compares direct backhaul, 850-to-1550 nm quantum frequency conversion (QFC), polarization to time-bin degree-of-freedom (DOF) conversion, and the combined QFC + DOF path. The results show that the preferred wavelength depends on the link section: 850 nm provides higher annual finite-key volume in the satellite free-space link, while 1550 nm is favorable for the fiber-extension layer. QFC connects the free-space-favorable 850 nm link to the fiber-favorable 1550 nm layer, and DOF conversion introduces conversion loss but suppresses polarization-drift-induced QBER. The framework is intended as an engineering-level tool for comparing candidate OGS sites and optical-conversion paths for satellite–fiber QKD network extension.

In 1995, a research team including P. G. Kwiat developed high-intensity entangled photon pair sources. In 2000, C. Simon and D. Bouwmeester theoretically studied the multi-photon effects of entangled photon pairs. Such research provides valuable tools for designing and analyzing longdistance entanglement distribution experiments in high-loss regime or satellite-to-ground QKD systems, which require high-intensity entangled photon pair sources. In this study, unlike previous papers that analyzed only specific scenarios, we numerically presented measurement results achievable through analysis in general scenarios. Based on these measured probability values, we confirmed that the results of state tomography. To prepare entangled photon pair sources, the spontaneous parametric down-conversion (SPDC) phenomenon is commonly utilized. A notable characteristic of this SPDC phenomenon is that as the intensity of the incident pump beam increases, the quantum state tends to take the form of a two-mode squeezed vacuum (TMSV) state. In this scenario, where photons corresponding to each arm of the quantum state, namely the idler and signal photons, are distributed to Alice and Bob, respectively, the quantum state undergoing loss channels can be analyzed in the beam splitter scheme. Leveraging this fact, we analytically calculated the probability values of measurement outcomes in a general scenario where Alice and Bob each have different loss channels and measurement bases are determined by their choices. Here, we assumed that both of Alice and Bob employ systems utilizing two threshold detectors for photon measurements. Analyzing the computed measurement outcomes reveals that the resulting state always takes the form of the Werner state, regardless of factors such as the intensity of the entangled photon pair source, channel losses, and the relative measurement basis angles of Alice and Bob. This suggests that even in long-distance entanglement distribution experiments in high-loss regime, the optimization and preservation of measurement bases using twirling techniques can be applied effectively, even when using high-intensity entangled photon pair sources.

Quantum information technologies that utilize entangled photon pairs assume a single- photon source. While this assumption poses no significant issues when the channel loss is low, high loss can have a detrimental impact on the system's performance. To overcome high loss, the most intuitive solution is to increase the gain of entangled photon pairs by sending a large quantity of them. However, high-gain sources tend to degrade the quantum quality of entangled photon pair sources. We derived the density matrix of the quantum state in the distribution of polarization-entangled photon pairs under the non- symmetric channel losses with threshold detectors. We analyzed the variation of the CHSH inequality parameter S and the effective photon state transfer probability 𝑁𝑚 by changing the non-linear gain γ. The increase and subsequent decrease in Nm with increasing γ can be interpreted as follows: when γ is small, the state is not properly transmitted due to high loss, but as γ increases, the error probability, such as double-click events, increases due to the influence of multi-photon events, leading to a decrease in Nm. This result indicates the need to optimize the brightness of the light source for practical implementation in quantum information technologies. This study is expected to contribute to the analysis of discrete variable quantum key distribution(DVQKD) systems like BBM92, E91, and long- distance quantum imaging systems in the future.