Heonoh Kim

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.