PUBLICATIONS

Telecommunication wavelength-entangled photon sources (EPS) are indispensable systems for a fiber-based quantum network. We developed a Sagnac-type spontaneous parametric down conversion systemadopting a Fresnel rhomb as a wideband and reasonable retarder. This novelty, to the best of ourknowledge, enables the generation of a highly nondegenerate two-photon entanglement comprising thetelecommunication wavelength (1550 nm) and quantum memory wavelength (606 nm for Pr:YSO) with onlyone nonlinear crystal. Quantum state tomography was performed to evaluate the degree of entanglement,and the fidelity with a Bell state |Φ+⟩ with a maximum of 94.4% was obtained. Therefore, this paper showsthe potential of nondegenerate EPSs that are compatible with both telecommunication wavelength andquantum-memory wavelength to be installed in quantum repeater architecture.

The implementation of quantum repeaters needed for long-distance quantum communication requires the generation of quantum entanglement distributed among the elementary links. These entanglements must be swapped among the quantum repeaters through Bell-state measurements. This study aims to improve the entanglement generation rate by frequency multiplexing the Bell-state measurements. As a preliminary step of the frequency-multiplexed Bell-state measurements, three frequency modes are mapped to a temporal mode by an atomic frequency comb prepared in Pr³⁺ ion-doped Y₂SiO₅ crystals using a weak coherent state, and Hong-Ou-Mandel interference, which is a measure of the indistinguishability of two inputs, is observed in each frequency mode by coincidence detection. The visibility for all the modes was 40%–42%（theoretically up to 50%）.Furthermore, we show that a mixture of different modes is avoided.The present results are connected to frequency-selective Bell-state measurements and therefore frequency-multiplexed quantum repeaters.

The ability to transmit quantum states over long distances is a fundamental requirement of the quantum internet and is reliant upon quantum repeaters. Quantum repeaters involve entangled photon sources that emit and deliver photonic entangled states at high rates and quantum memories that can temporarily store quantum states. Improvement of the entanglement distribution rate is essential for quantum repeaters, and multiplexing is expected to be a breakthrough. However, limited studies exist on multiplexed photon sources and their coupling with a multiplexed quantum memory. Here, we demonstrate the storing of a frequency-multiplexed two-photon source at telecommunication wavelengths in a quantum memory accepting visible wavelengths via wavelength conversion after 10-km distribution. To achieve this, quantum systems are connected via wavelength conversion with a frequency-stabilization system and a noise-reduction system. The developed system is stably operated for more than 42 h. Therefore, it can be applied to quantum repeater systems comprising various physical systems requiring long-term system stability.