PUBLICATIONS

In quantum communication with quantum repeaters, multiplexed quantum memory is expected to enhance communication rates. When using an atomic frequency comb (AFC) for on-demand storage, the frequency mode number is often limited by the optical power of the control pulses. Here, using a space-coupled waveguide electro-optic modulator, we increased the output power, allowing us to apply control pulses to multiple modes simultaneously. Further, through enhancement of an experimental setup that increases power density, we increased the number of modes. Consequently, we pioneered, to the best of our knowledge, on-demand storage using five modes of AFC. This technology is a significant achievement toward frequency-multiplexed on-demand quantum memory.

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.