PUBLICATIONS

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.

Frequency-multiplexed quantum communication usually requires a single-shot identification of the frequency mode of a single photon. In this paper, we propose a scheme that can identify the frequency mode with high resolution even for spontaneously emitted photons whose generation time is unknown, by combining the time-to-space and frequency-to-time mode mapping. We also demonstrate the mapping of the frequency mode (100 MHz intervals) to the temporal mode (435 ns intervals) for weak coherent pulses using atomic frequency combs. This frequency interval is close to the minimum frequency mode interval of the atomic frequency comb quantum memory with the Pr³⁺-ion-doped Y₂SiO₅ crystal, and the proposed scheme has the potential to maximize the frequency multiplexing of the quantum repeater scheme with the memory.

In long-distance quantum communication using quantum repeaters with quantum memories, entangled photons at telecommunication wavelengths that can be coupled to quantum memory with high efficiency are required. Typically, entangled photons are generated via spontaneous parametric down conversion (SPDC). However, the phase-matching bandwidth of SPDC is more than 100 GHz, which is much broader than the bandwidth of a Pr³⁺:Y₂SiO₅ quantum memory (with overall bandwidth of ∼10 GHz while the bandwidth of each frequency channel is ∼10 MHz) suitable for frequency-multiplexed quantum repeaters. In this study, nondegenerate SPDC (1550 nm and 995 nm) inside an optical cavity is used to obtain a narrow linewidth and cluster width of SPDC to match the Pr³⁺:Y₂SiO₅ bandwidth. We also developed a cavity control mechanism that can fulfill the doubly resonant condition. The developed two-photon source can maximize the coupling efficiency with Pr³⁺:Y₂SiO₅ by introducing wavelength conversion and is promising for use in a quantum repeater.