  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 was 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.
