This came up recently for an application proposal.
China Breaks Record for Quantum Memory Entanglement Distance
Tom's Hardware
Francisco Pires, August 17, 2022
Researchers in China have entangled two quantum memories across a record distance, bringing a quantum Internet a step closer to realization. The entangled memories could maintain coherence and transfer a photon between two separate laboratories, because the entangled quantum bits correlate so that their states cannot be described separately. The researchers entangled one quantum memory in the first lab and excited it with a laser, releasing the excess energy as a photon when the memory reverted to its ground state. A fiber-optic cable then transmitted the photon 12.5 kilometers (7.7 miles) from an original node to the destination node, allowing the researchers to entangle a new quantum memory. The researchers said they bypassed the photon's low energy using "the quantum frequency conversion technique to shift the photon's wavelength to 1,342 nm [nanometers] instead, which improves the overall transmission efficiency significantly." ... '
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