Researchers from the University of Science and Technology of China (USTC) have developed a multimodal quantum repeater based on absorptive memory and have successfully increased optical storage time to one hour—a huge increase over the previous record of one minute. This repeater uses high-performance solid-state memory, based on rare-earth-ion-doped crystals, which was developed by the research team. The new optical storage record is an important step toward realizing a quantum USB flash drive. 

Currently, optical fiber networks are spread all over the world, and light has become the basic carrier of modern information transmission. The speed of light is as high as 300,000 kilometers per second. To reduce the speed of light or even let the light stay, i.e., light storage, are the goals of the international academic community. 

Light storage is particularly important in the field of quantum communication. Quantum relays can be constructed based on light quantum storage, thus overcoming channel loss and establishing a large-scale quantum network. 

Another solution for long-distance quantum communication is a quantum USB flash drive, which would store photons in ultra-long-life quantum memory and then transmit quantum information by directly transporting the quantum USB flash drive. Quantum USB flash drives could be widely used in global satellite quantum communications and other fields. 

By employing a zero first-order-Zeeman magnetic field and dynamical decoupling to protect the spin coherence in a solid, the research teams of GUO Guangcan and ZHOU Zongquan demonstrated coherent storage of light in an atomic frequency comb memory for over one hour, suggesting a promising future for large-scale quantum communication based on long-lived solid-state quantum memory. 

Quantum repeaters based on absorptive quantum memory can overcome distance limitations on entanglement distribution by separating the quantum memory and quantum light sources. GUO and ZHOU’s teams present an experimental demonstration of heralded entanglement between absorptive quantum memories.  

They built two nodes separated by 3.5 meters, each containing a polarization-entangled photon-pair source and a solid-state quantum memory with bandwidth up to 1 gigahertz. The quantum nodes and channels served as an elementary quantum repeater link. 

Wideband absorptive quantum memories used in the nodes are compatible with deterministic entanglement sources and can simultaneously support multiplexing, thus paving the way for the construction of practical solid-state quantum repeaters and high-speed quantum networks. 

Results of the studies were published in Nature entitled "Noiseless photonic non-reciprocity via optically-induced magnetization" and "Heralded entanglement distribution between two absorptive quantum memories" and in Physical Review Letters.

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