The ability to distribute entangled photons over long distances in both free space and through optical fiber remains challenging. Even though methods are in development to extend that distance by preparing atom-photon entangled states in remote nodes and sending them to an intermediate node for interference, entanglement of stationary qubits (whether using single atoms, quantum dots, trapped ions, or other quantum ensembles) has only reached a maximum fiber distance of 1.7 km between two physically separated nodes.
Recognizing that quantum memories—the equivalent of computer memory where instead of 1s and 0s, entangled qubits are the information stream—are a crucial element in quantum information systems, Chinese researchers have been able to extend quantum memory entanglement over standard optical fibers to 22 km using two-photon interference techniques (and even to 50 km using single-photon interference).1 The breakthrough was a collaborative effort between researchers from the University of Science and Technology of China (USTC; Hefei, China), the Jinan Institute of Quantum Technology (Jinan, China), and the Shanghai Institute of Microsystem and Information Technology (SIMIT) at the Chinese Academy of Sciences (Shanghai, China).