Quantum networks play an extremely important role in quantum information science, with application to quantum communication, computation, metrology, and fundamental tests. One of the key challenges for implementing a quantum network is to distribute entangled flying qubits to spatially separated nodes, at which quantum interfaces or transducers map the entanglement onto stationary qubits. The stationary qubits at the separated nodes constitute quantum memories realized in matter while the flying qubits constitute quantum channels realized in photons. Dedicated efforts around the world for more than 20 years have resulted in both major theoretical and experimental progress toward entangling quantum nodes and ultimately building a global quantum network. Here, the development of quantum networks and the experimental progress over the past two decades leading to the current state of the art for generating entanglement of quantum nodes based on various physical systems such as single atoms, cold atomic ensembles, trapped ions, diamonds with nitrogen-vacancy centers, and solid-state host doped with rare-earth ions are reviewed. Along the way, the merits are discussed and the potential of each of these systems toward realizing a quantum network is compared.

Advanced 1.5-µm emitting materials that can be used to fabricate electrically driven light-emitting devices have the potential for developing cost-effective light sources for integrated silicon photonics. Sensitized erbium (Er 3+) in organic materials can give bright 1.5-µm luminescence and provide a route for realizing 1.5-µm organic light emitting diodes (OLEDs). However, the Er 3+ electroluminescence (EL) intensity needs to be further improved for device applications. Herein, an efficient 1.5-µm OLED made from a sensitized organic Er 3+ co-doped system is realized, where a "traditional" organic phosphorescent molecule with minimal triplet-triplet annihilation is used as a chromophore sensitizer. The chromophore provides efficient sensitization to a co-doped organic Er 3+ complex with a perfluorinatedligand shell. The large volume can protect the Er 3+ 1.5-µm luminescence from vibrational quenching. The average lifetime of the sensitized Er 3+ 1.5-µm luminescence reaches~0.86 ms, with a lifetime component of 2.65 ms, which is by far the longest Er 3+ lifetime in a hydrogen-abundant organic environment and can even compete with that obtained in the fully fluorinated organic Er 3+ system. The optimal sensitization enhances the Er 3+ luminescence by a factor of 1600 even with a high concentration of the phosphorescent molecule, and bright 1.5-µm OLEDs are obtained.

Metallophthalocyanines with ∼980 nm emission show sensitization effects on an erbium complex. A long lifetime of 1.05 ms, a PLQY of 13%, Er3+ 1.5 μm emission enhancement of 81 times are obtained in the perfluorinated organic erbium co-doped system.

A novel approach to sharp and persist NIR luminescence at 1 μm wavelength.

Intense organic neodymium (Nd3+) emission is obtained with near-infrared (NIR) emission equivalent in intensity to that of an organic semiconductor emitting material. The advantage of Nd3+ emission is its narrow line width and NIR emission, which is enhanced by ∼3000 times at low excitation power through an efficient sensitization effect from a composite organic sensitizer. This performance is optimized at high concentrations of Nd3+ ions, and the organic perfluorinated system provides the ion excitations with a quantum efficiency of ∼40%. The material system is applicable to thin films that are compatible with integrated optics applications.