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.

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We present a systematic study of the exciton capture, relaxation, and recombination processes in twodimensional quantum-dot superlattices ͑2D QDSL's͒ based on time-resolved photoluminescence measurements. Due to the formation of minibands in 2D QDSL's, the capture of excitons from the miniband into some large islands is found to be a quantum capture process. The capture time increases significantly with increasing excitation density. In addition, the excitons relax rapidly within the miniband. However, the relaxation becomes slower at high excitation densities. Furthermore, the fast recombination in the miniband indicates the drastic elongation of the exciton coherence length and the significant delocalization of the wave functions. Finally, the observation of radiative recombination over a wide energy region implies the relaxation of momentum conservation or the small exciton effective mass in 2D QDSL's. All phenomena suggest that the exciton dynamics in 2D QDSL's is governed by the exciton coherence length in the miniband.

Telecom-band–integrated quantum memory is an elementary building block for developing quantum networks compatible with fiber communication infrastructures. Toward such a network with large capacity, an integrated multimode photonic quantum memory at telecom band has yet been demonstrated. Here, we report a fiber-integrated multimode quantum storage of single photon at telecom band on a laser-written chip. The storage device is a fiber-pigtailed Er 3+ :LiNbO 3 waveguide and allows a storage of up to 330 temporal modes of heralded single photon with 4-GHz-wide bandwidth at 1532 nm and a 167-fold increasing of coincidence detection rate with respect to single mode. Our memory system with all-fiber addressing is performed using telecom-band fiber-integrated and on-chip components. The results represent an important step for the future quantum networks using integrated photonics devices.