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.

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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.

Spectrally uncorrelated biphoton state generated from the spontaneous nonlinear optical process is an important resource for quantum information. Currently such spectrally uncorrelated biphoton state can only be prepared from limited kinds of nonlinear media, thus limiting their wavelengths. In order to explore wider wavelength range, here we theoretically study the generation of spectrally uncorrelated biphoton state from 14 isomorphs of potassium dihydrogen phosphate (KDP) crystal. We find that 11 crystals from the 'KDP family' still maintain similar nonlinear optical properties of KDP, such as KDP, DKDP, ADP, DADP, ADA, DADA, RDA, DRDA, RDP, DRDP and KDA, which satisfy 3 kinds of the group-velocity matching conditions for spectrally uncorrelated biphoton state generation from near-infrared to telecom wavelengths. Based on the uncorrelated biphoton state, we investigate the generation of heralded pure-state single photon by detecting one member of the biphoton state to herald the output of the other. The purity of the heralded single photon is as high as 0.98 without using a narrow-band filter; the Hong-Ou-Mandel interference from independent sources can also achieve a visibility of 98%. This study may provide more and better single-photon sources for quantum information processing at near-infrared and telecom wavelengths.

On self-assembled In x Ga 1Ϫx As/GaAs͑311͒B quantum dots ͑QD's͒, photocurrent in the plane of QD arrays was measured under irradiation with wavelengths longer than 850 nm ͑1.46 eV͒. A sample with rather inhomogeneous QD sizes shows hopping conduction, indicating the localization of carriers in individual QD's. A two-dimensional QD superlattice, consisting of highly ordered and homogeneously sized QD's, exhibits negative differential conductance ͑NDC͒, i.e., photocurrent decrease with increasing applied voltage, in a limited electric-field range. The pre-NDC conduction is argued to arise from the miniband, which is evidenced by the photoluminescence, while the post-NDC conduction is found to be hopping as in a localized QD system, suggesting a miniband destruction under an in-plane electric field as low as ϳ10 3 V cm Ϫ1 . The miniband transport is likely controlled by two-dimensional acoustic-phonon scattering.

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.

Wide-temperature-range 10.3 Gbit/s operations of 1.3 mm distributed feedback (DFB) lasers using high-density quantum dots are presented. Clearly opened eye diagrams are obtained from 240 to 808C with extinction ratios of more than 5.2 dB.Introduction: Owing to their unique characteristics such as ultra-low threshold currents and temperature insensitivity [1], quantum-dot (QD) lasers have been actively investigated as next generation light sources. In particular, self-assembling growth techniques of In(Ga)As QDs on GaAs substrates have accelerated the development of the QD lasers emitting at 1.3 mm for optical communication systems [2,3]. Temperature-stable 10 Gbit/s operation under low-drive-current conditions has been demonstrated for Fabry-Pérot (FP) QD lasers [4]. For longer-distance applications, QD distributed feedback (DFB) lasers with single-longitudinal-mode oscillation have also been investigated. Most of them utilised laterally loss-coupled gratings [5, 6], while we adopted InGaP/GaAs index-coupled gratings and successfully exhibited temperature-stable single-longitudinal-mode operation with the sidemode suppression ratios (SMSR) of more than 45 dB [7]. In these QD DFB lasers, however, the limited operating temperature range and the modulation bandwidth still remain to be improved for practical applications.To improve these characteristics, the modal gain of the QD active layers must be increased. Higher modal gain will improve not only the light-current characteristics especially at high temperatures but also the modulation bandwidth, which is mainly limited by the large damping factor [4]. Recently, we have succeeded in increasing the quantum-dot density to 5.9 × 10 10 cm 22 by optimising the growth conditions and demonstrated 10 Gbit/s temperature-insensitive operation up to 1008C using FP lasers [8].In this Letter, we apply these high-density QDs to the DFB lasers with InGaP/GaAs gratings. In the fabricated lasers, we demonstrate 10.3 Gbit/s operation over a wide temperature range from 240 to 808C with clearly opened eye diagrams as well as the temperaturestable light-current characteristics under single-longitudinal-mode oscillation.

Recently, two-dimensional (2D) metal halides have triggered an enormous interest for their tunable mechanical, electronic, magnetic, and topological properties, greatly enriching the family of 2D materials. Here, based on first-principles calculations, we report a systematic study of group 11 transition-metal halide MX (M = Cu, Ag, Au; X = Cl, Br, I) monolayers. Among them, CuBr, CuI, AgBr, and AgI monolayers exhibit high thermodynamic, dynamic, and mechanic stability. The four stable monolayers have a direct band gap of ∼3.12–3.36 eV and possess high carrier mobility (∼103 cm2 V–1 s–1), suggestive of future photocatalysts for water splitting applications. What is more, the simulations of optical properties confirm that the stable MX monolayers hold the potential for further applications in ultraviolet optical devices and quantum cutting solar materials.