We report the observations of inelastic light scattering by the elementary excitations of the one-dimensional electron gas. The quantum wires are fabricated from a modulation-doped single GaAs/Al Gai, As quantum well by electron-beam lithography and shallow electron-cyclotronresonance reactive-ion etching. In spectra of intersubband excitations we observe a clear separation of spin-density and charge-density collective modes. The splitting associated with the lateral potential of the wires is also seen in transitions to states of the first excited level of the parent quantum well. Determinations of electron-electron interactions in intersubband excitations of the one-dimensional electron gas show large corrections due to the exchange terms (or vertex corrections) that represent the excitonic binding of the particle-hole pairs. Narrow quantum wires of width less than 100 nm are currently obtained by state-of-the-art semiconductor nanofabrication technologies. ' The behavior of free carriers in such nanostructures presents fundamental characteristics due to their motional confinement to one dimension. Electron-electron interactions in one dimension are expected to change drastically both elementary excitations and screening, and the question of whether the onedimensional (1D) electron system is better described as a Fermi liquid or as a Luttinger liquid is still open. ' Intersubband transitions and elementary excitations of the 1D electron gas have been studied by far-infrared (FIR) optical absorption ' and inelastic light scattering. ' FIR measurements can detect charge-density excitations (CDE's), if they carry an electrical dipole moment. s s io In contrast, inelastic light scattering experiments are specially well suited to observe CDE's, ' spin-density excitations (SDE's), ' and single-particle excitations (SPE's), r s i2 which can be identified by simple polarization selection rules.The observation of spin-density excitations that are well separated from the single-particle continuum reveals the strength of vertex corrections (or excitonic shifts) due to exchange terms of the Coulomb interaction in 2D systems. ' These terms represent the excitonic binding of the particle-hole pair states in intersubband excitations.The strength of the exchange interaction depends strongly on the confinement of the electron gas and on its density. In two dimensions and at very low densities exchange eÃects are predominant over the Hartree terms, whereas in the high-density regime with more than one subband occupied by electrons, SDE's are suppressed.Recent light scattering work in semiconductor quantum wires found indications for an enhancement of excitonic shifts in one dimension.However, in these experiments well-defined intersubband spin-density excitations were not observed in the absence of an external magnetic field and the strength of vertex corrections in one dimension could not be assessed.In this paper we report observations of excitations of the one-dimensional electron gas in GaAs quantum wires by resonant inelast...

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-Single photon emitters (SPEs) are at the basis of many applications for quantum information management. Semiconductor-based SPEs are best suited for practical implementations because of high design flexibility, scalability and integration potential in practical devices. Single photon emission from ordered arrays of InGaN nano-disks embedded in GaN nanowires is reported. Intense and narrow optical emission lines from quantum dot-like recombination centers are observed in the blue-green spectral range. Characterization by electron microscopy, cathodoluminescence and micro-photoluminescence indicate that single photons are emitted from regions of high In concentration in the nano-disks due to alloy composition fluctuations. Single photon emission is determined by photon correlation measurements showing deep antibunching minima in the second order correlation function. The present results are a promising step towards the realization of on-site/on-demand single photon sources in the blue-green spectral range operating in the GHz frequency range at high temperatures.Introduction. -Single photons are ideal "flying" qubits to convey quantum information between distant nodes of a quantum network. Reliable and controlled generation of single photons is therefore a crucial step to develop applications for quantum communication, quantum information processing and quantum metrology [1,2]. Single photons can be emitted in principle by material entities possessing discrete energy levels, as they need a finite time to "recharge" after emission of one photon. The standard method to assess single photon emission is to measure the second order photon correlation function by Hanbury-Brown and Twiss (HBT) interferometry. As shown in Fig. 1, single photons are either reflected or transmitted by a beam splitter, so that the probability of simultaneous detection in the two detectors of the interferometer is zero. The detection events are stored in a Time-Correlated Single Photon Counter (TCSPC), and the resulting correlation function g 2 (τ) shows an

Long distance ͑1.4 m͒ interaction of two different InAs/GaAs quantum dots in a photonic crystal microcavity is observed. Simultaneous coupling of both quantum dots to the cavity is demonstrated by Purcell effect measurements. Resonant optical excitation in the p state of any of the quantum dots, results in an increase in the s-state emission of the other one. The cavity-mediated coupling can be controlled by varying the excitation intensity. These results represent an experimental step toward the realization of quantum logic operations using distant solid-state qubits. DOI: 10.1103/PhysRevB.81.193301 PACS number͑s͒: 78.67.Hc, 42.50.Ex, 78.67.De Efficient quantum information applications require qubits with low decoherence rates, fast manipulation times, and easy scalability.1 These requirements are met by qubits based on electron spins or excitons in semiconductor quantum dots ͑QDs͒. Coupling of single semiconductor QD excitons to a microcavity confined electromagnetic mode has different advantages depending on the coupling strength. Weak coupling allows enhanced optical efficiency associated to the exciton decay time reduction by the Purcell effect.2 In the strong-coupling regime, the system presents entangled lightmatter states that can be used as building blocks for transmission of quantum information, 3 qubit readout, 4 production of entangled pairs by compensation of the natural exciton fine structure splitting, 5 and lasing. 6 Single QD-cavity coupling has been demonstrated in the past years, 7-13 showing interesting cavity-quantum electrodynamics effects. The possibility of using two or more qubits coupled by a single optical microcavity is appealing for it can provide techniques for long distance, fast interactions between qubits.14-16 New dynamical phenomena are expected in these systems, which are dependent on the relative energy scales of the coupling between qubits and between qubits and the cavity mode ͑CM͒. In randomly distributed QDs samples it is statistically difficult to have two or more QDs both spatially and spectrally coupled to a microcavity mode. Some approaches have been proposed to obtain this type of coupled system, [17][18][19] which rely on the deterministic location of the QD in the cavity.12,20 Coupling of several QDs to a single cavity mode has been reported as the origin of lasing at very low threshold. 21In this Brief Report, we show that exciton states of two semiconductor quantum dots with large lateral separation interact through a microcavity confined optical mode. Individual and simultaneous coupling of the QDs to the CM is demonstrated by changes in photoluminescence ͑PL͒ emission intensity and spontaneous emission rate ͑Purcell effect͒ when the QD excitons are brought into resonance with the CM. Cavity-mediated inter-QD interaction is demonstrated by PL excitation ͑PLE͒ measurements, in which resonant excitation at the p state of any of the QDs increases the s-state emission of the other one. The microcavity-mediated interaction of the two QDs can be controlled by vary...

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