One-dimensional plasmons in ultrathin metallic silicide wires of finite width

The acoustic dispersion of plasmons (PLs) in narrow (4 nm) and ultrathin (one unit cell) metallic DySi2 wires, grown by self-assembly on vicinal Si(100)-[011] 4° turns out to be unidirectional. We observed the lowest intersubband PL as well as the acoustic PL. These PLs are specific for narrow metallic strips of finite width. Our experimental and theoretical analysis suggests that only one of two electron pockets in the surface Brillouin zone makes a substantial contribution to the PLs because the other pocket has a much smaller conductive character due to a strong admixture of electronic states with d character. © 2010 The American Physical Society

A solid-state source of single and entangled photons at diamond SiV−^--center transitions operating at 80K

Large-scale quantum networks require the implementation of long-lived quantum memories as stationary nodes interacting with qubits of light. Epitaxially grown quantum dots hold great potential for the on-demand generation of single and entangled photons with high purity and indistinguishability. Coupling these emitters to memories with long coherence times enables the development of hybrid nanophotonic devices incorporating the advantages of both systems. Here we report the first GaAs/AlGaAs quantum dots grown by droplet etching and nanohole infilling method, emitting single photons with a narrow wavelength distribution (736.2 $\pm$ 1.7 nm) close to the zero-phonon line of Silicon-vacancy centers. Polarization entangled photons are generated via the biexciton-exciton cascade with a fidelity of (0.73 $\pm$ 0.09). High single photon purity is maintained from 4 K (g$^($$^2$$^)$(0) = 0.07 $\pm$ 0.02) up to 80 K (g$^($$^2$$^)$(0) = 0.11 $\pm$ 0.01), therefore making this hybrid system technologically attractive for real-world quantum photonic applications

Local droplet etching on InAlAs/InP surfaces with InAl droplets

GaAs quantum dots (QDs) grown by local droplet etching (LDE) have been studied extensively in recent years. The LDE method allows for high crystallinity, as well as precise control of the density, morphology, and size of QDs. These properties make GaAs QDs an ideal candidate as single photon and entangled photon sources at short wavelengths (<800 nm). For technologically important telecom wavelengths, however, it is still unclear whether LDE grown QDs can be realized. Controlling the growth conditions does not enable shifting the wavelength of GaAs QDs to the telecom region. New recipes will have to be established. In this work, we study Indium–Aluminum (InAl) droplet etching on ultra-smooth In0.55Al0.45As surfaces on InP substrates, with a goal to lay the foundation for growing symmetrical and strain-free telecom QDs using the LDE method. We report that both droplets start to etch nanoholes at a substrate temperature above 415 °C, showing varying nanohole morphology and rapidly changing density (by more than one order of magnitude) at different temperatures. Al and In droplets are found to not intermix during etching, and instead etch nanoholes individually. The obtained nanoholes show a symmetric profile and very low densities, enabling infilling of lattice-matched InGaAs QDs on InxAl1−xAs/InP surfaces in further works

Statistical limits for entanglement swapping with semiconductor entangled photon sources

Semiconductor quantum dots are promising building blocks for quantum communication applications. Al- though deterministic, efficient, and coherent emission of entangled photons has been realized, implementing a practical quantum repeater remains outstanding. Here we explore the statistical limits for entanglement swapping with sources of polarization-entangled photons from the commonly used biexciton-exciton cascade. We stress the necessity of tuning the exciton fine structure, and explain why the often observed time evolution of photonic entanglement in quantum dots is not applicable for large quantum networks. We identify the critical, statistically distributed device parameters for entanglement swapping based on two sources. A numerical model for benchmarking the consequences of device fabrication, dynamic tuning techniques, and statistical effects is developed, in order to bring the realization of semiconductor-based quantum networks one step closer to reality. ©2022 American Physical Societ

Experimental investigation of two-dimensional plasmons in a DySi 2 monolayer on Si(111)

DySi2 monolayers were prepared by thermal evaporation of Dy at room temperature followed by annealing to 500°C on Si(111), which yields a perfect (1×1) low-energy electron diffraction pattern. These monolayers of DySi2 were investigated by electron energy loss spectroscopy with both high energy and momentum resolution. A low-energy acoustic-like dispersion was found with very small anisotropy in reciprocal space, consistent with the characteristic losses due to a two-dimensional plasmon in the hexagonal monolayer structure of DySi2, and effective hole densities of N2d =4.1× 1013 cm-2 with an effective mass of m* =0.37 me. Deviations of the expected dispersion due to single-particle excitations in the Si substrate were found above q∥ =0.08 Å-1. On (111) facets with terrace widths of 12 lattice constants, these properties change little along the terraces, but no plasmon wave propagation across step edges is possible, leading to strong suppression of characteristic loss signals in the direction normal to the steps. © 2008 The American Physical Society

Charge reconfiguration in arrays of quantum dots

Semiconductor quantum dots are potential building blocks for scalable qubit architectures. Efficient control over the exchange interaction and the possibility of coherently manipulating electron states are essential ingredients towards this goal. We studied experimentally the shuttling of electrons trapped in serial quantum dot arrays isolated from the reservoirs. The isolation hereby enables a high degree of control over the tunnel couplings between the quantum dots, while electrons can be transferred through the array by gate voltage variations. Model calculations are compared with our experimental results for double, triple, and quadruple quantum dot arrays. We are able to identify all transitions observed in our experiments, including cotunneling transitions between distant quantum dots. The shuttling of individual electrons between quantum dots along chosen paths is demonstrated

Electrical transport in ultrathin Cs layers on Si(001)

Electrical transport in ultrathin Cs layers on Si(001) has been studied combining macroscopic conductivity measurements with low-energy electron diffraction, energy loss spectroscopy, and measurements of the work function. At temperatures around 150K, growth of the first three atomic layers proceeds layer-by-layer. The completion of each layer correlates with stepwise increases of the surface sheet conductance with coverage. Calibrating the Cs coverage by combined conductivity and work function measurements, the areal density of a single atomic layer is determined as 0.5 monolayers (3.39×1014cm-2). Electron spectroscopy reveals a semiconductor-metal transition of the surface upon completion of the first atomic layer, which correlates with the onset of a macroscopically measured sheet conductance in the 10-5Ω-1 range. While the conductance can be ascribed to electrical transport within surface states, its dependence on temperature indicates an activation barrier, which, most likely, is due to domain boundaries. At coverages of one monolayer and beyond, the Cs Si(001) surface exhibits a high metal-like conductance in the 10-3Ω-1 range. © 2005 The American Physical Society

High efficiency grating couplers for strain tunable GaAs quantum dot based entangled photon sources

The on-chip integration of single photon and entangled photon emitters such as epitaxially grown semiconductor quantum dots into photonic frameworks is a rapidly evolving research field. GaAs quantum dots offer high purity and a high degree of entanglement due to, in part, exhibiting very small fine structure splitting along with short radiative lifetimes. Integrating strain-tunable quantum dots into nanostructures enhances the quantum optical fingerprint, i.e., radiative lifetimes and coupling of these sources, and allows for on-chip manipulation and routing of the generated quantum states of light. Efficient out-coupling of photons for off-chip processing and detection requires carefully engineered mesoscopic structures. Here, we present numerical studies of highly efficient grating couplers reaching up to over 90% transmission. A 2D Gaussian mode overlap of 83.39% for enhanced out-coupling of light from within strain-tunable photonic nanostructures for free-space transmission and single-mode fiber coupling is shown. The photon wavelength under consideration is 780 nm, corresponding to the emission from GaAs quantum dots resembling the 87Rb D2 line. The presented numerical study helps implement such sources for applications in complex quantum optical networks