Photon-pair generation by non-instantaneous spontaneous four-wave mixing

We present a general model, based on a Hamiltonian approach, for the joint quantum state of photon pairs generated through pulsed spontaneous four-wave mixing, including nonlinear phase-modulation and a finite material response time. For the case of a silica fiber, it is found that the pair-production rate depends weakly on the waveguide temperature, due to higher-order Raman scattering events, and more strongly on pump-pair frequency detuning. From the analytical model, a numerical scheme is derived, based on the well-known split-step method. This scheme allows computation of joint states where nontrivial effects are included, such as group-velocity dispersion and Raman scattering. In this work, the numerical model is used to study the impact of the non-instantaneous response on the pre-filtering purity of heralded single photons. We find that for pump pulses shorter than 1 ps, a significant detuning-dependent change in quantum-mechanical purity may be observed in silica

Long-term Framework for Electricity Distribution Access Charges

In order to achieve overall economic efficiency, incentive regulation of electricity distribution utilities must address two important and inter-related issues. First, the utilities’ allowed revenues need to be set at correct levels. Second, the access charging mechanism by which the utilities recover the allowed revenues must give the correct economic signals to generation and load connected to the network. This paper is concerned with the latter aspect of regulation. The paper discusses the main economic principles that should form the basis on which a distribution access charging model is developed. The charging model should have a number of attributes: be calibrated to each existing network; contain an asset register; be able to determine assets needed to meet new demand; find least-cost system expansion; compute network losses and handle ancillary services; estimate incremental operating and maintenance costs; be available to users; and be simple enough for external users to understand

Hybridization at superconductor-semiconductor interfaces

Hybrid superconductor-semiconductor devices are currently one of the most promising platforms for realizing Majorana zero modes. Their topological properties are controlled by the band alignment of the two materials, as well as the electrostatic environment, which are currently not well understood. Here, we pursue to fill in this gap and address the role of band bending and superconductor-semiconductor hybridization in such devices by analyzing a gated single Al-InAs interface using a self-consistent Schrodinger-Poisson approach. Our numerical analysis shows that the band bending leads to an interface quantum well, which localizes the charge in the system near the superconductor-semiconductor interface. We investigate the hybrid band structure and analyze its response to varying the gate voltage and thickness of the Al layer. This is done by studying the hybridization degrees of the individual subbands, which determine the induced pairing and effective $g$-factors. The numerical results are backed by approximate analytical expressions which further clarify key aspects of the band structure. We find that one can obtain states with strong superconductor-semiconductor hybridization at the Fermi energy, but this requires a fine balance of parameters, with the most important constraint being on the width of the Al layer. In fact, in the regime of interest, we find an almost periodic dependence of the hybridization degree on the Al width, with a period roughly equal to the thickness of an Al monolayer. This implies that disorder and shape irregularities, present in realistic devices, may play an important role for averaging out this sensitivity and, thus, may be necessary for stabilizing the topological phase.Comment: 10 Figures. 16 pages. Published versio

Engineering spectrally unentangled photon pairs from nonlinear microring resonators through pump manipulation

The future of integrated quantum photonics relies heavily on the ability to engineer refined methods for preparing the quantum states needed to implement various quantum protocols. An important example of such states are quantum-correlated photon pairs, which can be efficiently generated using spontaneous nonlinear processes in integrated microring-resonator structures. In this work, we propose a method for generating spectrally unentangled photon pairs from a standard microring resonator. The method utilizes interference between a primary and a delayed secondary pump pulse to effectively increase the pump spectral width inside the cavity. This enables on-chip generation of heralded single photons with state purities in excess of 99 % without spectral filtering.Comment: 5 pages, 5 figure

Weak Coulomb blockade effect in quantum dots

We develop the general non-equilibrium theory of transport through a quantum dot, including Coulomb Blockade effects via a 1/N expansion, where N is the number of scattering channels. At lowest order we recover the Landauer formula for the current plus a self-consistent equation for the dot potential. We obtain the leading corrections and compare with earlier approaches. Finally, we show that to leading and next leading order in 1/N there is no interaction correction to the weak localization, in contrast to previous theories, but consistent with experiments by Huibers et al. [Phys. Rev. Lett. 81, 1917 (1998)], where N=4.Comment: 4 pages, 2 figures. Published versio

Spectrally pure heralded single photons by spontaneous four-wave mixing in a fiber: reducing impact of dispersion fluctuations

We model the spectral quantum-mechanical purity of heralded single photons from a photon-pair source based on nondegenerate spontaneous four-wave mixing taking the impact of distributed dispersion fluctuations into account. The considered photon-pair-generation scheme utilizes pump-pulse walk-off to produce pure heralded photons and phase matching is achieved through the dispersion properties of distinct spatial modes in a few-mode silica step-index fiber. We show that fiber-core-radius fluctuations in general severely impact the single-photon purity. Furthermore, by optimizing the fiber design we show that generation of single photons with very high spectral purity is feasible even in the presence of large core-radius fluctuations. At the same time, contamination from spontaneous Raman scattering is greatly mitigated by separating the single-photon frequency by more than 32 THz from the pump frequency

Unraveling the acoustic electron-phonon interaction in graphene

Using a first-principles approach we calculate the acoustic electron-phonon couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic phonons. Analytic forms of the coupling matrix elements valid in the long-wavelength limit are found to give an almost quantitative description of the first-principles based matrix elements even at shorter wavelengths. Using the analytic forms of the coupling matrix elements, we study the acoustic phonon-limited carrier mobility for temperatures 0-200 K and high carrier densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective acoustic deformation potential of graphene is \Xi_eff = 6.8 eV and that the temperature dependence of the mobility \mu ~ T^{-\alpha} increases beyond an \alpha = 4 dependence even in the absence of screening when the full coupling matrix elements are considered. The large disagreement between our calculated deformation potential and those extracted from experimental measurements (18-29 eV) indicates that additional or modified acoustic phonon-scattering mechanisms are at play in experimental situations.Comment: 7 pages, 3 figure

Gravitino Dark Matter in the CMSSM With Improved Constraints from BBN

In the framework of the Constrained MSSM we re--examine the gravitino as the lightest superpartner and a candidate for cold dark matter in the Universe. Unlike in other recent studies, we include both a thermal contribution to its relic population from scatterings in the plasma and a non--thermal one from neutralino or stau decays after freeze--out. Relative to a previous analysis [1] we update, extend and considerably improve our treatment of constraints from observed light element abundances on additional energy released during BBN in association with late gravitino production. Assuming the gravitino mass in the GeV to TeV range, and for natural ranges of other supersymmetric parameters, the neutralino region is excluded, while for smaller values of the gravitino mass it becomes allowed again. The gravitino relic abundance is consistent with observational constraints on cold dark matter from BBN and CMB in some well defined domains of the stau region but, in most cases, only due to a dominant contribution of the thermal population. This implies, depending on the gravitino mass, a large enough reheating temperature. If \mgravitino>1 GeV then $T_R>10^7$ GeV, if allowed by BBN and other constraints but, for light gravitinos, if \mgravitino>100 keV then $T_R>3\times 10^3$ GeV. On the other hand, constraints mostly from BBN imply an upper bound T_R \lsim {a few}x 10^8\times10^9 GeV which appears inconsistent with thermal leptogenesis. Finally, most of the preferred stau region corresponds to the physical vacuum being a false vacuum. The scenario can be partially probed at the LHC.Comment: Version with Erratum. Numerical bug fixed. An upper bound on the reheating temperature strengthened by about an order of magnitud