Random unitary dynamics of quantum networks

We investigate the asymptotic dynamics of quantum networks under repeated applications of random unitary operations. It is shown that in the asymptotic limit of large numbers of iterations this dynamics is generally governed by a typically low dimensional attractor space. This space is determined completely by the unitary operations involved and it is independent of the probabilities with which these unitary operations are applied. Based on this general feature analytical results are presented for the asymptotic dynamics of arbitrarily large cyclic qubit networks whose nodes are coupled by randomly applied controlled-NOT operations.Comment: 4 pages, 2 figure

Calculation of the Raman G peak intensity in monolayer graphene: role of Ward identities

The absolute integrated intensity of the single-phonon Raman peak at 1580 cm^{-1} is calculated for a clean graphene monolayer. The resulting intensity is determined by the trigonal warping of the electronic bands and the anisotropy of the electron-phonon coupling, and is proportional to the second power of the excitation frequency. The main contribution to the process comes from the intermediate electron-hole states with typical energies of the order of the excitation frequency, contrary to what has been reported earlier. This occurs because of strong cancellations between different terms of the perturbation theory, analogous to Ward identities in quantum electrodynamics

Backaction in metasurface etalons

We consider the response of etalons created by a combination of a conventional mirror and a metasurface, composed of a periodic lattice of metal scatterers with a resonant response. This geometry has been used previously for perfect absorption, in so-called Salisbury screens, and for hybridization of localized plasmons with Fabry-Perot resonances. The particular aspect we address is if one can assume an environment-independent reflectivity for the metasurface when calculating the reflectivity of the composite system, as in a standard Fabry-Perot analysis, or whether the fact that the metasurface interacts with its own mirror image renormalizes its response. Using lattice sum theory, we take into account all possible retarded dipole-dipole interactions of scatterers in the metasurface amongst each other, and through the mirror. We show that while a layer-by-layer Fabry-Perot formalism captures the main qualitative features of metasurface etalons, in fact the mirror modifies both the polarizability and reflectivity of the metasurface in a fashion that is akin to Drexhage's modification of the radiative properties of a single dipole.Comment: 10 pages, 5 figure

Asymptotic Evolution of Random Unitary Operations

We analyze the asymptotic dynamics of quantum systems resulting from large numbers of iterations of random unitary operations. Although, in general, these quantum operations cannot be diagonalized it is shown that their resulting asymptotic dynamics is described by a diagonalizable superoperator. We prove that this asymptotic dynamics takes place in a typically low dimensional attractor space which is independent of the probability distribution of the unitary operations applied. This vector space is spanned by all eigenvectors of the unitary operations involved which are associated with eigenvalues of unit modulus. Implications for possible asymptotic dynamics of iterated random unitary operations are presented and exemplified in an example involving random controlled-not operations acting on two qubits

Statistical properties of spontaneous emission near a rough surface

We study the lifetime of the excited state of an atom or molecule near a plane surface with a given random surface roughness. In particular, we discuss the impact of the scattering of surface modes within the rough surface. Our study is completed by considering the lateral correlation length of the decay rate and the variance discussing its relation to the C0 correlation

Spin Hall effect of light in photon tunneling

We resolve the breakdown of angular momentum conservation on two-dimensional photon tunneling by considering spin Hall effect (SHE) of light. This interesting effect manifests itself as polarization-dependent transverse shifts for a classic wave packet tunneling through a prism-air-prism barrier. For a certain circularly polarized component, the transverse shifts can be modulated by altering the refractive index gradient associated with the two prisms. We find that the SHE in conventional beam refraction can be evidently enhanced via photon tunneling mechanism. The polarization-dependent transverse shift is governed by the total angular momentum conservation law, while the polarization-dependent angular shift is governed by the total linear momentum law. These findings open the possibility for developing new nano-photonic devices and can be extrapolated to other physical systems.Comment: 8 pages, 5 figure

Quality factor of thin-film Fabry-Perot resonators: dependence on interface roughness

Thin-film Fabry-Perot (F-P) optical resonators are studied for application as wavelength-selecting elements in on-chip spectrometers. The interface roughness between the different resonator layers (Al /PECVD SiO2 / Ag) is identified to be the primary source of light scattering and energy losses. It is demonstrated that conventional IC fabrication yields layers with RMS interface roughness easily exceeding 10 nm. When applied to the visible spectral range, such a roughness causes significant degradation of the F-P filter quality factor. Moreover, the scattered light contributes to transmittance outside the narrow resonance band to which the F-P filter is tuned and overall device performance is decreased

Nanoplasmonic Renormalization and Enhancement of Coulomb Interactions

Nanostructured plasmonic metal systems are known to enhance greatly variety of radiative and nonradiative optical processes, both linear and nonlinear, which are due to the interaction of an electron in a molecule or semiconductor with the enhanced local optical field of the surface plasmons. Principally different are numerous many-body phenomena that are due to the Coulomb interaction between charged particles: carriers (electrons and holes) and ions. These include carrier-carrier or carrier-ion scattering, energy and momentum transfer (including the drag effect), thermal equilibration, exciton formation, impact ionization, Auger effects, etc. It is not widely recognized that these and other many-body effects can also be modified and enhanced by the surface-plasmon local fields. A special but extremely important class of such many-body phenomena is constituted by chemical reactions at metal surfaces, including catalytic reactions. Here, we propose a general and powerful theory of the plasmonic enhancement of the many-body phenomena resulting in a closed expression for the surface plasmon-dressed Coulomb interaction. We illustrate this theory by computing this dressed interaction explicitly for an important example of metal-dielectric nanoshells, which exhibits a reach resonant behavior in both the magnitude and phase. This interaction is used to describe the nanoplasmonic-enhanced Foerster energy transfer between nanocrystal quantum dots in the proximity of a plasmonic nanoshell. Catalysis at nanostructured metal surfaces, nonlocal carrier scattering and surface-enhanced Raman scattering are discussed among other effects and applications where the nanoplasmonic renormalization of the Coulomb interaction may be of principal importance

Fluorescence quenching in graphene: a fundamental ruler and evidence for transverse plasmons

Graphene's fluorescence quenching is studied as a function of distance. Transverse decay channels, full retardation and graphene-field coupling to all orders are included, extending previous instantaneous results. For neutral graphene, a virtually exact analytical expression for the fluorescence yield is derived, valid for arbitrary distances and only based on the fine structure constant $\alpha$, the fluorescent wavelength $\lambda$, and distance $z$. Thus graphene's fluorescence quenching measurements provide a fundamental distance ruler. For doped graphene and at appropriate energies, the fluorescence yield at large distances is dominated by transverse plasmons, providing a platform for their detection.Comment: 6 pages, 2 figure