Coexistence of full which-path information and interference in Wheelers delayed choice experiment with photons

We present a computer simulation model that is a one-to-one copy of an experimental realization of Wheeler's delayed choice experiment that employs a single photon source and a Mach-Zehnder interferometer composed of a 50/50 input beam splitter and a variable output beam splitter with adjustable reflection coefficient $R$ (V. Jacques {\sl et al.}, Phys. Rev. Lett. 100, 220402 (2008)). For $0\le R\le 0.5$, experimentally measured values of the interference visibility $V$ and the path distinguishability $D$, a parameter quantifying the which-path information WPI, are found to fulfill the complementary relation $V^2+D^2\le 1$, thereby allowing to obtain partial WPI while keeping interference with limited visibility. The simulation model that is solely based on experimental facts, that satisfies Einstein's criterion of local causality and that does not rely on any concept of quantum theory or of probability theory, reproduces quantitatively the averages calculated from quantum theory. Our results prove that it is possible to give a particle-only description of the experiment, that one can have full WPI even if D=0, V=1 and therefore that the relation $V^2+D^2\le 1$ cannot be regarded as quantifying the notion of complementarity.Comment: Physica E, in press; see also http://www.compphys.ne

Quantum Decoherence at Finite Temperatures

We study measures of decoherence and thermalization of a quantum system $S$ in the presence of a quantum environment (bath) $E$. The whole system is prepared in a canonical thermal state at a finite temperature. Applying perturbation theory with respect to the system-environment coupling strength, we find that under common Hamiltonian symmetries, up to first order in the coupling strength it is sufficient to consider the uncoupled system to predict decoherence and thermalization measures of $S$. This decoupling allows closed form expressions for perturbative expansions for the measures of decoherence and thermalization in terms of the free energies of $S$ and of $E$. Numerical results for both coupled and decoupled systems with up to 40 quantum spins validate these findings.Comment: 5 pages, 3 figure

Modeling electronic structure and transport properties of graphene with resonant scattering centers

We present a detailed numerical study of the electronic properties of single-layer graphene with resonant ("hydrogen") impurities and vacancies within a framework of noninteracting tight-binding model on a honeycomb lattice. The algorithms are based on the numerical solution of the time-dependent Schr\"{o}dinger equation and applied to calculate the density of states, \textit{quasieigenstates}, AC and DC conductivities of large samples containing millions of atoms. Our results give a consistent picture of evolution of electronic structure and transport properties of functionalized graphene in a broad range of concentration of impurities (from graphene to graphane), and show that the formation of impurity band is the main factor determining electrical and optical properties at intermediate impurity concentrations, together with a gap opening when approaching the graphane limit.Comment: 17 pages, 17 figures, expanded version to appear in PR

Chromospheric seismology above sunspot umbrae

The acoustic resonator is an important model for explaining the three-minute oscillations in the chromosphere above sunspot umbrae. The steep temperature gradients at the photosphere and transition region provide the cavity for the acoustic resonator, which allows waves to be both partially transmitted and partially reflected. In this paper, a new method of estimating the size and temperature profile of the chromospheric cavity above a sunspot umbra is developed. The magnetic field above umbrae is modelled numerically in 1.5D with slow magnetoacoustic wave trains travelling along magnetic fieldlines. Resonances are driven by applying the random noise of three different colours---white, pink and brown---as small velocity perturbations to the upper convection zone. Energy escapes the resonating cavity and generates wave trains moving into the corona. Line of sight (LOS) integration is also performed to determine the observable spectra through SDO/AIA. The numerical results show that the gradient of the coronal spectra is directly correlated with the chromosperic temperature configuration. As the chromospheric cavity size increases, the spectral gradient becomes shallower. When LOS integrations is performed, the resulting spectra demonstrate a broadband of excited frequencies that is correlated with the chromospheric cavity size. The broadband of excited frequencies becomes narrower as the chromospheric cavity size increases. These two results provide a potentially useful diagnostic for the chromospheric temperature profile by considering coronal velocity oscillations

Computer simulation of Wheeler's delayed choice experiment with photons

We present a computer simulation model of Wheeler's delayed choice experiment that is a one-to-one copy of an experiment reported recently (V. Jacques {\sl et al.}, Science 315, 966 (2007)). The model is solely based on experimental facts, satisfies Einstein's criterion of local causality and does not rely on any concept of quantum theory. Nevertheless, the simulation model reproduces the averages as obtained from the quantum theoretical description of Wheeler's delayed choice experiment. Our results prove that it is possible to give a particle-only description of Wheeler's delayed choice experiment which reproduces the averages calculated from quantum theory and which does not defy common sense.Comment: Europhysics Letters (in press

Multigap Superconductivity in Y2_2C3_3: A 13^{13}C-NMR Study

We report on the superconducting (SC) properties of Y$_2$C$_3$ with a relatively high transition temperature $T_{\rm c}=15.7$ K investigated by $^{13}$C nuclear-magnetic-resonance (NMR) measurements under a magnetic field. The $^{13}$C Knight shift has revealed a significant decrease below $T_{\rm c}$, suggesting a spin-singlet superconductivity. From an analysis of the temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ in the SC state, Y$_2$C$_3$ is demonstrated to be a multigap superconductor that exhibits a large gap $2\Delta/k_{\rm B}T_{\rm c}=5$ at the main band and a small gap $2\Delta/k_{\rm B}T_{\rm c}=2$ at other bands. These results have revealed that Y$_2$C$_3$ is a unique multigap s-wave superconductor similar to MgB$_2$.Comment: 4 pages, 5 figure

Phenomenological study on the significance of the scalar potential and Lamb shift

We indicated in our previous work that for QED the contributions of the scalar potential which appears at the loop level is much smaller than that of the vector potential and in fact negligible. But the situation may be different for QCD, one reason is that the loop effects are more significant because $\alpha_s$ is much larger than $\alpha$, and secondly the non-perturbative QCD effects may induce the scalar potential. In this work, we phenomenologically study the contribution of the scalar potential to the spectra of charmonia. Taking into account both vector and scalar potentials, by fitting the well measured charmonia spectra, we re-fix the relevant parameters and test them by calculating other states of the charmonia family. We also consider the role of the Lamb shift and present the numerical results with and without involving the Lamb shift

More on volume dependence of spectral weight function

Spectral weight functions are easily obtained from two-point correlation functions and they might be used to distinguish single-particle from multi-particle states in a finite-volume lattice calculation, a problem crucial for many lattice QCD simulations. In previous studies, it is shown that the spectral weight function for a broad resonance shares the typical volume dependence of a two-particle scattering state i.e. proportional to $1/L^3$ in a large cubic box of size $L$ while the narrow resonance case requires further investigation. In this paper, a generalized formula is found for the spectral weight function which incorporates both narrow and broad resonance cases. Within L\"uscher's formalism, it is shown that the volume dependence of the spectral weight function exhibits a single-particle behavior for a extremely narrow resonance and a two-particle behavior for a broad resonance. The corresponding formulas for both $A^+_1$ and $T^-_1$ channels are derived. The potential application of these formulas in the extraction of resonance parameters are also discussed