9,552 research outputs found
Fully quantum mechanical dynamic analysis of single-photon transport in a single-mode waveguide coupled to a traveling-wave resonator
We analyze the dynamics of single photon transport in a single-mode waveguide
coupled to a micro-optical resonator using a fully quantum mechanical model. We
examine the propagation of a single-photon Gaussian packet through the system
under various coupling conditions. We review the theory of single photon
transport phenomena as applied to the system and we develop a discussion on the
numerical technique we used to solve for dynamical behavior of the quantized
field. To demonstrate our method and to establish robust single photon results,
we study the process of adiabatically lowering or raising the energy of a
single photon trapped in an optical resonator under active tuning of the
resonator. We show that our fully quantum mechanical approach reproduces the
semi-classical result in the appropriate limit and that the adiabatic invariant
has the same form in each case. Finally, we explore the trapping of a single
photon in a system of dynamically tuned, coupled optical cavities.Comment: 24 pages, 10 figure
Generalized Stacking Fault Energy Surfaces and Dislocation Properties of Silicon: A First-Principles Theoretical Study
The generalized stacking fault (GSF) energy surfaces have received
considerable attention due to their close relation to the mechanical properties
of solids. We present a detailed study of the GSF energy surfaces of silicon
within the framework of density functional theory. We have calculated the GSF
energy surfaces for the shuffle and glide set of the (111) plane, and that of
the (100) plane of silicon, paying particular attention to the effects of the
relaxation of atomic coordinates. Based on the calculated GSF energy surfaces
and the Peierls-Nabarro model, we obtain estimates for the dislocation
profiles, core energies, Peierls energies, and the corresponding stresses for
various planar dislocations of silicon.Comment: 9 figures (not included; send requests to [email protected]
Rapid state purification protocols for a Cooper pair box
We propose techniques for implementing two different rapid state purification
schemes, within the constraints present in a superconducting charge qubit
system. Both schemes use a continuous measurement of charge (z) measurements,
and seek to minimize the time required to purify the conditional state. Our
methods are designed to make the purification process relatively insensitive to
rotations about the x-axis, due to the Josephson tunnelling Hamiltonian. The
first proposed method, based on the scheme of Jacobs [Phys. Rev. A 67,
030301(R) (2003)] uses the measurement results to control bias (z) pulses so as
to rotate the Bloch vector onto the x-axis of the Bloch sphere. The second
proposed method, based on the scheme of Wiseman and Ralph [New J. Phys. 8, 90
(2006)] uses a simple feedback protocol which tightly rotates the Bloch vector
about an axis almost parallel with the measurement axis. We compare the
performance of these and other techniques by a number of different measures.Comment: 14 pages, 14 figures. v2: Revised version after referee comments.
Accepted for publication by Physical Review
Spectroscopy of a Cooper-Pair box in the Autler-Townes configuration
A theoretical spectroscopic analysis of a microwave driven superconducting
charge qubit (Cooper-pair box coupled) to an RLC oscillator model is performed.
By treating the oscillator as a probe through the backreaction effect of the
qubit on the oscillator circuit, we extract frequency splitting features
analogous to the Autler-Townes effect from quantum optics, thereby extending
the analogies between superconducting and quantum optical phenomenology. These
features are found in a frequency band that avoids the need for high frequency
measurement systems and therefore may be of use in qubit characterization and
coupling schemes. In addition we find this frequency band can be adjusted to
suit an experimental frequency regime by changing the oscillator frequency.Comment: 13 pages, 7 figures. v2: Revised version after referee comments.
Accepted for publication by Physical Review
Interacting damage models mapped onto Ising and percolation models
We introduce a class of damage models on regular lattices with isotropic
interactions, as e.g. quasistatic fiber bundles. The system starts intact with
a surface-energy threshold required to break any cell sampled from an
uncorrelated quenched-disorder distribution. The evolution of this
heterogeneous system is ruled by Griffith's principle which states that a cell
breaks when the release in elastic energy in the system exceeds the
surface-energy barrier necessary to break the cell. By direct integration over
all possible realizations of the quenched disorder, we obtain the probability
distribution of each damage configuration at any level of the imposed external
deformation. We demonstrate an isomorphism between the distributions so
obtained and standard generalized Ising models, in which the coupling constants
and effective temperature in the Ising model are functions of the nature of the
quenched-disorder distribution and the extent of accumulated damage. In
particular, we show that damage models with global load sharing are isomorphic
to standard percolation theory, that damage models with local load sharing rule
are isomorphic to the standard Ising model, and draw consequences thereof for
the universality class and behavior of the autocorrelation length of the
breakdown transitions corresponding to these models. We also treat damage
models having more general power-law interactions, and classify the breakdown
process as a function of the power-law interaction exponent. Last, we also show
that the probability distribution over configurations is a maximum of Shannon's
entropy under some specific constraints related to the energetic balance of the
fracture process, which firmly relates this type of quenched-disorder based
damage model to standard statistical mechanics.Comment: 16 pages, 3 figure
Scattering in an environment
The cross section of elastic electron-proton scattering taking place in an
electron gas is calculated within the Closed Time Path method. It is found to
be the sum of two terms, one being the expression in the vacuum except that it
involves dressing due to the electron gas. The other term is due to the
scattering particles-electron gas entanglement. This term dominates the usual
one when the exchange energy is in the vicinity of the Fermi energy.
Furthermore it makes the trajectories of the colliding particles more
consistent and the collision more irreversible, rendering the scattering more
classical in this regime.Comment: final version to appear in Phys. Rev.
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Experimental Evidence for Different Strain Regimes of Crack Populations in a Clay Model
We report results from clay extension experiments used as a model for the evolution of fault populations due to stress interactions. At yielding cracks begin to appear and the brittle strain due to them quickly reaches a rate matching the applied stretching rate. The crack density (number of cracks per unit area) initially increases apace, then reaches a maximum at a critical strain, decreasing thereafter. At low strains, where the crack population is dilute, a power law length distribution is observed, which at high strain, gradually transitions to an exponential. This agrees with fault populations data observed in low and high strain settings. These results indicate that fault populations ranging from power law to exponential size-frequency distributions reflect the population evolution with increased strain
The effects of entry on incumbent innovation and productivity
How does firm entry affect innovation incentives in incumbent firms? Microdata suggest that there is heterogeneity across industries. Specifically, incumbent productivity growth and patenting is positively correlated with lagged greenfield foreign firm entry in technologically advanced industries, but not in laggard industries. In this paper we provide evidence that these correlations arise from a causal effect predicted by Schumpeterian growth theory—the threat of technologically advanced entry spurs innovation incentives in sectors close to the technology frontier, where successful innovation allows incumbents to survive the threat, but discourages innovation in laggard sectors, where the threat reduces incumbents' expected rents from innovating. We find that the empirical patterns hold using rich micro panel data for the United Kingdom. We control for the endogeneity of entry by exploiting major European and U.K. policy reforms, and allow for endogeneity of additional factors. We complement the analysis for foreign entry with evidence for domestic entry and entry through imports
Microscopic origin of diagonal stripe phases in doped nickelates
We investigate the electron density distribution and the stability of stripe
phases in the realistic two-band model with hopping elements between e_g
orbitals at Ni sites on the square lattice, and compare these results with
those obtained for the doubly degenerate Hubbard model with two equivalent
orbitals and diagonal hopping. For both models we determine the stability
regions of filled and half-filled stripe phases for increasing hole doping
x=2-n in the range of x<0.4, using Hartree-Fock approximation for large
clusters. In the parameter range relevant to the nickelates, we obtain the most
stable diagonal stripe structures with filling of nearly one hole per atom, as
observed experimentally. In contrast, for the doubly degenerate Hubbard model
the most stable stripes are somewhat reminiscent of the cuprates, with
half-filled atoms at the domain wall sites. This difference elucidates the
crucial role of the off-diagonal e_g hopping terms for the stripe formation in
La_2-xSr_xNiO_4. The influence of crystal field is discussed as well.Comment: 15 pages, 12 figure
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