259 research outputs found
A New Approach to Analytic, Non-Perturbative and Gauge-Invariant QCD
Following a previous calculation of quark scattering in eikonal
approximation, this paper presents a new, analytic and rigorous approach to the
calculation of QCD phenomena. In this formulation a basic distinction between
the conventional "idealistic" description of QCD and a more "realistic"
description is brought into focus by a non-perturbative, gauge-invariant
evaluation of the Schwinger solution for the QCD generating functional in terms
of the exact Fradkin representations of the Green's functional and the vacuum
functional. Because quarks exist asymptotically only in bound states, their
transverse coordinates can never be measured with arbitrary precision; the
non-perturbative neglect of this statement leads to obstructions that are
easily corrected by invoking in the basic Lagrangian a probability amplitude
which describes such transverse imprecision.
The second result of this non-perturbative analysis is the appearance of a
new and simplifying output called "Effective Locality", in which the
interactions between quarks by the exchange of a "gluon bundle" - which
"bundle" contains an infinite number of gluons, including cubic and quartic
gluon interactions - display an exact locality property that reduces the
several functional integrals of the formulation down to a set of ordinary
integrals. It should be emphasized that "non-perturbative" here refers to the
effective summation of all gluons between a pair of quark lines, but does not
(yet) include a summation over all closed-quark loops which are tied by
gluon-bundle exchange to the rest of the "Bundle Diagram". As an example of the
power of these methods we offer as a first analytic calculation the
quark-antiquark binding potential of a pion, and the corresponding three-quark
binding potential of a nucleon, obtained in a simple way from relevant eikonal
scattering approximations.Comment: 38 pages, 3 figures in REVTeX. Collections of follow-on work of Eur.
Phys. J. C65, pp. 395-411 (2010). arXiv admin note: text overlap with
arXiv:1103.4179, arXiv:1104.4663, arXiv:1003.293
Quantum feedback control of a solid-state qubit
We have studied theoretically the basic operation of a quantum feedback loop
designed to maintain a desired phase of quantum coherent oscillations in a
single solid-state qubit. The degree of oscillations synchronization with
external harmonic signal is calculated as a function of feedback strength,
taking into account available bandwidth and coupling to environment.
The feedback can efficiently suppress the dephasing of oscillations if the
qubit coupling to the detector is stronger than coupling to environment.Comment: Extended version of cond-mat/0107280 (5 pages, 5 figures); to be
published in PRB (RC
Removal of a single photon by adaptive absorption
We present a method to remove, using only linear optics, exactly one photon
from a field-mode. This is achieved by putting the system in contact with an
absorbing environment which is under continuous monitoring. A feedback
mechanism then decouples the system from the environment as soon as the first
photon is absorbed. We propose a possible scheme to implement this process and
provide the theoretical tools to describe it
Microscopic calculation of the spin-dependent neutron scattering lengths on 3He
We report on the spin.dependent neutron scattering length on 3He from a
microscopic calculation of p-3H, n-3He, and d-2H scattering employing the
Argonne v18 nucleon-nucleon potential with and without additional three-nucleon
force. The results and that of a comprehensive R-matrix analysis are compared
to a recent measurement. The overall agreement for the scattering lengths is
quite good. The imaginary parts of the scattering lengths are very sensitive to
the inclusion of three-nucleon forces, whereas the real parts are almost
insensitive.Comment: 9 pages, 1 figur
Causality in quantum teleportation: information extraction and noise effects in entanglement distribution
Quantum teleportation is possible because entanglement allows a definition of
precise correlations between the non-commuting properties of a local system and
corresponding non-commuting properties of a remote system. In this paper, the
exact causality achieved by maximal entanglement is analyzed and the results
are applied to the transfer of effects acting on the entanglement distribution
channels to the teleported output state. In particular, it is shown how
measurements performed on the entangled system distributed to the sender
provide information on the teleported state while transferring the
corresponding back-action to the teleported quantum state.Comment: 14 pages, including three figures, discussion of fidelity adde
Elastic p-3He and n-3H scattering with two- and three-body forces
We report on a microscopic calculation of n-3H and p-3He scattering employing
the Argonne v_{18} and v_8' nucleon-nucleon potentials with and without
additional three-nucleon force. An R-matrix analysis of the p-3He and n-3H
scattering data is presented. Comparisons are made for the phase shifts and a
selection of measurements in both scattering systems. Differences between our
calculation and the R-matrix results or the experimental data can be attributed
to only two partial waves (3P0 and 3P2). We find the effect of the Urbana IX
and the Texas-Los Alamos three-nucleon forces on the phase shifts to be
negligible.Comment: submitted to Phys. Rev.
Pattern matching and pattern discovery algorithms for protein topologies
We describe algorithms for pattern matching and pattern
learning in TOPS diagrams (formal descriptions of protein topologies).
These problems can be reduced to checking for subgraph isomorphism
and finding maximal common subgraphs in a restricted class of ordered
graphs. We have developed a subgraph isomorphism algorithm for
ordered graphs, which performs well on the given set of data. The
maximal common subgraph problem then is solved by repeated
subgraph extension and checking for isomorphisms. Despite the
apparent inefficiency such approach gives an algorithm with time
complexity proportional to the number of graphs in the input set and is
still practical on the given set of data. As a result we obtain fast
methods which can be used for building a database of protein
topological motifs, and for the comparison of a given protein of known
secondary structure against a motif database
Non-Markovian homodyne-mediated feedback on a two-level atom: a quantum trajectory treatment
Quantum feedback can stabilize a two-level atom against decoherence
(spontaneous emission), putting it into an arbitrary (specified) pure state.
This requires perfect homodyne detection of the atomic emission, and
instantaneous feedback. Inefficient detection was considered previously by two
of us. Here we allow for a non-zero delay time in the feedback circuit.
Because a two-level atom is a nonlinear optical system, an analytical solution
is not possible. However, quantum trajectories allow a simple numerical
simulation of the resulting non-Markovian process. We find the effect of the
time delay to be qualitatively similar to that of inefficient detection. The
solution of the non-Markovian quantum trajectory will not remain fixed, so that
the time-averaged state will be mixed, not pure. In the case where one tries to
stabilize the atom in the excited state, an approximate analytical solution to
the quantum trajectory is possible. The result, that the purity () of the average state is given by (where
is the spontaneous emission rate) is found to agree very well with the
numerical results.Comment: Changed content, Added references and Corrected typo
Microscopic Calculation of the System
We report on a consistent, microscopic calculation of the bound and
scattering states in the 4He system employing a realistic nucleon-nucleon
potential in the framework of the resonating group model (RGM). We present for
comparison with these microscopic RGM calculations the results from a
charge-independent, Coulomb-corrected R-matrix analysis of all types of data
for reactions in the A=4 system. Comparisons are made between the phase shifts,
and with a selection of measurements from each reaction, as well as between the
resonance spectra obtained from both calculations. In general, the comparisons
are favorable, but distinct differences are observed between the RGM
calculations and some of the polarisation data. The partial-wave decomposition
of the experimental data produced by the R-matrix analysis shows that these
differences can be attributed to just a few S-matrix elements, for which
inadequate tensor-force strength in the N-N interaction used appears to be
responsible.Comment: 53 pages, PS document can also be found at
ftp://theorie3.physik.uni-erlangen.de/pub/publications/he4.ps.g
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