39,245 research outputs found
Ginsparg-Wilson Relation and Ultralocality
It is shown that it is impossible to construct a free theory of fermions on
infinite hypercubic Euclidean lattice in four dimensions that is: (a)
ultralocal, (b) respects symmetries of hypercubic lattice, (c) corresponding
kernel satisfies D gamma5 + gamma5 D = D gamma5 D (Ginsparg-Wilson relation),
(d) describes single species of massless Dirac fermions in the continuum limit.Comment: 4 pages, REVTEX; few minor change
Collective Fields for QCD
A gauge-symmetric approach to effective Lagrangians is described with special
emphasis on derivations of effective low-energy Lagrangians from QCD. The
examples we discuss are based on exact rewritings of cut-off QCD in terms of
new collective degrees of freedom. These cut-off Lagrangians are thus
``effective'' in the sense that they explicitly contain some of the physical
long-distance degrees of freedom from the outset.(Talk presented by P.H.
Damgaard at the workshop on ``Quantum Field Theoretical Methods in High Energy
Physics'', Kyffhauser, Germany, Sept. 1993. To appear in those proceedings).Comment: LaTeX, 12 pages, CERN--TH-7035/9
Trace functions as Laplace transforms
We study trace functions on the form t\to\tr f(A+tB) where is a
real function defined on the positive half-line, and and are
matrices such that is positive definite and is positive
semi-definite. If is non-negative and operator monotone decreasing, then
such a trace function can be written as the Laplace transform of a positive
measure. The question is related to the Bessis-Moussa-Villani conjecture.
Key words: Trace functions, BMV-conjecture.Comment: Minor change of style, update of referenc
Ginsparg-Wilson-Luscher Symmetry and Ultralocality
Important recent discoveries suggest that Ginsparg-Wilson-Luscher (GWL)
symmetry has analogous dynamical consequences for the theory on the lattice as
chiral symmetry does in the continuum. While it is well known that inherent
property of lattice chiral symmetry is fermion doubling, we show here that
inherent property of GWL symmetry is that the infinitesimal symmetry
transformation couples fermionic degrees of freedom at arbitrarily large
lattice distances (non-ultralocality). The consequences of this result for
ultralocality of symmetric actions are discussed.Comment: 18 pages, LATEX. For clarity changed to infinitesimal
transformations, typos corrected, explicit hypothesis adde
Maintaining Quantum Coherence in the Presence of Noise through State Monitoring
Unsharp POVM measurements allow the estimation and tracking of quantum
wavefunctions in real-time with minimal disruption of the dynamics. Here we
demonstrate that high fidelity state monitoring, and hence quantum control, is
possible even in the presence of classical dephasing and amplitude noise, by
simulating such measurements on a two-level system undergoing Rabi
oscillations. Finite estimation fidelity is found to persist indefinitely long
after the decoherence times set by the noise fields in the absence of
measurement.Comment: 5 pages, 4 figure
Signatures of orbital loop currents in the spatially resolved local density of states
Polarized neutron scattering measurements have suggested that intra-unit cell
antiferromagnetism may be associated with the pseudogap phase. Assuming that
loop current order is responsible for the observed magnetism, we calculate some
signatures of such circulating currents in the local density of states around a
single non-magnetic impurity in a coexistence phase with superconductivity. We
find a distinct C4 symmetry breaking near the disorder which is also detectable
in the resulting quasi-particle interference patterns.Comment: 5 pages, 3 figure
Lie Algebras and Suppression of Decoherence in Open Quantum Systems
Since there are many examples in which no decoherence-free subsystems exist
(among them all cases where the error generators act irreducibly on the system
Hilbert space), it is of interest to search for novel mechanisms which suppress
decoherence in these more general cases. Drawing on recent work
(quant-ph/0502153) we present three results which indicate decoherence
suppression without the need for noiseless subsystems. There is a certain
trade-off; our results do not necessarily apply to an arbitrary initial density
matrix, or for completely generic noise parameters. On the other hand, our
computational methods are novel and the result--suppression of decoherence in
the error-algebra approach without noiseless subsystems--is an interesting new
direction.Comment: 7 page
Unstable Modes in Three-Dimensional SU(2) Gauge Theory
We investigate SU(2) gauge theory in a constant chromomagnetic field in three
dimensions both in the continuum and on the lattice. Using a variational method
to stabilize the unstable modes, we evaluate the vacuum energy density in the
one-loop approximation. We compare our theoretical results with the outcomes of
the numerical simulations.Comment: 24 pages, REVTEX 3.0, 3 Postscript figures included. (the whole
postscript file (text+figures) is available on request from
[email protected]
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