239,728 research outputs found
Instanton solutions mediating tunneling between the degenerate vacua in curved space
We investigate the instanton solution between the degenerate vacua in curved
space. We show that there exist -symmetric solutions not only in de
Sitter but also in both flat and anti-de Sitter space. The geometry of the new
type of solutions is finite and preserves the symmetry. The nontrivial
solution corresponding to the tunneling is possible only if gravity is taken
into account. The numerical solutions as well as the analytic computations
using the thin-wall approximation are presented. We expect that these solutions
do not have any negative mode as in the instanton solution.Comment: Some typos are corrected and references are added with respect to the
published version. 17pages, 11fi
Quantifying mixed-state quantum entanglement by optimal entanglement witness
We develop an approach of quantifying entanglement in mixed quantum states by
the optimal entanglement witness operator. We identify the convex set of mixed
states for which a single witness provides the exact value of an entanglement
measure, and show that the convexity, properties, and symmetries of
entanglement or of a target state considerably fix the form of the optimal
witness. This greatly reduces difficulty in computing and experimentally
determining entanglement measures. As an example, we show how to experimentally
quantify bound entanglement in four-qubit noisy Smolin states and three-qubit
Greenberger-Horne-Zeilinger (GHZ) entanglement under white noise. For general
measures and states, we provide a numerical method to efficiently optimize
witness.Comment: Supplemental material is include
Universal pulse sequence to minimize spin dephasing in the central spin decoherence problem
We present a remarkable finding that a recently discovered [G. S. Uhrig,
Phys. Rev. Lett. 98, 100504 (2007)] series of pulse sequences, designed to
optimally restore coherence to a qubit in the spin-boson model of decoherence,
is in fact completely model-independent and generically valid for arbitrary
dephasing Hamiltonians given sufficiently short delay times between pulses. The
series maximizes qubit fidelity versus number of applied pulses for
sufficiently short delay times because the series, with each additional pulse,
cancels successive orders of a time expansion for the fidelity decay. The
"magical" universality of this property, which was not appreciated earlier,
requires that a linearly growing set of "unknowns" (the delay times) must
simultaneously satisfy an exponentially growing set of nonlinear equations that
involve arbitrary dephasing Hamiltonian operators.Comment: Published in PRL, revise
Dynamics of warped compactifications and the shape of the warped landscape
The dynamics of warped/flux compactifications is studied, including warping
effects, providing a firmer footing for investigation of the "landscape." We
present a general formula for the four-dimensional potential of warped
compactifications in terms of ten-dimensional quantities. This allows a
systematic investigation of moduli-fixing effects and potentials for mobile
branes. We provide a necessary criterion, "slope-dominance," for evading
"no-go" results for de Sitter vacua. We outline the ten-dimensional derivation
of the non-perturbative effects that should accomplish this in KKLT examples,
and outline a systematic discussion of their corrections. We show that
potentials for mobile branes receive generic contributions inhibiting slow-roll
inflation. We give a linearized analysis of general scalar perturbations of
warped IIB compactifications, revealing new features for both time independent
and dependent moduli, and new aspects of the kinetic part of the
four-dimensional effective action. The universal Kahler modulus is found_not_
to be a simple scaling of the internal metric, and a prescription is given for
defining holomorphic Kahler moduli, including warping effects. In the presence
of mobile branes, this prescription elucidates couplings between bulk and brane
fields. Our results are thus relevant to investigations of the existence of de
Sitter vacua in string theory, and of their phenomenology, cosmology, and
statistics.Comment: (80 pages; two appendices; harvmac. v3: minor corrections, and
references added. v4: argument that pure Kahler deformations are flat.
Remarks on the Reeh-Schlieder property for higher spin free fields on curved spacetimes
The existence of states enjoying a weak form of the Reeh-Schlieder property
has been recently established on curved backgrounds and in the framework of
locally covariant quantum field theory. Since only the example of a real scalar
field has been discussed, we extend the analysis to the case of massive and
massless free fields either of spin 1/2 or of spin 1. In the process, it is
also shown that both the vector potential and the Proca field can be described
as a locally covariant quantum field theory.Comment: 28 pages, references and remarks added, typos correcte
Oscillating instanton solutions in curved space
We investigate oscillating instanton solutions of a self-gravitating scalar
field between degenerate vacua. We show that there exist O(4)-symmetric
oscillating solutions in a de Sitter background. The geometry of this solution
is finite and preserves the symmetry. The nontrivial solution
corresponding to tunneling is possible only if the effect of gravity is taken
into account. We present numerical solutions of this instanton, including the
phase diagram of solutions in terms of the parameters of the present work and
the variation of energy densities. Our solutions can be interpreted as
solutions describing an instanton-induced domain wall or braneworld-like object
rather than a kink-induced domain wall or braneworld. The oscillating instanton
solutions have a thick wall and the solutions can be interpreted as a mechanism
providing nucleation of the thick wall for topological inflation. We remark
that invariant solutions also exist in a flat and anti-de Sitter
background, though the physical significance is not clear.Comment: 25 pages, 11 figues. Some typos corrected, references added, and Ch3.
modified according to referee's comment
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