51 research outputs found
Linear growth of the entanglement entropy and the Kolmogorov-Sinai rate
The rate of entropy production in a classical dynamical system is
characterized by the Kolmogorov-Sinai entropy rate given by
the sum of all positive Lyapunov exponents of the system. We prove a quantum
version of this result valid for bosonic systems with unstable quadratic
Hamiltonian. The derivation takes into account the case of time-dependent
Hamiltonians with Floquet instabilities. We show that the entanglement entropy
of a Gaussian state grows linearly for large times in unstable systems,
with a rate determined by the Lyapunov exponents and
the choice of the subsystem . We apply our results to the analysis of
entanglement production in unstable quadratic potentials and due to periodic
quantum quenches in many-body quantum systems. Our results are relevant for
quantum field theory, for which we present three applications: a scalar field
in a symmetry-breaking potential, parametric resonance during post-inflationary
reheating and cosmological perturbations during inflation. Finally, we
conjecture that the same rate appears in the entanglement growth of
chaotic quantum systems prepared in a semiclassical state.Comment: 50+17 Pages, 11 figure
Gravitational quantum switch on a superposition of spherical shells
The existence of states describing quantum superpositions of distinct
spacetime geometries is a feature that is generally expected in theories of
quantum gravity. Phenomenological models have recently been explored for the
analysis of possible effects due to a superposition of geometries, including
the occurrence of processes with indefinite order. In a gravitational quantum
switch, in particular, the order of operations applied by two agents on a
target system is entangled with the state of the geometry. We consider a model
describing the superposition of geometries produced by distinct arrangements of
spherical mass shells, and show that a protocol for the implementation of a
gravitational quantum switch can be formulated in such a system. The geometries
in superposition are identical in an exterior region outside a given radius,
and differ within such a radius. The exterior region provides a classical frame
from which the superposition of geometries in the interior region can be
probed. One of the agents crosses the interior region and becomes entangled
with the geometry, which is explored as a resource for the implementation of
the quantum switch. Novel features of the protocol include the superposition of
nonisometric geometries, the existence of a region with a definite geometry,
and the fact that the agent that experiences the superposition of geometries is
in free fall, preventing information on the global geometry to be obtained from
within its laboratory.Comment: 22 pages, 5 figures. References adde
Topological low-temperature limit of Z(2) spin-gauge theory in three dimensions
We study Z(2) lattice gauge theory on triangulations of a compact 3-manifold.
We reformulate the theory algebraically, describing it in terms of the
structure constants of a bidimensional vector space H equipped with algebra and
coalgebra structures, and prove that in the low-temperature limit H reduces to
a Hopf Algebra, in which case the theory becomes equivalent to a topological
field theory. The degeneracy of the ground state is shown to be a topological
invariant. This fact is used to compute the zeroth- and first-order terms in
the low-temperature expansion of Z for arbitrary triangulations. In finite
temperatures, the algebraic reformulation gives rise to new duality relations
among classical spin models, related to changes of basis of H.Comment: 10 pages, no figure
In vivo single cell analysis reveals Gata2 dynamics in cells transitioning to hematopoietic fate
Cell fate is established through coordinated gene expression programs in individual cells. Regulatory networks that include the Gata2 transcription factor play central roles in hematopoietic fate establishment. Although Gata2 is essential to the embryonic development and function of hematopoietic stem cells that form the adult hierarchy, little is known about the in vivo expression dynamics of Gata2 in single cells. Here, we examine Gata2 expression in single aortic cells as they establish hematopoietic fate in Gata2Venus mouse embryos. Time-lapse imaging reveals rapid pulsatile level changes in Gata2 reporter expression in cells undergoing endothelial-to-hematopoietic transition. Moreover, Gata2 reporter pulsatile expression is dramatically altered in Gata2+/- aortic cells, which undergo fewer transitions and are reduced in hematopoietic potential. Our novel finding of dynamic pulsatile expression of Gata2 suggests a highly unstable genetic state in single cells concomitant with their transition to hematopoietic fate. This reinforces the notion that threshold levels of Gata2 influence fate establishment and has implications for transcription factor-related hematologic dysfunctions
Cell signalling by reactive lipid species: new concepts and molecular mechanisms
The process of lipid peroxidation is widespread in biology and is mediated through both enzymatic and non-enzymatic pathways. A significant proportion of the oxidized lipid products are electrophilic in nature, the RLS (reactive lipid species), and react with cellular nucleophiles such as the amino acids cysteine, lysine and histidine. Cell signalling by electrophiles appears to be limited to the modification of cysteine residues in proteins, whereas non-specific toxic effects involve modification of other nucleophiles. RLS have been found to participate in several physiological pathways including resolution of inflammation, cell death and induction of cellular antioxidants through the modification of specific signalling proteins. The covalent modification of proteins endows some unique features to this signalling mechanism which we have termed the ‘covalent advantage’. For example, covalent modification of signalling proteins allows for the accumulation of a signal over time. The activation of cell signalling pathways by electrophiles is hierarchical and depends on a complex interaction of factors such as the intrinsic chemical reactivity of the electrophile, the intracellular domain to which it is exposed and steric factors. This introduces the concept of electrophilic signalling domains in which the production of the lipid electrophile is in close proximity to the thiol-containing signalling protein. In addition, we propose that the role of glutathione and associated enzymes is to insulate the signalling domain from uncontrolled electrophilic stress. The persistence of the signal is in turn regulated by the proteasomal pathway which may itself be subject to redox regulation by RLS. Cell death mediated by RLS is associated with bioenergetic dysfunction, and the damaged proteins are probably removed by the lysosome-autophagy pathway
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