2,706,767 research outputs found
Entanglement of Atomic Ensembles by Trapping Correlated Photon States
We describe a general technique that allows for an ideal transfer of quantum
correlations between light fields and metastable states of matter. The
technique is based on trapping quantum states of photons in coherently driven
atomic media, in which the group velocity is adiabatically reduced to zero. We
discuss possible applications such as quantum state memories, generation of
squeezed atomic states, preparation of entangled atomic ensembles and quantum
information processing
On the information entropy of matter-waves in quasi-disorder potentials
We consider ultracold Bose gases in quasi-random potentials and quantify
localization of matter waves by means of Shannon information entropy. We
explicitly examine the role of quasi-random potentials in producing localized
states in the linear and nonlinear regimes. It is seen that the information
entropic-based approach can be more useful to quantify localization of
different types of states observed in Bose-Einstein condensates.Comment: 8 pages, 6 figure
Quantum information cannot be completely hidden in correlations: implications for the black-hole information paradox
The black-hole information paradox has fueled a fascinating effort to
reconcile the predictions of general relativity and those of quantum mechanics.
Gravitational considerations teach us that black holes must trap everything
that falls into them. Quantum mechanically the mass of a black hole leaks away
as featureless (Hawking) radiation, but if the black hole vanishes, where is
the information about the matter that made it? We treat the states of the
in-fallen matter quantum mechanically and show that the black-hole information
paradox becomes more severe. Our formulation of the paradox rules out one of
the most conservative resolutions: that the state of the in-falling matter
might be hidden in correlations between semi-classical Hawking radiation and
the internal states of the black hole. As a consequence, either unitarity or
Hawking's semi-classical predictions must break down. Any resolution of the
black-hole information crisis must elucidate one of these possibilities.Comment: We first obtained this result two years ag
WIMPonium and Boost Factors for Indirect Dark Matter Detection
We argue that WIMP dark matter can annihilate via long-lived "WIMPonium"
bound states in reasonable particle physics models of dark matter (DM).
WIMPonium bound states can occur at or near threshold leading to substantial
enhancements in the DM annihilation rate, closely related to the Sommerfeld
effect. Large "boost factor" amplifications in the annihilation rate can thus
occur without large density enhancements, possibly preferring colder less dense
objects such as dwarf galaxies as locations for indirect DM searches. The
radiative capture to and transitions among the WIMPonium states generically
lead to a rich energy spectrum of annihilation products, with many distinct
lines possible in the case of 2-body decays to or
final states. The existence of multiple radiative capture modes further
enhances the total annihilation rate, and the detection of the lines would give
direct over-determined information on the nature and self-interactions of the
DM particles.Comment: 8 pages, PDFLaTex, 5 figures, references added, published version
Limitations of quantum computing with Gaussian cluster states
We discuss the potential and limitations of Gaussian cluster states for
measurement-based quantum computing. Using a framework of Gaussian projected
entangled pair states (GPEPS), we show that no matter what Gaussian local
measurements are performed on systems distributed on a general graph, transport
and processing of quantum information is not possible beyond a certain
influence region, except for exponentially suppressed corrections. We also
demonstrate that even under arbitrary non-Gaussian local measurements, slabs of
Gaussian cluster states of a finite width cannot carry logical quantum
information, even if sophisticated encodings of qubits in continuous-variable
(CV) systems are allowed for. This is proven by suitably contracting tensor
networks representing infinite-dimensional quantum systems. The result can be
seen as sharpening the requirements for quantum error correction and fault
tolerance for Gaussian cluster states, and points towards the necessity of
non-Gaussian resource states for measurement-based quantum computing. The
results can equally be viewed as referring to Gaussian quantum repeater
networks.Comment: 13 pages, 7 figures, details of main argument extende
Generalizations of entanglement based on coherent states and convex sets
Unentangled pure states on a bipartite system are exactly the coherent states
with respect to the group of local transformations. What aspects of the study
of entanglement are applicable to generalized coherent states? Conversely, what
can be learned about entanglement from the well-studied theory of coherent
states? With these questions in mind, we characterize unentangled pure states
as extremal states when considered as linear functionals on the local Lie
algebra. As a result, a relativized notion of purity emerges, showing that
there is a close relationship between purity, coherence and (non-)entanglement.
To a large extent, these concepts can be defined and studied in the even more
general setting of convex cones of states. Based on the idea that entanglement
is relative, we suggest considering these notions in the context of partially
ordered families of Lie algebras or convex cones, such as those that arise
naturally for multipartite systems. The study of entanglement includes notions
of local operations and, for information-theoretic purposes, entanglement
measures and ways of scaling systems to enable asymptotic developments. We
propose ways in which these may be generalized to the Lie-algebraic setting,
and to a lesser extent to the convex-cones setting. One of our original
motivations for this program is to understand the role of entanglement-like
concepts in condensed matter. We discuss how our work provides tools for
analyzing the correlations involved in quantum phase transitions and other
aspects of condensed-matter systems.Comment: 37 page
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