873 research outputs found
Mott insulator states of ultracold atoms in optical resonators
We study the low temperature physics of an ultracold atomic gas in the
potential formed inside a pumped optical resonator. Here, the height of the
cavity potential, and hence the quantum state of the gas, depends not only on
the pump parameters, but also on the atomic density through a dynamical
a.c.-Stark shift of the cavity resonance. We derive the Bose-Hubbard model in
one dimension, and use the strong coupling expansion to determine the parameter
regime in which the system is in the Mott-insulator state. We predict the
existence of overlapping, competing Mott states, and bistable behavior in the
vicinity of the shifted cavity resonance, controlled by the pump parameters.
Outside these parameter regions, the state of the system is in most cases
superfluid.Comment: 4 pages, 3 figures. Substantially revised version. To appear in Phys.
Rev. Let
Generalized qudit Choi maps
Following the linear programming prescription of Ref. \cite{PRA72}, the
Bell diagonal entanglement witnesses are provided. By using
Jamiolkowski isomorphism, it is shown that the corresponding positive maps are
the generalized qudit Choi maps. Also by manipulating particular
Bell diagonal separable states and constructing corresponding bound entangled
states, it is shown that thus obtained BDEW's (consequently qudit
Choi maps) are non-decomposable in certain range of their parameters.Comment: 22 page
Separable approximations of density matrices of composite quantum systems
We investigate optimal separable approximations (decompositions) of states
rho of bipartite quantum systems A and B of arbitrary dimensions MxN following
the lines of Ref. [M. Lewenstein and A. Sanpera, Phys. Rev. Lett. 80, 2261
(1998)]. Such approximations allow to represent in an optimal way any density
operator as a sum of a separable state and an entangled state of a certain
form. For two qubit systems (M=N=2) the best separable approximation has a form
of a mixture of a separable state and a projector onto a pure entangled state.
We formulate a necessary condition that the pure state in the best separable
approximation is not maximally entangled. We demonstrate that the weight of the
entangled state in the best separable approximation in arbitrary dimensions
provides a good entanglement measure. We prove in general for arbitrary M and N
that the best separable approximation corresponds to a mixture of a separable
and an entangled state which are both unique. We develop also a theory of
optimal separable approximations for states with positive partial transpose
(PPT states). Such approximations allow to decompose any density operator with
positive partial transpose as a sum of a separable state and an entangled PPT
state. We discuss procedures of constructing such decompositions.Comment: 12 pages, 2 figure
Robustness of Fractional Quantum Hall States with Dipolar Atoms in Artificial Gauge Fields
The robustness of fractional quantum Hall states is measured as the energy
gap separating the Laughlin ground-state from excitations. Using thermodynamic
approximations for the correlation functions of the Laughlin state and the
quasihole state, we evaluate the gap in a two-dimensional system of dipolar
atoms exposed to an artificial gauge field. For Abelian fields, our results
agree well with the results of exact diagonalization for small systems, but
indicate that the large value of the gap predicted in [Phys. Rev. Lett. 94,
070404 (2005)] was overestimated. However, we are able to show that the small
gap found in the Abelian scenario is dramatically increased if we turn to
non-Abelian fields squeezing the Landau levels
Classification of mixed three-qubit states
We introduce a classification of mixed three-qubit states, in which we define
the classes of separable, biseparable, W- and GHZ-states. These classes are
successively embedded into each other. We show that contrary to pure W-type
states, the mixed W-class is not of measure zero. We construct witness
operators that detect the class of a mixed state. We discuss the conjecture
that all entangled states with positive partial transpose (PPTES) belong to the
W-class. Finally, we present a new family of PPTES "edge" states with maximal
ranks.Comment: 4 pages, 1 figur
Scaling of entanglement at quantum phase transition for two-dimensional array of quantum dots
With Hubbard model, the entanglement scaling behavior in a two-dimensional
itinerant system is investigated. It has been found that, on the two sides of
the critical point denoting an inherent quantum phase transition (QPT), the
entanglement follows different scalings with the size just as an order
parameter does. This fact reveals the subtle role played by the entanglement in
QPT as a fungible physical resource
Multipartite entanglement percolation
We present percolation strategies based on multipartite measurements to
propagate entanglement in quantum networks. We consider networks spanned on
regular lattices whose bonds correspond to pure but non-maximally entangled
pairs of qubits, with any quantum operation allowed at the nodes. Despite
significant effort in the past, improvements over naive (classical) percolation
strategies have been found for only few lattices, often with restrictions on
the initial amount of entanglement in the bonds. In contrast, multipartite
entanglement percolation outperform the classical percolation protocols, as
well as all previously known quantum ones, over the entire range of initial
entanglement and for every lattice that we considered.Comment: revtex4, 4 page
Entanglement and coherence in quantum state merging
Understanding the resource consumption in distributed scenarios is one of the
main goals of quantum information theory. A prominent example for such a
scenario is the task of quantum state merging where two parties aim to merge
their parts of a tripartite quantum state. In standard quantum state merging,
entanglement is considered as an expensive resource, while local quantum
operations can be performed at no additional cost. However, recent developments
show that some local operations could be more expensive than others: it is
reasonable to distinguish between local incoherent operations and local
operations which can create coherence. This idea leads us to the task of
incoherent quantum state merging, where one of the parties has free access to
local incoherent operations only. In this case the resources of the process are
quantified by pairs of entanglement and coherence. Here, we develop tools for
studying this process, and apply them to several relevant scenarios. While
quantum state merging can lead to a gain of entanglement, our results imply
that no merging procedure can gain entanglement and coherence at the same time.
We also provide a general lower bound on the entanglement-coherence sum, and
show that the bound is tight for all pure states. Our results also lead to an
incoherent version of Schumacher compression: in this case the compression rate
is equal to the von Neumann entropy of the diagonal elements of the
corresponding quantum state.Comment: 9 pages, 1 figure. Lemma 5 in Appendix D of the previous version was
not correct. This did not affect the results of the main tex
Edge Transport in 2D Cold Atom Optical Lattices
We theoretically study the observable response of edge currents in two
dimensional cold atom optical lattices. As an example we use Gutzwiller
mean-field theory to relate persistent edge currents surrounding a Mott
insulator in a slowly rotating trapped Bose-Hubbard system to time of flight
measurements. We briefly discuss an application, the detection of Chern number
using edge currents of a topologically ordered optical lattice insulator
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