663 research outputs found
Entanglement criteria for Dicke states
Dicke states are a family of multi-qubit quantum states with interesting
entanglement properties and have been observed in many experiments. We
construct entanglement witnesses for detecting genuine multiparticle
entanglement in the vicinity of these states. We use the approach of PPT
mixtures to derive the conditions analytically. For nearly all cases, our
criteria are stronger than all conditions previously known.Comment: 18 pages, 4 figures; updated to match published versio
RXJ0848.6+4453: The Evolution of Galaxy Sizes and Stellar Populations in a z=1.27 Cluster
RXJ0848.6+4453 (Lynx W) at redshift 1.27 is part of the Lynx Supercluster of
galaxies. Our analysis of stellar populations and star formation history in the
cluster covers 24 members and is based on deep optical spectroscopy from Gemini
North and imaging data from HST. Focusing on the 13 bulge-dominated galaxies
for which we can determine central velocity dispersions, we find that these
show a smaller evolution of sizes and velocity dispersions than reported for
field galaxies and galaxies in poorer clusters. The galaxies in RXJ0848.6+4453
populate the Fundamental Plane similar to that found for lower redshift
clusters with a zero point offset corresponding to an epoch of last star
formation at z_form= 1.95+-0.2. The spectra of the galaxies in RXJ0848.6+4453
are dominated by young stellar populations at all galaxy masses and in many
cases show emission indicating low level on-going star formation. The average
age of the young stellar populations (estimated from H-zeta) is consistent with
a major star formation episode 1-2 Gyr prior, which in turn agrees with
z_form=1.95. Galaxies dominated by young stellar populations are distributed
throughout the cluster. We speculate that low level star formation has not yet
been fully quenched in the center of this cluster may be because the cluster is
significantly poorer than other clusters previously studied at similar
redshifts, which appear to have very little on-going star formation in their
centers.Comment: Accepted for publication in Astronomical Journal. High-resolution
figures available from the first author by reques
RXJ0152.7-1357: Stellar populations in an X-ray luminous galaxy cluster at z=0.83
We present a study of the stellar populations of galaxies in the cluster
RXJ0152.7-1357 at a redshift of 0.83. The study is based on new high S/N
spectroscopy of 29 cluster members covering the wavelength range 5000-10000A as
well as r'i'z' photometry of the cluster. The scaling relations between
velocity dispersions, luminosities and Balmer line strengths appear to be in
agreement with pure passive evolution of the stellar populations with a
formation redshift z=4. However, the strengths of the D4000 indices and the
metal indices do not support this interpretation. Compared to z=0, the metal
indices (C4668, Fe4383, CN3883, G4300 and CN2) show that at least half of the
non-emission line galaxies in RXJ0152.7-1357 have [alpha/Fe] of 0.2 dex higher,
and about half of the galaxies have significantly lower metal content. The
differences in stellar populations of the galaxies are associated with the
location of the galaxies relative to the X-ray emission. The galaxies with weak
C4668 and G4300, as well as galaxies with weak [OII] emission, are located in
areas of low X-ray luminosity. It is possible that these galaxies are
experiencing the effect of the cluster merger taking place in RXJ0152.7-1357 as
(short) episodes of star formation, while the galaxies in the cores of the
X-ray sub-clumps are unaffected by the merger. The spectroscopy of the
RXJ0152.7-1357 galaxies shows for the first time galaxies in a rich cluster at
intermediate redshift that cannot evolve passively into the present day galaxy
population in rich clusters. Additional physical processes may be at work and
we speculate that merging with infalling (disk) galaxies in which stars have
formed over an extended period might produce the required reduction in
[alpha/Fe]. (abridged)Comment: Accepted for publication in Astronomical Journal, 39 pages, uses
emulateapj.cls, 18 figures. High resolution color figures available by e-mail
request to the first autho
Nonlinear quantum error correction
We introduce a theory of quantum error correction (QEC) for a subclass of
states within a larger Hilbert space. In the standard theory of QEC, the set of
all encoded states is formed by an arbitrary linear combination of the
codewords. However, this can be more general than required for a given quantum
protocol which may only traverse a subclass of states within the Hilbert space.
Here we propose the concept of nonlinear QEC (NLQEC), where the encoded states
are not necessarily a linear combination of codewords. We introduce a
sufficiency criterion for NLQEC with respect to the subclass of states. The new
criterion gives a more relaxed condition for the formation of a QEC code, such
that under the assumption that the states are within the subclass of states,
the errors are correctable. This allows us, for instance, to effectively
circumvent the no-go theorems regarding optical QEC for Gaussian states and
channels, for which we present explicit examples
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