8,557 research outputs found
Quantum measurement of the degree of polarization of a light beam
We demonstrate a coherent quantum measurement for the determination of the
degree of polarization (DOP). This method allows to measure the DOP in the
presence of fast polarization state fluctuations, difficult to achieve with the
typically used polarimetric technique. A good precision of the DOP measurements
is obtained using 8 type II nonlinear crystals assembled for spatial walk-off
compensation.Comment: 4 pages, 3 figure
High fidelity readout scheme for rare-earth solid state quantum computing
We propose and analyze a high fidelity readout scheme for a single instance
approach to quantum computing in rare-earth-ion-doped crystals. The scheme is
based on using different species of qubit and readout ions, and it is shown
that by allowing the closest qubit ion to act as a readout buffer, the readout
error can be reduced by more than an order of magnitude. The scheme is shown to
be robust against certain experimental variations, such as varying detection
efficiencies, and we use the scheme to predict the expected quantum fidelity of
a CNOT gate in these solid state systems. In addition, we discuss the potential
scalability of the protocol to larger qubit systems. The results are based on
parameters which we believed are experimentally feasible with current
technology, and which can be simultaneously realized.Comment: 7 pages, 5 figure
Response of the large-scale subglacial drainage system of Northeast Greenland to surface elevation changes
The influence of subglacial water on the dynamics of ice flow has been the
object of increasing interest in the past decade. In this study we focus on
large-scale, long-term changes in surface elevation over Northeast
Greenland and the corresponding changes in subglacial water routeways. Our
results show that over timescales ranging from decades to millennia the area
may experience redistribution of and fluctuation in subglacial water outflux
under the main glacier outlets. The fluctuations in subglacial water routing
occur even in the absence of external forcing. Based on these results we
conclude that changes in the subglacial water routeways are an intrinsic part
of the drainage basin dynamics, where the subglacial system is likely always
in a transient state. The results also imply that fluctuations at the margins
observed at present might originate from changes several hundred kilometres
upstream. Since surface elevation changes may propagate upstream over
timescales much longer than the observational period, the cause of the
fluctuations may not be present in current observational records
GW approximation with self-screening correction
The \emph{GW} approximation takes into account electrostatic self-interaction
contained in the Hartree potential through the exchange potential. However, it
has been known for a long time that the approximation contains self-screening
error as evident in the case of the hydrogen atom. When applied to the hydrogen
atom, the \emph{GW} approximation does not yield the exact result for the
electron removal spectra because of the presence of self-screening: the hole
left behind is erroneously screened by the only electron in the system which is
no longer present. We present a scheme to take into account self-screening and
show that the removal of self-screening is equivalent to including exchange
diagrams, as far as self-screening is concerned. The scheme is tested on a
model hydrogen dimer and it is shown that the scheme yields the exact result to
second order in where and are respectively
the onsite and offsite Hubbard interaction parameters and the hopping
parameter.Comment: 9 pages, 2 figures; Submitted to Phys. Rev.
Experimental investigation of a coherent quantum measurement of the degree of polarization of a single mode light beam
A novel method for the direct measurement of the degree of polarization is
described. It is one of the first practical implementations of a coherent
quantum measurement, the projection on the singlet state. Our first results
demonstrate the successful operation of the method. However, due to the
nonlinear crystals used presently, its application is limited to spectral
widths larger than ~8nm.Comment: 23 pages, 9 figures, submitted to Journal of Modern Optic
The Hamburg/ESO R-process Enhanced Star survey (HERES) VI. The Galactic Chemical Evolution of Silicon
We determined the silicon abundances of 253 metal-poor stars in the
metallicity range , based on non-local thermodynamic
equilibrium (NLTE) line formation calculations of neutral silicon and
high-resolution spectra obtained with VLT-UT2/UVES. The
dependence of [Si/Fe] noticed in previous investigation is diminished in our
abundance analysis due to the inclusion of NLTE effects. An increasing slope of
[Si/Fe] towards decreasing metallicity is present in our results, in agreement
with Galactic chemical evolution models. The small intrinsic scatter of [Si/Fe]
in our sample may imply that these stars formed in a region where the yields of
type II supernovae were mixed into a large volume, or that the formation of
these stars was strongly clustered, even if the ISM was enriched by single SNa
II in a small mixing volume. We identified two dwarfs with
: HE 01313953, and HE 14301123. These
main-sequence turnoff stars are also carbon-enhanced. They might have been
pre-enriched by sub-luminous supernovae.Comment: 12 pages, 9 figures, 1 electronical table. Accepted by A &
Beaver and Naked Mole Rat Genomes Reveal Common Paths to Longevity
Long-lived rodents have become an attractive model for the studies on aging. To understand evolutionary paths to long life, we prepare chromosome-level genome assemblies of the two longest-lived rodents, Canadian beaver (Castor canadensis) and naked mole rat (NMR, Heterocephalus glaber), which were scaffolded with in vitro proximity ligation and chromosome conformation capture data and complemented with long-read sequencing. Our comparative genomic analyses reveal that amino acid substitutions at disease-causing sites are widespread in the rodent genomes and that identical substitutions in long-lived rodents are associated with common adaptive phenotypes, e.g., enhanced resistance to DNA damage and cellular stress. By employing a newly developed substitution model and likelihood ratio test, we find that energy and fatty acid metabolism pathways are enriched for signals of positive selection in both long-lived rodents. Thus, the high-quality genome resource of long-lived rodents can assist in the discovery of genetic factors that control longevity and adaptive evolution
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