21,998 research outputs found
Electron spin decoherence of single Nitrogen-Vacancy defects in diamond
We present a theoretical analysis of the electron spin decoherence in single
Nitrogen-Vacancy defects in ultra-pure diamond. The electron spin decoherence
is due to the interactions with Carbon-13 nuclear spins in the diamond lattice.
Our approach takes advantage of the low concentration (1.1%) of Carbon-13 and
their random distribution in the diamond lattice by an algorithmic aggregation
of spins into small, strongly interacting groups. By making use of this
\emph{disjoint cluster} approach, we demonstrate a possibility of non-trival
dynamics of the electron spin that can not be described by a single time
constant. This dependance is caused by a strong coupling between the electron
and few nuclei and results, in particular, in a substantial echo signal even at
microsecond time scales. Our results are in good agreement with recent
experimental observations
Hominin palaeoecology in Late Pliocene Malawi : first insights from isotopes (13C, 18O) in mammal teeth
Carbon-13 and oxygen-18 abundances were measured in large mammal skeletal remains (tooth enamel, dentine and bone) from the Chiwondo Beds in Malawi, which were dated by biostratigraphic correlation to ca. 2.5 million years ago. The biologic isotopic patterns, in particular the difference in carbon-13 abundances between grazers and browsers and the difference in oxygen-18 abundances between semi-aquatic and terrestrial herbivores, were preserved in enamel, but not in dentine and bone. The isotopic results obtained from the skeletal remains from the Chiwondo Beds indicate a dominance of savannah habitats with some trees and shrubs. This environment was more arid than the contemporaneous Ndolanya Beds in Tanzania. The present study confirms that robust australopithecines were able to live in relatively arid environments and were not confined to more mesic environments elsewhere in southern Africa
One-second coherence for a single electron spin coupled to a multi-qubit nuclear-spin environment
Single electron spins coupled to multiple nuclear spins provide promising
multi-qubit registers for quantum sensing and quantum networks. The obtainable
level of control is determined by how well the electron spin can be selectively
coupled to, and decoupled from, the surrounding nuclear spins. Here we realize
a coherence time exceeding a second for a single electron spin through
decoupling sequences tailored to its microscopic nuclear-spin environment. We
first use the electron spin to probe the environment, which is accurately
described by seven individual and six pairs of coupled carbon-13 spins. We
develop initialization, control and readout of the carbon-13 pairs in order to
directly reveal their atomic structure. We then exploit this knowledge to store
quantum states for over a second by carefully avoiding unwanted interactions.
These results provide a proof-of-principle for quantum sensing of complex
multi-spin systems and an opportunity for multi-qubit quantum registers with
long coherence times
The effects of thermohaline mixing on low-metallicity asymptotic giant branch stars
We examine the effects of thermohaline mixing on the composition of the
envelopes of low-metallicity asymptotic giant branch (AGB) stars. We have
evolved models of 1, 1.5 and 2 solar masses from the pre-main sequence to the
end of the thermally pulsing asymptotic giant branch with thermohaline mixing
applied throughout the simulations. In agreement with other authors, we find
that thermohaline mixing substantially reduces the abundance of helium-3 on the
upper part of the red giant branch in our lowest mass model. However, the small
amount of helium-3 that remains is enough to drive thermohaline mixing on the
AGB. We find that thermohaline mixing is most efficient in the early thermal
pulses and its efficiency drops from pulse to pulse. Nitrogen is not
substantially affected by the process, but we do see substantial changes in
carbon-13. The carbon-12 to carbon-13 ratio is substantially lowered during the
early thermal pulses but the efficacy of the process is seen to diminish
rapidly. As the process stops after a few pulses, the carbon-12 to carbon-13
ratio is still able to reach values of 10^3-10^4, which is inconsistent with
the values measured in carbon-enhanced metal-poor stars. We also note a
surprising increase in the lithium-7 abundance, with log epsilon(Li-7) reaching
values of over 2.5 in the 1.5 solar mass model. It is thus possible to get
stars which are both C- and Li-rich at the same time. We compare our models to
measurements of carbon and lithium in carbon-enhanced metal-poor stars which
have not yet reached the giant branch. These models can simultaneously
reproduced the observed C and Li abundances of carbon-enhanced metal-poor
turn-off stars that are Li-rich, but the observed nitrogen abundances still
cannot be matched.Comment: Accepted for publication in MNRAS. 12 pages, 7 figure
Carbon-13 n.m.r. investigation on the nitrogen methylation of the mono- and diazanaphthalenes
The 13C n.m.r. spectra of the N-methylated mono- and diazanaphthalenes have been recorded and analysed. It has been shown that N-methylation as well as N-protonation in cinnoline occur predominantly at the -nitrogen atom. N-methylation and N-protonation show a similar effect on the 13C chemical shift
Substituent effects on the nitrogen-15 and carbon-13 shieldings of some N-arylguanidinium chlorides
The 13C and 15N chemical shifts of five N-arylguanidinium chlorides carrying polar substituents, ranging in character from 4-methoxy to 4-nitro groups, have been determined by NMR spectroscopy at the natural-abundance level of 13C and 15N in dimethyl sulfoxide solution. Comparison of the 13C shifts of these salts with those of monosubstituted benzenes shows that the guanidinium group induces an average downfield shift of -5.8 ppm of the resonance of the aryl carbon to which it is attached (C1), an average upfield shift of +4.2 ppm for C2 and C6, and a small upfield shift of +1.9 ppm for C4. The shifts of C3 and C5 are small and erratic relative to the corresponding carbons in monosubstituted benzenes. The 15N resonances of the guanidinium nitrogens are quite sensitive to electric effects resulting from substitution of polar groups at C4. The 15N shift of the ==NAr nitrogen relative to that of the salts suggests that the predominant tautomer for N-arylguanidines is (H2N)2C==NAr. The 15N shifts of the (NH2) 2 nitrogens correlate rather well with σp- parameters, whereas the shifts of the -NHAr nitrogens seem to correlate only with R values derived from the σp- substituent constants
Experimental Implementation of Remote State Preparation by Nuclear Magnetic Resonance
We have experimentally implemented remote state preparation (RSP) of a qubit
from a hydrogen to a carbon nucleus in molecules of carbon-13 labeled
chloroform CHCl over interatomic distances using liquid-state
nuclear magnetic resonance (NMR) technique. Full RSP of a special ensemble of
qubits, i.e., a qubit chosen from equatorial and polar great circles on a Bloch
sphere with Pati's scheme, was achieved with one cbit communication. Such a RSP
scheme can be generalized to prepare a large number of qubit states and may be
used in other quantum information processing and quantum computing.Comment: 10 pages,5 PS figure
Diagnosis of Helicobacter pylori by carbon-13 urea breath test using a portable mass spectrometer
CONTEXT: In the non-invasive detection of markers of disease, mass spectrometry is able to detect small quantities of volatile markers in exhaled air. However, the problem of size, expense and immobility of conventional mass spectrometry equipment has restricted its use. Now, a smaller, less expensive, portable quadrupole mass spectrometer system has been developed. Helicobacter pylori has been implicated in the development of chronic gastritis, gastric and duodenal ulcers and gastric cancer. OBJECTIVES: To compare the results obtained from the presence of H. pylori by a carbon-13 urea test using a portable quadrupole mass spectrometer system with those from a fixed mass spectrometer in a hospital-based clinical trial. METHODS: Following ethical approval, 45 patients attending a gastroenterology clinic at the Royal Liverpool University Hospital exhaled a breath sample into a Tedlar gas sampling bag. They then drank an orange juice containing urea radiolabelled with carbon and 30 min later gave a second breath sample. The carbon-13 content of both samples was measured using both quadrupole mass spectrometer systems. If the post-drink level exceeded the pre-drink level by 3% or more, a positive diagnosis for the presence of H. pylori was made. RESULTS: The findings were compared to the results using conventional isotope ratio mass spectrometry using a laboratory-based magnetic sector instrument off-site. The results showed agreement in 39 of the 45 patients. CONCLUSIONS: This study suggests that a portable quadrupole mass spectrometer is a potential alternative to the conventional centralised testing equipment. Future development of the portable quadrupole mass spectrometer to reduce further its size and cost is indicated, together with further work to validate this new equipment and to enhance its use in mass spectrometry diagnosis of other medical conditions
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