151 research outputs found
Optical Excitation of Nuclear Spin Coherence in Tm3+:YAG
A thulium-doped crystal is experimentally shown to be an excellent candidate
for broadband quantum storage in a solid-state medium. For the first time,
nuclear spin coherence is optically excited, detected and characterized in such
a crystal. The lifetime of the spin coherence -- the potential storage entity
-- is measured by means of Raman echo to be about 300 s over a wide range
of ground state splittings. This flexibility, attractive for broadband
operation, and well fitted to existing quantum sources, results from the simple
hyperfine structure, contrasting with Pr- and Eu- doped crystals
Optical Excitation of Nuclear Spin Coherence in Tm3+:YAG
A thulium-doped crystal is experimentally shown to be an excellent candidate
for broadband quantum storage in a solid-state medium. For the first time,
nuclear spin coherence is optically excited, detected and characterized in such
a crystal. The lifetime of the spin coherence -- the potential storage entity
-- is measured by means of Raman echo to be about 300 s over a wide range
of ground state splittings. This flexibility, attractive for broadband
operation, and well fitted to existing quantum sources, results from the simple
hyperfine structure, contrasting with Pr- and Eu- doped crystals
Efficient optical pumping of Zeeman spin levels in Nd3+:YVO4
We demonstrate that Zeeman ground-state spin levels in Nd3+:YVO4 provides the
possibility to create an efficient lambda-system for optical pumping
experiments. The branching ratio R in the lambda-system is measured
experimentally via absorption spectroscopy and is compared to a theoretical
model. We show that R can be tuned by changing the orientation of the magnetic
field. These results are applied to optical pumping experiments, where
significant improvement is obtained compared to previous experiments in this
system. The tunability of the branching ratio in combination with its good
coherence properties and the high oscillator strength makes Nd3+:YVO4 an
interesting candidate for various quantum information protocols.Comment: 8 pages, 6 figure
Emission spectra and intrinsic optical bistability in a two-level medium
Scattering of resonant radiation in a dense two-level medium is studied
theoretically with account for local field effects and renormalization of the
resonance frequency. Intrinsic optical bistability is viewed as switching
between different spectral patterns of fluorescent light controlled by the
incident field strength. Response spectra are calculated analytically for the
entire hysteresis loop of atomic excitation. The equations to describe the
non-linear interaction of an atomic ensemble with light are derived from the
Bogolubov-Born-Green-Kirkwood-Yvon hierarchy for reduced single particle
density matrices of atoms and quantized field modes and their correlation
operators. The spectral power of scattered light with separated coherent and
incoherent constituents is obtained straightforwardly within the hierarchy. The
formula obtained for emission spectra can be used to distinguish between
possible mechanisms suggested to produce intrinsic bistability.Comment: 18 pages, 5 figure
Développer la capacité des conseillers à agir face à la diversité des situations de conseil en grande culture
Face aux enjeux d’une transition agro-écologique, le conseil en grande culture s’ouvre à de nouvelles formes de raisonnement agronomique et implique de nouvelles compétences, ou plus exactement de nouvelles capacités à agir des conseillers. Les auteurs proposent une analyse de l’activité de conseil destiné à comprendre l’origine des difficultés que les conseillers rencontrent dans leurs interactions avec les agriculteurs, en postulant que de nouvelles capacités à agir se développent dans une réflexion de ces difficultés. En comparant trois situations de conseil réalisées par un même conseiller, les auteurs pointent les réussites ou les défauts de coordination et identifient ce que le conseiller peut faire évoluer dans la construction de son action de conseil.Arable farming advisors have to develop new agronomic reasoning and new skills to address the challenges of an agro-ecological transition. To support this professional development, we propose an analytical framework of the advisory activity. It stresses the operations carried out by an advisor in order to orient his performance and highlights the need for the advisor to reflect on his way to co-ordinate with his environment and ”the others” for building a common action. We analyse three advisory situations and show that this framework enables us to identify the obstacles met in the coordination process and in the coupling of the advisor’s activity and the situation. We discuss how such results can support a reflexive process oriented towards the professional development of the advisors
Emission of photon echoes in a strongly scattering medium
We observe the two- and three-pulse photon echo emission from a scattering
powder, obtained by grinding a Pr:YSiO rare earth doped single
crystal. We show that the collective emission is coherently constructed over
several grains. A well defined atomic coherence can therefore be created
between randomly placed particles. Observation of photon echo on powders as
opposed to bulk materials opens the way to faster material development. More
generally, time-domain resonant four-wave mixing offers an attractive approach
to investigate coherent propagation in scattering media
Spectroscopic investigations of a Ti:Tm:LiNbO3 waveguide for photon-echo quantum memory
We report the fabrication and characterization of a
Ti:Tm:LiNbO optical waveguide in view of photon-echo quantum
memory applications. In particular, we investigated room- and
cryogenic-temperature properties via absorption, spectral hole burning, photon
echo, and Stark spectroscopy. We found radiative lifetimes of 82 s and 2.4
ms for the H and F levels, respectively, and a 44% branching
ratio from the H to the F level. We also measured an optical
coherence time of 1.6 s for the HH, 795 nm
wavelength transition, and investigated the limitation of spectral diffusion to
spectral hole burning. Upon application of magnetic fields of a few hundred
Gauss, we observed persistent spectral holes with lifetimes up to seconds.
Furthermore, we measured a linear Stark shift of 25 kHzcm/V. Our results
are promising for integrated, electro-optical, waveguide quantum memory for
photons.Comment: 11 pages, 14 figure
An agile laser with ultra-low frequency noise and high sweep linearity
We report on a fiber-stabilized agile laser with ultra-low frequency noise.
The frequency noise power spectral density is comparable to that of an
ultra-stable cavity stabilized laser at Fourier frequencies higher than 30 Hz.
When it is chirped at a constant rate of ~ 40 MHz/s, the max non-linearity
frequency error is about 50 Hz peak-to-peak over more than 600 MHz tuning
range. The Rayleigh backscattering is found to be a significant frequency noise
source dependent on fiber length, chirping rate and the power imbalance of the
interferometer arms. We analyze this effect both theoretically and
experimentally and put forward techniques to reduce this noise contribution.Comment: 14 pages, 11 figures, accepted by Optics Expres
Using fine-scale spatial genetics of Norway rats to improve control efforts and reduce leptospirosis risk in urban slum environments
The Norway rat (Rattus norvegicus) is a key pest species globally and responsible for seasonal outbreaks of the zoonotic bacterial disease leptospirosis in the tropics. The city of Salvador, Brazil, has seen recent and dramatic increases in human population residing in slums, where conditions foster high rat density and increasing leptospirosis infection rates. Intervention campaigns have been used to drastically reduce rat numbers. In planning these interventions, it is important to define the eradication units ‐ the spatial scale at which rats constitute continuous populations and from where rats are likely recolonizing, post‐intervention. To provide this information, we applied spatial genetic analyses to 706 rats collected across Salvador and genotyped at 16 microsatellite loci. We performed spatially explicit analyses and estimated migration levels to identify distinct genetic units and landscape features associated with genetic divergence at different spatial scales, ranging from valleys within a slum community to city‐wide analyses. Clear genetic breaks exist between rats not only across Salvador but also between valleys of slums separated by <100 m—well within the dispersal capacity of rats. The genetic data indicate that valleys may be considered separate units and identified high‐traffic roads as strong impediments to rat movement. Migration data suggest that most (71–90%) movement is contained within valleys, with no clear source population contributing to migrant rats. We use these data to recommend eradication units and discuss the importance of carrying out individual‐based analyses at different spatial scales in urban landscapes
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