25 research outputs found
Cavity QED with Diamond Nanocrystals and Silica Microspheres
Normal mode splitting is observed in a cavity QED system, in which nitrogen
vacancy centers in diamond nanocrystals are coupled to whispering gallery modes
in a silica microsphere. The composite nanocrystal-microsphere system takes
advantage of the exceptional spin properties of nitrogen vacancy centers as
well as the ultra high quality factor of silica microspheres. The observation
of the normal mode splitting indicates that the dipole optical interaction
between the relevant nitrogen vacancy center and whispering gallery mode has
reached the strong coupling regime of cavity QED
Metastability in Josephson transmission lines
Thermal activation and macroscopic quantum tunneling in current-biased
discrete Josephson transmission lines are studied theoretically. The degrees of
freedom under consideration are the phases across the junctions which are
coupled to each other via the inductances of the system. The resistively
shunted junctions that we investigate constitute a system of N interacting
degrees of freedom with an overdamped dynamics. We calculate the decay rate
within exponential accuracy as a function of temperature and current. Slightly
below the critical current, the decay from the metastable state occurs via a
unique ("rigid") saddlepoint solution of the Euclidean action describing the
simultaneous decay of the phases in all the junctions. When the current is
reduced, a crossover to a regime takes place, where the decay occurs via an
"elastic" saddlepoint solution and the phases across the junctions leave the
metastable state one after another. This leads to an increased decay rate
compared with the rigid case both in the thermal and the quantum regime. The
rigid-to-elastic crossover can be sharp or smooth analogous to first- or
second- order phase transitions, respectively. The various regimes are
summarized in a current-temperature decay diagram.Comment: 11 pages, RevTeX, 3 PS-figures, revised versio
Crossovers in the thermal decay of metastable states in discrete systems
The thermal decay of linear chains from a metastable state is investigated. A
crossover from rigid to elastic decay occurs when the number of particles, the
single particle energy barrier or the coupling strength between the particles
is varied. In the rigid regime, the single particle energy barrier is small
compared to the coupling strength and the decay occurs via a uniform
saddlepoint solution, with all degrees of freedom decaying instantly.
Increasing the barrier one enters the elastic regime, where the decay is due to
bent saddlepoint configurations using the elasticity of the chain to lower
their activation energy. Close to the rigid-to-elastic crossover, nucleation
occurs at the boundaries of the system. However, in large systems, a second
crossover from boundary to bulk nucleation can be found within the elastic
regime, when the single particle energy barrier is further increased. We
compute the decay rate in the rigid and in the elastic regimes within the
Gaussian approximation. Around the rigid-to-elastic crossover, the calculations
are performed beyond the steepest descent approximation. In this region, the
prefactor exhibits a scaling property. The theoretical results are discussed in
the context of discrete Josephson transmission lines and pancake vortex stacks
that are pinned by columnar defects.Comment: 13 pages, RevTeX, 7 PS-figure
Relaxation nucléaire de 3He ↑ dans un champ magnétique inhomogène
This article gives a theoretical study of the effects of weak magnetic field gradients on the spin relaxation of a dilute atomic gas. The theory is valid for low or moderate values of the field (the relevant correlation time of the gas is then the diffusion time across the container) as well as for high fields (the intercollision time then plays the most important role). Quantum effects due to particle indistinguishability are included. No consequences of the divergence of the atomic mean free path in a polarized gas at zero temperature are found, in contrast to the situation for viscosity and heat conductivity. On the other hand, substantial effects due to « identical spin rotation effects » are predicted.On présente dans cet article une étude théorique de la relaxation de spin d'un gaz atomique dilué, produite par un gradient de champ magnétique de faible intensité. La théorie est valide à la fois en champ faible ou modéré (cas où le temps de corrélation du gaz qui joue un rôle est le temps de diffusion dans son récipient) et en champ fort (c'est alors le temps entre collisions qui joue le rôle essentiel). Elle tient compte des effets quantiques d'indiscernabilité des atomes. Si les effets de divergence du libre parcours moyen dans un gaz polarisé à température nulle ne jouent ici aucun rôle, contrairement à ce qui se produit pour la viscosité par exemple, les effets de « rotation des spins identiques » peuvent être importants
Nuclear polarization through optical pumping of gaseous 3He below 1 K
We have extended to temperatures below 1 K the optical pumping technique for polarizing the nuclei pf ground state 3He atoms in the gas phase. A superfluid 4He film covering the surface of the pyrex cell protected the 3He atoms from the rapid depolarization processes that would be experienced in the presence of bare pyrex walls. Relatively long relaxation times of order 103 s were observed at temperatures down to 500 mK, the lowest temperature accessible with the present apparatus.La méthode du pompage optique qui permet d'orienter nucléairement 3He gazeux a pu être étendue au domaine des températures en dessous de 1 K. L'utilisation d'un enduit superfluide de 4He couvrant la paroi interne de la cellule a permis de réduire très fortement la relaxation nucléaire intense qui serait produite par du pyrex nu. On obtient ainsi des temps de relaxation nucléaires relativement longs (Tr ≽ 103 s), même aux températures les plus basses accessibles dans l'expérience (T ≅ 500 mK)
Thermal bistability-based method for real-time optimization of ultralow-threshold whispering gallery mode microlasers
Region Ile-de-France; China Scholarship CouncilA method based on thermal bistability for ultralow-threshold microlaser optimization is demonstrated. When sweeping the pump laser frequency across a pump resonance, the dynamic thermal bistability slows down the power variation. The resulting line shape modification enables a real-time monitoring of the laser characteristic. We demonstrate this method for a functionalized microsphere exhibiting a submicrowatt laser threshold. This approach is confirmed by comparing the results with a step-by-step recording in quasi-static thermal conditions. (C) 2012 Optical Society of Americ
Room temperature emission at 1.54 µm from Er-doped silicon-rich oxide microcrotoroidal cavities
International audienc