Coexistence of thermal noise and squeezing in the intensity fluctuations of small laser diodes

The intensity fluctuations of laser light are derived from photon number rate equations. In the limit of short times, the photon statistics for small laser devices such as typical semiconductor laser diodes show thermal characteristics even above threshold. In the limit of long time averages represented by the low frequency component of the noise, the same devices exhibit squeezing. It is shown that squeezing and thermal noise can coexist in the multi-mode output field of laser diodes. This result implies that the squeezed light generated by regularly pumped semiconductor laser diodes is qualitatively different from single mode squeezed light. In particular, no entanglement between photons can be generated using this type of collective multi-mode squeezing.Comment: 9 pages, 8 figures, submitted to J. Opt. Soc. Am. B, added references and clarifications of the contex

Saturation of Magnetorotational Instability through Magnetic Field Generation

The saturation mechanism of Magneto-Rotational Instability (MRI) is examined through analytical quasilinear theory and through nonlinear computation of a single mode in a rotating disk. We find that large-scale magnetic field is generated through the alpha effect (the correlated product of velocity and magnetic field fluctuations) and causes the MRI mode to saturate. If the large-scale plasma flow is allowed to evolve, the mode can also saturate through its flow relaxation. In astrophysical plasmas, for which the flow cannot relax because of gravitational constraints, the mode saturates through field generation only.Comment: 9 pages, 10 figures to appear in ApJ, Jun 2009, 10 v69

The early history of protostellar disks, outflows, and binary stars

In star formation, magnetic fields act as a cosmic angular momentum extractor that increases mass accretion rates onto protostars and in the process, creates spectacular outflows. However, recently it has been argued that this magnetic brake is so strong that early protostellar disks -- the cradles of planet formation -- cannot form. Our three-dimensional numerical simulations of the early stages of collapse (\lesssim 10^5 yr) of overdense star--forming clouds form early outflows and have magnetically regulated and rotationally dominated disks (inside 10 AU) with high accretion rates, despite the slip of the field through the mostly neutral gas. We find that in three dimensions, magnetic fields suppress gravitationally driven instabilities which would otherwise prevent young, well ordered disks from forming. Our simulations have surprising consequences for the early formation of disks, their density and temperature structure, the mechanism and structure of early outflows, the flash heating of dust grains through ambipolar diffusion, and the origin of planets and binary stars.Comment: 12 pages, 3 figures, accepted by ApJ Letters; corrected text to match journal version; movies can be found at http://www.physics.mcmaster.ca/~duffindf/movies.htm

Enhanced quasiparticle heat conduction of the multigap superconductor Lu2Fe3Si5

The thermal transport measurements have been made on the Fe-based superconductor Lu2Fe3Si5 (Tc ~ 6 K) down to a very low temperature Tc/120. The field and temperature dependences of the thermal conductivity confirm the multigap superconductivity with fully opened gaps on the whole Fermi surfaces. In comparison to MgB2 as a typical example of the multigap superconductor in a p-electron system, Lu2Fe3Si5 reveals a remarkably enhanced quasiparticle heat conduction in the mixed state. The results can be interpreted as a consequence of the electronic correlations derived from Fe 3d-electrons.Comment: 5 pages, 4 figure

Observability inequalities for transport equations through Carleman estimates

We consider the transport equation \ppp_t u(x,t) + H(t)\cdot \nabla u(x,t) = 0 in \OOO\times(0,T), where $T>0$ and \OOO\subset \R^d is a bounded domain with smooth boundary \ppp\OOO. First, we prove a Carleman estimate for solutions of finite energy with piecewise continuous weight functions. Then, under a further condition which guarantees that the orbits of $H$ intersect \ppp\OOO, we prove an energy estimate which in turn yields an observability inequality. Our results are motivated by applications to inverse problems.Comment: 18 pages, 3 figure

Method and apparatus for instantaneous band ratioing in a reflectance radiometer

A hand-held instrument is provided to compare information from selected infrared and visible bands in the 0.4 to 2.5 micrometer range, to perform ratioing via a dividing circuit (17) and to directly read out, via a display system (18), ratio values in a continuous digital display. The dual-beam, ratioing radiometer contains two optical trains (10, 12), each having two repeater lenses (L1a, L1b and L2a, L2b) and a cooled lead sulfide detector (D1, D2). One of the trains (10) is pivotal to facilitate measurements at distances ranging from about 1 meter to infinity. The optical trains are intersected by a set of two coaxially-mounted filter wheels (F1, F2), each containing up to five interference filters and slits to pass radiation filtered by the other. Filters with band passes as narrow as 0.01 micrometer are used in the region 0.4 to 2.5 micrometers. The total time for a calibration and measurement is only a few seconds. It is known from previous field studies using prior art devices, that materials, e.g., clay minerals, and carbonate minerals such as limestone, have unique spectral properties in the 2.0 to 2.5 micrometer region. Using properly chosen spectral filters, and ratioing the signals to remove the effect of topography on the brightness measured, the instrument can be used for real-time analysis of reflecting materials in the field. Other materials in the broader range of 0.4 to 2.5 micrometers (and even beyond) could be similarly identified once the reflectance spectrum of the material is established by any means

Vortex nucleation in rotating BEC: the role of the boundary condition for the order parameter

We study the process of vortex nucleation in rotating two-dimensional BEC confined in a harmonic trap. We show that, within the Gross-Pitaevskii theory with the boundary condition of vanishing of the order parameter at infinity, topological defects nucleation occurs via the creation of vortex-antivortex pairs far from the cloud center, where the modulus of the order parameter is small. Then, vortices move towards the center of the cloud and antivortices move in the opposite direction but never disappear. We also discuss the role of surface modes in this process.Comment: 6 pages, 2 figure

Distribution of the spacing between two adjacent avoided crossings

We consider the frequency at which avoided crossings appear in an energy level structure when an external field is applied to a quantum chaotic system. The distribution of the spacing in the parameter between two adjacent avoided crossings is investigated. Using a random matrix model, we find that the distribution of these spacings is well fitted by a power-law distribution for small spacings. The powers are 2 and 3 for the Gaussian orthogonal ensemble and Gaussian unitary ensemble, respectively. We also find that the distributions decay exponentially for large spacings. The distributions in concrete quantum chaotic systems agree with those of the random matrix model.Comment: 11 page