22,383 research outputs found
Drivers and food web effects of Gonyostomum semen blooms
The flagellate Gonyostomum semen forms dense late-summer blooms in humic lakes and is a nuisance to swimmers because it forms a slimy coat on the skin, causing irritation in sensitive individuals. Increasing occurrence and bloom incidence of G. semen has been reported during recent decades, but it is not clear which factors affect the distribution and bloom formation of this alga. Large cell size, ejection of long, slimy threads (trichocysts), and nighttime migration to the hypolimnion may limit grazing on G. semen by herbivorous zooplankton, resulting in a decreased coupling between phytoplankton and higher trophic levels during blooms. The studies included in this thesis investigate which factors affect G. semen occurrence and bloom formation and how G. semen blooms affect the community composition and trophic interactions in boreal, humic lakes.
The occurrence of G. semen has increased between 1995 and 2010, especially in southern Sweden. Bloom incidence and total biomass did not increase continually, but fluctuated among years and peaked in the middle of the study period. Temperature and length of the growing season affected the occurrence and, to a lesser extent, bloom formation of G. semen, but local factors such as pH and water colour were more important for bloom formation. More lakes may become suitable habitats with the ongoing increase in water colour and increasing temperatures may result in a more frequent occurrence and bloom formation of G. semen.
Blooms resulted in a shift in zooplankton assemblages toward predominance by small cladocerans, which were not able to feed on G. semen but instead fed more on heterotrophic food resources, supporting the hypothesis of a reduced coupling between phytoplankton and zooplankton. Zooplankton assemblages predominated by small animals feeding on low-quality resources may reduce the food quality for planktivorous fish. Instead, the invertebrate predator C. flavicans appeared to benefit from G. semen blooms, as indicated by its high abundance in bloom-lakes. Calanoid copepods and a large cladoceran fed efficiently on G. semen in the laboratory, indicating that there is, however, some trophic coupling between G. semen and higher trophic levels. This supports the use of biomanipulation of fish communities for controlling G. semen blooms
Steady state entanglement of two superconducting qubits engineered by dissipation
We present a scheme for the dissipative preparation of an entangled steady
state of two superconducting qubits in a circuit QED setup. Combining resonator
photon loss, a dissipative process already present in the setup, with an
effective two-photon microwave drive, we engineer an effective decay mechanism
which prepares a maximally entangled state of the two qubits. This state is
then maintained as the steady state of the driven, dissipative evolution. The
performance of the dissipative state preparation protocol is studied
analytically and verified numerically. In view of the experimental
implementation of the presented scheme we investigate the effects of potential
experimental imperfections and show that our scheme is robust to small
deviations in the parameters. We find that high fidelities with the target
state can be achieved both with state-of-the-art 3D, as well as with the more
commonly used 2D transmons. The promising results of our study thus open a
route for the demonstration of an entangled steady state in circuit QED.Comment: 12 pages, 5 figures; close to published versio
Readout methods and devices for Josephson-junction-based solid-state qubits
We discuss the current situation concerning measurement and readout of
Josephson-junction based qubits. In particular we focus attention of dispersive
low-dissipation techniques involving reflection of radiation from an oscillator
circuit coupled to a qubit, allowing single-shot determination of the state of
the qubit. In particular we develop a formalism describing a charge qubit read
out by measuring its effective (quantum) capacitance. To exemplify, we also
give explicit formulas for the readout time.Comment: 20 pages, 7 figures. To be published in J. Phys.: Condensed Matter,
18 (2006) Special issue: Quantum computin
Resonant ratcheting of a Bose-Einstein condensate
We study the rectification process of interacting quantum particles in a
periodic potential exposed to the action of an external ac driving. The
breaking of spatio-temporal symmetries leads to directed motion already in the
absence of interactions. A hallmark of quantum ratcheting is the appearance of
resonant enhancement of the current (Europhys. Lett. 79 (2007) 10007 and Phys.
Rev. A 75 (2007) 063424). Here we study the fate of these resonances within a
Gross-Pitaevskii equation which describes a mean field interaction between many
particles. We find, that the resonance is i) not destroyed by interactions, ii)
shifting its location with increasing interaction strength. We trace the
Floquet states of the linear equations into the nonlinear domain, and show that
the resonance gives rise to an instability and thus to the appearance of new
nonlinear Floquet states, whose transport properties differ strongly as
compared to the case of noninteracting particles
Optical Versus Mid-Infrared Spectroscopic Classification of Ultraluminous Infrared Galaxies
The origin of huge infrared luminosities of ultraluminous infrared galaxies
(ULIGs) is still in question. Recently, Genzel et al. made mid-infrared (MIR)
spectroscopy of a large number of ULIGs and found that the major energy source
in them is massive stars formed in the recent starburst activity; i.e.,
70% -- 80% of the sample are predominantly powered by the starburst. However,
it is known that previous optical spectroscopic observations showed that the
majority of ULIGs are classified as Seyferts or LINERs (low-ionization nuclear
emission-line regions). In order to reconcile this difference, we compare types
of emission-line activity for a sample of ULIGs which have been observed in
both optical and MIR. We confirm the results of previous studies that the
majority of ULIGs classified as LINERs based on the optical emission-line
diagnostics turn to be starburst-dominated galaxies based on the MIR ones.
Since the MIR spectroscopy can probe more heavily-reddened, inner parts of the
ULIGs, it is quite unlikely that the inner parts are powered by the starburst
while the outer parts are powered by non-stellar ionization sources. The most
probable resolution of this dilemma is that the optical emission-line nebulae
with the LINER properties are powered predominantly by shock heating driven by
the superwind activity; i.e., a blast wave driven by a collective effect of a
large number of supernovae in the central region of galaxy mergers.Comment: 15 pages, 2 tables, and 3 eps figures. The Astrophysical Journal
(Part 1), in pres
Molecular clouds in the centers of galaxies: Constraints from HCN and CO-13 line emission
We have searched for HCN J=1-0 line emission in the centers of 12 galaxies and have detected it in 10 of them. We have obtained complementary data on J=1-0 and 2-1 transitions of CO-12 and CO-13 in these systems. The ratio of integrated intensities, I(CO 1-0)/I(HCN 1-0) = 25 +/- 11 for this sample. We find that HCN emission of this strength can be produced under conditions of subthermal excitation. In combination with the line ratios in CO and CO-13, HCN puts constraints on the mean conditions of molecular clouds and on the mix of cloud types within the projected beam
Levitated droplet dye laser
We present the first observation, to our knowledge, of lasing from a
levitated, dye droplet. The levitated droplets are created by computer
controlled pico-liter dispensing into one of the nodes of a standing ultrasonic
wave (100 kHz), where the droplet is trapped. The free hanging droplet forms a
high quality optical resonator. Our 750 nL lasing droplets consist of Rhodamine
6G dissolved in ethylene glycol, at a concentration of 0.02 M. The droplets are
optically pumped at 532 nm light from a pulsed, frequency doubled Nd:YAG laser,
and the dye laser emission is analyzed by a fixed grating spectrometer. With
this setup we have achieved reproducible lasing spectra in the visible
wavelength range from 610 nm to 650 nm. The levitated droplet technique has
previously successfully been applied for a variety of bio-analytical
applications at single cell level. In combination with the lasing droplets, the
capability of this high precision setup has potential applications within
highly sensitive intra-cavity absorbance detection.Comment: 6 pages including 3 figure
Infrared FeII Emission in Narrow-Line Seyfert 1 Galaxies
We obtained 0.8-2.4 micron spectra at a resolution of 320 km/s of four
narrow-line Seyfert 1 galaxies in order to study the near-infrared properties
of these objects. We focus on the analysis of the FeII emission in that region
and the kinematics of the low-ionization broad lines. We found that the 1
micron FeII lines (9997 A, 10501 A, 10863 A and 11126 A) are the strongest FeII
lines in the observed interval. For the first time, primary cascade lines of
FeII arising from the decay of upper levels pumped by Ly-alpha fluorescence are
resolved and identified in active galactic nuclei. Excitation mechanisms
leading to the emission of the 1 micron FeII features are discussed. A
combination of Ly-alpha fluorescence and collisional excitation are found to be
the main contributors. The flux ratio between near-IR FeII lines varies from
object to object, in contrast to what is observed in the optical region. A good
correlation between the 1 micron and optical FeII emission is found. This
suggests that the upper z4Fo and z4Do levels from which the bulk of the optical
lines descend are mainly populated by the transitions leading to the 1 micron
lines. The width and profile shape of FeII 11127, CaII 8642 and OI 8446 are
very similar but significantly narrower than Pa-beta, giving strong
observational support to the hypothesis that the region where FeII, CaII and OI
are produced are co-spatial, interrelated kinematically and most probably
located in the outermost portion of the BLR.Comment: Accepted for publication in ApJ - 35 page
Dynamical Casimir effect entangles artificial atoms
We show that the physics underlying the dynamical Casimir effect may generate
multipartite quantum correlations. To achieve it, we propose a circuit quantum
electrodynamics (cQED) scenario involving superconducting quantum interference
devices (SQUIDs), cavities, and superconducting qubits, also called artificial
atoms. Our results predict the generation of highly entangled states for two
and three superconducting qubits in different geometric configurations with
realistic parameters. This proposal paves the way for a scalable method of
multipartite entanglement generation in cavity networks through dynamical
Casimir physics.Comment: Improved version and references added. Accepted for publication in
Physical Review Letter
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