12,438 research outputs found
Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory
We use polarization-entangled photon pairs to demonstrate quantum nonlocality
in an experiment suitable for advanced undergraduates. The photons are produced
by spontaneous parametric downconversion using a violet diode laser and two
nonlinear crystals. The polarization state of the photons is tunable. Using an
entangled state analogous to that described in the Einstein-Podolsky-Rosen
``paradox,'' we demonstrate strong polarization correlations of the entanged
photons. Bell's idea of a hidden variable theory is presented by way of an
example and compared to the quantum prediction. A test of the Clauser, Horne,
Shimony and Holt version of the Bell inequality finds , in
clear contradiciton of hidden variable theories. The experiments described can
be performed in an afternoon.Comment: 10 pages, 6 figure
The Metallicity of the Redshift 4.16 Quasar BR2248-1242
We estimate the metallicity in the broad emission-line region of the redshift
z=4.16 quasar, BR2248-1242, by comparing line ratios involving nitrogen to
theoretical predictions. BR2248-1242 has unusually narrow emission lines with
large equivalent widths, thus providing a rare opportunity to measure several
line-ratio abundance diagnostics. The combined diagnostics indicate a
metallicity of ~2 times solar. This result suggests that an episode of vigorous
star formation occurred near BR2248-1242 prior to the observed z=4.16 epoch.
The time available for this enrichment episode is only ~1.5 Gyr at z=4.16 (for
H_{0}=65 km s^-1 Mpc^-1, Omega_{m}=0.3 and Omega_Lambda ~< 1). This evidence
for high metallicities and rapid star formation is consistent with the expected
early-epoch evolution of dense galactic nuclei.Comment: 8 pages, 3 figures. Prepared in AAStex. Submitted to the
Astrophysical Journal Revised version: added 1 referenc
Liquid drop in a cone - line tension effects
The shape of a liquid drop placed in a cone is analyzed macroscopically.
Depending on the values of the cone opening angle, the Young angle and the line
tension four different interfacial configurations may be realized. The phase
diagram in these variables is constructed and discussed; it contains both the
first- and the second-order transition lines. In particular, the tricritical
point is found and the value of the critical exponent characterizing the
behaviour of the system along the line of the first-order transitions in the
neighbourhood of this point is determined.Comment: 11 pages, 4 figure
Precision measurement of the branching ratio in the 6P3/2 decay of BaII with a single trapped ion
We present a measurement of the branching ratios from the 6P3/2 state of BaII
into all dipoleallowed decay channels (6S1/2, 5D3/2 and 5D5/2). Measurements
were performed on single 138Ba+ ions in a linear Paul trap with a
frequency-doubled mode-locked Ti:Sapphire laser resonant with the 6S1/2->6P3/2
transition at 455 nm by detection of electron shelving into the dark 5D5/2
state. By driving a pi Rabi rotation with a single femtosecond pulse, a
absolute measurement of the branching ratio to 5D5/2 state was performed.
Combined with a measurement of the relative decay rates into 5D3/2 and 5D5/2
states performed with long trains of highly attenuated 455 nm pulses, it
allowed the extraction of the absolute ratios of the other two decays. Relative
strengths normalized to unity are found to be 0.756+/-0.046, 0.0290+/-0.0015
and 0.215+/-0.0064 for 6S1/2, 5D3/2 and 5D5/2 respectively. This approximately
constitutes a threefold improvement over the best previous measurements and is
a sufficient level of precision to compare to calculated values for dipole
matrix elements.Comment: 6 pages, 5 figures, 1 tabl
Ohmic contacts to GaAs for high-temperature device applications
Ohmic contacts to n-type GaAs were developed for high temperature device applications up to 300 C. Refractory metallizations were used with epitaxial Ge layers to form the contacts: TiW/Ge/GaAs, Ta/Ge/GaAs, Mo/Ge/GaAs, and Ni/Ge/GaAs. Contacts with high dose Si or Se ion implantation of the Ge/GaAs interface were also investigated. The contacts were fabricated on epitaxial GaAs layer grown on N+ or semi-insulating GaAs substrates. Ohmic contact was formed by both thermal annealing (at temperatures up to 700 C) and laser annealing (pulsed Ruby). Examination of the Ge/GaAs interface revealed Ge migration into GaAs to form an N+ doping layer. The specific contact resistances of specimens annealed by both methods are given
Measurement of Lande g factor of 5D5/2 state of BaII with a single trapped ion
We present the first terrestrial measurement of the Lande g factor of the
5D5/2 state of singly ionized barium. Measurements were performed on single
Doppler-cooled 138Ba+ ions in a linear Paul trap. A frequency-stabilized fiber
laser with nominal wavelength 1.762 um was scanned across the 6S1/25D5/2
transition to spectroscopically resolve transitions between Zeeman sublevels of
the ground and excited states. From the relative positions of the four narrow
transitions observed at several different values for the applied magnetic
field, we find a value of 1.2020+/-0.0005 for g of 5D5/2.Comment: 3 figure
Magnetic friction in Ising spin systems
A new contribution to friction is predicted to occur in systems with magnetic
correlations: Tangential relative motion of two Ising spin systems pumps energy
into the magnetic degrees of freedom. This leads to a friction force
proportional to the area of contact. The velocity and temperature dependence of
this force are investigated. Magnetic friction is strongest near the critical
temperature, below which the spin systems order spontaneously.
Antiferromagnetic coupling leads to stronger friction than ferromagnetic
coupling with the same exchange constant. The basic dissipation mechanism is
explained. If the coupling of the spin system to the heat bath is weak, a
surprising effect is observed in the ordered phase: The relative motion acts
like a heat pump cooling the spins in the vicinity of the friction surface.Comment: 4 pages, 4 figure
Quasar Evolution and the Baldwin Effect in the Large Bright Quasar Survey
From a large homogeneous sample of optical/UV emission line measurements for
993 quasars from the Large Bright Quasar Survey (LBQS), we study correlations
between emission line equivalent width and both restframe ultraviolet
luminosity (i.e., the Baldwin Effect) and redshift. Our semi-automated spectral
fitting accounts for absorption lines, fits blended iron emission, and provides
upper limits to weak emission lines. Use of a single large, well-defined sample
and consistent emission line measurements allows us to sensitively detect many
correlations, most of which have been previously noted. A new finding is a
significant Baldwin Effect in UV iron emission. Further analysis reveals that
the primary correlation of iron emission strength is probably with redshift,
implying an evolutionary rather than a luminosity effect. We show that for most
emission lines with a significant Baldwin Effect, and for some without,
evolution dominates over luminosity effects. This may reflect evolution in
abundances, in cloud covering factors, or overall cloud conditions such as
density and ionization. We find that in our sample, a putative correlation
between Baldwin Effect slope and the ionization potential is not significant.
Uniform measurements of other large quasar samples will extend the luminosity
and redshift range of such spectral studies and provide even stronger tests of
spectral evolution.Comment: 16 pages, Latex, emulateapj style, including 3 tables and 6 figures.
Accepted April 02, 2001 for publication in ApJ Main Journal. See also
http://hea-www.harvard.edu/~pgreen/Papers.htm
Fluid structure in the immediate vicinity of an equilibrium three-phase contact line and assessment of disjoining pressure models using density functional theory
We examine the nanoscale behavior of an equilibrium three-phase contact line
in the presence of long-ranged intermolecular forces by employing a statistical
mechanics of fluids approach, namely density functional theory (DFT) together
with fundamental measure theory (FMT). This enables us to evaluate the
predictive quality of effective Hamiltonian models in the vicinity of the
contact line. In particular, we compare the results for mean field effective
Hamiltonians with disjoining pressures defined through (I) the adsorption
isotherm for a planar liquid film, and (II) the normal force balance at the
contact line. We find that the height profile obtained using (I) shows good
agreement with the adsorption film thickness of the DFT-FMT equilibrium density
profile in terms of maximal curvature and the behavior at large film heights.
In contrast, we observe that while the height profile obtained by using (II)
satisfies basic sum rules, it shows little agreement with the adsorption film
thickness of the DFT results. The results are verified for contact angles of
20, 40 and 60 degrees
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