618 research outputs found
A high-flux source of polarization-entangled photons from a periodically-poled KTP parametric downconverter
We have demonstrated a high-flux source of polarization-entangled photons
using a type-II phase-matched periodically-poled KTP parametric downconverter
in a collinearly propagating configuration. We have observed quantum
interference between the single-beam downconverted photons with a visibility of
99% and a measured coincidence flux of 300/s/mW of pump. The
Clauser-Horne-Shimony-Holt version of Bell's inequality was violated with a
value of 2.711 +/- 0.017.Comment: 7 pages submitted to Physical Review
Matter-induced vertices for photon splitting in a weakly magnetized plasma
We evaluate the three-photon vertex functions at order and in a
weak constant magnetic field at finite temperature and density with on shell
external lines. Their application to the study of the photon splitting process
leads to consider high energy photons whose dispersion relations are not
changed significantly by the plasma effects. The absorption coefficient is
computed and compared with the perturbative vacuum result. For the values of
temperature and density of some astrophysical objects with a weak magnetic
field, the matter effects are negligible.Comment: 14 pages, 1 figure. Accepted for publication in PR
Dynamics of an anisotropic Haldane antiferromagnet in strong magnetic field
We report the results of elastic and inelastic neutron scattering experiments
on the Haldane gap quantum antiferromagnet Ni(C5D14N2)2N3(PF6) performed at mK
temperatures in a wide range of magnetic field applied parallel to the S = 1
spin chains. Even though this geometry is closest to an ideal axially symmetric
configuration, the Haldane gap closes at the critical field Hc~4T, but reopens
again at higher fields. The field dependence of the two lowest magnon modes is
experimentally studied and the results are compared with the predictions of
several theoretical models. We conclude that of several existing theories, only
the recently proposed model [Zheludev et al., cond-mat/0301424 ] is able to
reproduce all the features observed experimentally for different field
orientations.Comment: 11 pages 8 figures submitted to Phys. Rev.
Preparation of facilities for fundamental research with ultracold neutrons at PNPI
The WWR-M reactor of PNPI offers a unique opportunity to prepare a source for
ultracold neutrons (UCN) in an environment of high neutron flux (about 3*10^12
n/cm^2/s) at still acceptable radiation heat release (about 4*10^-3 W/g). It
can be realized within the reactor thermal column situated close to the reactor
core. With its large diameter of 1 m, this channel allows to install a 15 cm
thick bismuth shielding, a graphite premoderator (300 dm^3 at 20 K), and a
superfluid helium converter (35 dm^3). At a temperature of 1.2 K it is possible
to remove the heat release power of about 20 W. Using the 4pi flux of cold
neutrons within the reactor column can bring more than a factor 100 of cold
neutron flux incident on the superfluid helium with respect to the present cold
neutron beam conditions at the ILL reactor. The storage lifetime for UCN in
superfluid He at 1.2 K is about 30 s, which is sufficient when feeding
experiments requiring a similar filling time. The calculated density of UCN
with energy between 50 neV and 250 neV in an experimental volume of 40 liters
is about 10^4 n/cm^3. Technical solutions for realization of the project are
discussed.Comment: 10 pages, more detail
Massive triplet excitations in a magnetized anisotropic Haldane spin chain
Inelastic neutron scattering experiments on the Haldane-gap quantum
antiferromagnet \nd are performed at mK temperatures in magnetic fields of
almost twice the critical field applied perpendicular to the spin cahins.
Above a re-opening of the spin gap is clearly observed. In the high-field
N\'eel-ordered state the spectrum is dominated by three distinct long-lived
excitation branches. Several field-theoretical models are tested in a
quantitative comparison with the experimental data.Comment: 4 pages, 3 figure
Purity of Gaussian states: measurement schemes and time-evolution in noisy channels
We present a systematic study of the purity for Gaussian states of
single-mode continuous variable systems. We prove the connection of purity to
observable quantities for these states, and show that the joint measurement of
two conjugate quadratures is necessary and sufficient to determine the purity
at any time. The statistical reliability and the range of applicability of the
proposed measurement scheme is tested by means of Monte Carlo simulated
experiments. We then consider the dynamics of purity in noisy channels. We
derive an evolution equation for the purity of general Gaussian states both in
thermal and squeezed thermal baths. We show that purity is maximized at any
given time for an initial coherent state evolving in a thermal bath, or for an
initial squeezed state evolving in a squeezed thermal bath whose asymptotic
squeezing is orthogonal to that of the input state.Comment: 9 Pages, 6 Figures; minor errors correcte
Magnetic Braking in Differentially Rotating, Relativistic Stars
We study the magnetic braking and viscous damping of differential rotation in
incompressible, uniform density stars in general relativity. Differentially
rotating stars can support significantly more mass in equilibrium than
nonrotating or uniformly rotating stars. The remnant of a binary neutron star
merger or supernova core collapse may produce such a "hypermassive" neutron
star. Although a hypermassive neutron star may be stable on a dynamical
timescale, magnetic braking and viscous damping of differential rotation will
ultimately alter the equilibrium structure, possibly leading to delayed
catastrophic collapse. Here we consider the slow-rotation, weak-magnetic field
limit in which E_rot << E_mag << W, where E_rot is the rotational kinetic
energy, E_mag is the magnetic energy, and W is the gravitational binding energy
of the star. We assume the system to be axisymmetric and solve the MHD
equations in both Newtonian gravitation and general relativity. Toroidal
magnetic fields are generated whenever the angular velocity varies along the
initial poloidal field lines. We find that the toroidal fields and angular
velocities oscillate independently along each poloidal field line, which
enables us to transform the original 2+1 equations into 1+1 form and solve them
along each field line independently. The incoherent oscillations on different
field lines stir up turbulent-like motion in tens of Alfven timescales ("phase
mixing"). In the presence of viscosity, the stars eventually are driven to
uniform rotation, with the energy contained in the initial differential
rotation going into heat. Our evolution calculations serve as qualitative
guides and benchmarks for future, more realistic MHD simulations in full 3+1
general relativity.Comment: 26 pages, 27 graphs, 1 table, accepted for publication by Phys. Rev.
Magnetism in Dense Quark Matter
We review the mechanisms via which an external magnetic field can affect the
ground state of cold and dense quark matter. In the absence of a magnetic
field, at asymptotically high densities, cold quark matter is in the
Color-Flavor-Locked (CFL) phase of color superconductivity characterized by
three scales: the superconducting gap, the gluon Meissner mass, and the
baryonic chemical potential. When an applied magnetic field becomes comparable
with each of these scales, new phases and/or condensates may emerge. They
include the magnetic CFL (MCFL) phase that becomes relevant for fields of the
order of the gap scale; the paramagnetic CFL, important when the field is of
the order of the Meissner mass, and a spin-one condensate associated to the
magnetic moment of the Cooper pairs, significant at fields of the order of the
chemical potential. We discuss the equation of state (EoS) of MCFL matter for a
large range of field values and consider possible applications of the magnetic
effects on dense quark matter to the astrophysics of compact stars.Comment: To appear in Lect. Notes Phys. "Strongly interacting matter in
magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A.
Schmitt, H.-U. Ye
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