44,658 research outputs found
Relevance of the weak equivalence principle and experiments to test it: lessons from the past and improvements expected in space
Tests of the Weak Equivalence Principle (WEP) probe the foundations of
physics. Ever since Galileo in the early 1600s, WEP tests have attracted some
of the best experimentalists of any time. Progress has come in bursts, each
stimulated by the introduction of a new technique: the torsion balance, signal
modulation by Earth rotation, the rotating torsion balance. Tests for various
materials in the field of the Earth and the Sun have found no violation to the
level of about 1 part in 1e13. A different technique, Lunar Laser Ranging
(LLR), has reached comparable precision. Today, both laboratory tests and LLR
have reached a point when improving by a factor of 10 is extremely hard. The
promise of another quantum leap in precision rests on experiments performed in
low Earth orbit. The Microscope satellite, launched in April 2016 and currently
taking data, aims to test WEP in the field of Earth to 1e-15, a 100-fold
improvement possible thanks to a driving signal in orbit almost 500 times
stronger than for torsion balances on ground. The `Galileo Galilei' (GG)
experiment, by combining the advantages of space with those of the rotating
torsion balance, aims at a WEP test 100 times more precise than Microscope, to
1e-17. A quantitative comparison of the key issues in the two experiments is
presented, along with recent experimental measurements relevant for GG. Early
results from Microscope, reported at a conference in March 2017, show
measurement performance close to the expectations and confirm the key role of
rotation with the advantage (unique to space) of rotating the whole spacecraft.
Any non-null result from Microscope would be a major discovery and call for
urgent confirmation; with 100 times better precision GG could settle the matter
and provide a deeper probe of the foundations of physics.Comment: To appear: Physics Letters A, special issue in memory of Professor
Vladimir Braginsky, 2017. Available online:
http://dx.doi.org/10.1016/j.physleta.2017.09.02
Generation of Rb-resonant bright two-mode squeezed light with four-wave mixing
Squeezed states of light have found their way into a number of applications
in quantum-enhanced metrology due to their reduced noise properties. In order
to extend such an enhancement to metrology experiments based on atomic
ensembles, an efficient light-atom interaction is required. Thus, there is a
particular interest in generating narrow-band squeezed light that is on atomic
resonance. This will make it possible not only to enhance the sensitivity of
atomic based sensors, but also to deterministically entangle two distant atomic
ensembles. We generate bright two-mode squeezed states of light, or twin beams,
with a non-degenerate four-wave mixing (FWM) process in hot Rb in a
double-lambda configuration. Given the proximity of the energy levels in the D1
line of Rb and Rb, we are able to operate the FWM in Rb in
a regime that generates two-mode squeezed states in which both modes are
simultaneously on resonance with transitions in the D1 line of Rb, one
mode with the to transition and the other one with the to
transition. For this configuration, we obtain an intensity difference
squeezing level of dB. Moreover, the intensity difference squeezing
increases to dB and dB when only one of the modes of the squeezed
state is resonant with the D1 to or to transition of
Rb, respectively
Infinite chain of N different deltas: a simple model for a Quantum Wire
We present the exact diagonalization of the Schrodinger operator
corresponding to a periodic potential with N deltas of different couplings, for
arbitrary N. This basic structure can repeat itself an infinite number of
times. Calculations of band structure can be performed with a high degree of
accuracy for an infinite chain and of the correspondent eigenlevels in the case
of a random chain. The main physical motivation is to modelate quantum wire
band structure and the calculation of the associated density of states. These
quantities show the fundamental properties we expect for periodic structures
although for low energy the band gaps follow unpredictable patterns. In the
case of random chains we find Anderson localization; we analize also the role
of the eigenstates in the localization patterns and find clear signals of
fractality in the conductance. In spite of the simplicity of the model many of
the salient features expected in a quantum wire are well reproduced.Comment: 28 pages, LaTeX, 13 eps figures (3 color
Screening of point charges in Si quantum dots
The screening of point charges in hydrogenated Si quantum dots ranging in
diameter from 10 A to 26 A has been studied using first-principles
density-functional methods. We find that the main contribution to the screening
function originates from the electrostatic field set up by the polarization
charges at the surface of the nanocrystals. This contribution is well described
by a classical electrostatics model of dielectric screening
Rejoinder to "Support Vector Machines with Applications"
Rejoinder to ``Support Vector Machines with Applications'' [math.ST/0612817]Comment: Published at http://dx.doi.org/10.1214/088342306000000501 in the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Simple model for a Quantum Wire II. Correlations
In a previous paper (Eur. Phys. J. B 30, 239-251 (2002)) we have presented
the main features and properties of a simple model which -in spite of its
simplicity- describes quite accurately the qualitative behaviour of a quantum
wire. The model was composed of N distinct deltas each one carrying a different
coupling. We were able to diagonalize the Hamiltonian in the periodic case and
yield a complete and analytic description of the subsequent band structure.
Furthermore the random case was also analyzed and we were able to describe
Anderson localization and fractal structure of the conductance. In the present
paper we go one step further and show how to introduce correlations among the
sites of the wire. The presence of a correlated disorder manifests itself by
altering the distribution of states and the localization of the electrons
within the systemComment: RevTex, 7 pages, 9 figures (3 greyscale, 6 coloured
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