1,436 research outputs found
Observation of modified radiative properties of cold atoms in vacuum near a dielectric surface
We have observed a distance-dependent absorption linewidth of cold Rb
atoms close to a dielectric-vacuum interface. This is the first observation of
modified radiative properties in vacuum near a dielectric surface. A cloud of
cold atoms was created using a magneto-optical trap (MOT) and optical molasses
cooling. Evanescent waves (EW) were used to observe the behavior of the atoms
near the surface. We observed an increase of the absorption linewidth with up
to 25% with respect to the free-space value. Approximately half the broadening
can be explained by cavity-quantum electrodynamics (CQED) as an increase of the
natural linewidth and inhomogeneous broadening. The remainder we attribute to
local Stark shifts near the surface. By varying the characteristic EW length we
have observed a distance dependence characteristic for CQED.Comment: 6 pages, 6 figures, some minor revision
Slowing heavy, ground-state molecules using an alternating gradient decelerator
Cold supersonic beams of molecules can be slowed down using a switched
sequence of electrostatic field gradients. The energy to be removed is
proportional to the mass of the molecules. Here we report deceleration of YbF,
which is 7 times heavier than any molecule previously decelerated. We use an
alternating gradient structure to decelerate and focus the molecules in their
ground state. We show that the decelerator exhibits the axial and transverse
stability required to bring these molecules to rest. Our work significantly
extends the range of molecules amenable to this powerful method of cooling and
trapping.Comment: 4 pages, 5 figure
Coherent radiation from neutral molecules moving above a grating
We predict and study the quantum-electrodynamical effect of parametric
self-induced excitation of a molecule moving above the dielectric or conducting
medium with periodic grating. In this case the radiation reaction force
modulates the molecular transition frequency which results in a parametric
instability of dipole oscillations even from the level of quantum or thermal
fluctuations. The present mechanism of instability of electrically neutral
molecules is different from that of the well-known Smith-Purcell and transition
radiation in which a moving charge and its oscillating image create an
oscillating dipole.
We show that parametrically excited molecular bunches can produce an easily
detectable coherent radiation flux of up to a microwatt.Comment: 4 page
Electric dipole moment of the electron in YbF molecule
Ab initio calculation of the hyperfine, P-odd, and P,T-odd constants for the
YbF molecule was performed with the help of the recently developed technique,
which allows to take into account correlations and polarization in the
outercore region. The ground state electronic wave function of the YbF molecule
is found with the help of the Relativistic Effective Core Potential method
followed by the restoration of molecular four-component spinors in the core
region of ytterbium in the framework of a non-variational procedure. Core
polarization effects are included with the help of the atomic Many Body
Perturbation Theory for Yb atom. For the isotropic hyperfine constant A,
accuracy of our calculation is about 3% as compared to the experimental datum.
The dipole constant Ad (which is much smaller in magnitude), though better than
in all previous calculations, is still underestimated by almost 23%. Being
corrected within a semiempirical approach for a perturbation of 4f-shell in the
core of Yb due to the bond making, this error is reduced to 8%. Our value for
the effective electric field on the unpaired electron is 4.9 a.u.=2.5E+10 V/cm.Comment: 7 pages, REVTE
Two-dimensional atom trapping in field-induced adiabatic potentials
We show how to create a novel two-dimensional trap for ultracold atoms from a conventional magnetic trap. We achieve this by utilizing rf-induced adiabatic potentials to enhance the trapping potential in one direction. We demonstrate the loading process and discuss the experimental conditions under which it might be possible to prepare a 2D Bose condensate. A scheme for the preparation of coherent matterwave bubbles is also discussed
Adiabatic transfer of light in a double cavity and the optical Landau-Zener problem
We analyze the evolution of an electromagnetic field inside a double cavity
when the difference in length between the two cavities is changed, e.g. by
translating the common mirror. We find that this allows photons to be moved
deterministically from one cavity to the other. We are able to obtain the
conditions for adiabatic transfer by first mapping the Maxwell wave equation
for the electric field onto a Schroedinger-like wave equation, and then using
the Landau-Zener result for the transition probability at an avoided crossing.
Our analysis reveals that this mapping only rigorously holds when the two
cavities are weakly coupled (i.e. in the regime of a highly reflective common
mirror), and that, generally speaking, care is required when attempting a
hamiltonian description of cavity electrodynamics with time-dependent boundary
conditions.Comment: 24 pages, 18 figures. Version 2 includes a new section (Sec. VIII) on
the regimes of validity of the Schroedinger-like equations and also of the
adiabatic approximation, together with a new figure (Fig. 10). The discussion
section (Sec. XI) has also been enhance
Vacuum-field level shifts in a single trapped ion mediated by a single distant mirror
A distant mirror leads to a vacuum-induced level shift in a laser-excited
atom. This effect has been measured with a single mirror 25 cm away from a
single, trapped barium ion. This dispersive action is the counterpart to the
mirror's dissipative effect, which has been shown earlier to effect a change in
the ion's spontaneous decay [J. Eschner et al., Nature 413, 495-498 (2001)].
The experimental data are well described by 8-level optical Bloch equations
which are amended to take into account the presence of the mirror according to
the model in [U. Dorner and P. Zoller, Phys. Rev. A 66, 023816 (2002)].
Observed deviations from simple dispersive behavior are attributed to
multi-level effects.Comment: version accepted by PR
Spontaneous emission of an atom in front of a mirror
Motivated by a recent experiment [J. Eschner {\it et al.}, Nature {\bf 413},
495 (2001)], we now present a theoretical study on the fluorescence of an atom
in front of a mirror. On the assumption that the presence of the distant mirror
and a lens imposes boundary conditions on the electric field in a plane close
to the atom, we derive the intensities of the emitted light as a function of an
effective atom-mirror distance. The results obtained are in good agreement with
the experimental findings.Comment: 8 pages, 6 figures, revised version, references adde
Protein folding using contact maps
We present the development of the idea to use dynamics in the space of
contact maps as a computational approach to the protein folding problem. We
first introduce two important technical ingredients, the reconstruction of a
three dimensional conformation from a contact map and the Monte Carlo dynamics
in contact map space. We then discuss two approximations to the free energy of
the contact maps and a method to derive energy parameters based on perceptron
learning. Finally we present results, first for predictions based on threading
and then for energy minimization of crambin and of a set of 6 immunoglobulins.
The main result is that we proved that the two simple approximations we studied
for the free energy are not suitable for protein folding. Perspectives are
discussed in the last section.Comment: 29 pages, 10 figure
Calculation of T_ odd effects in $"" sup 205_TIF including electron correlation
A method and codes for two-step correlation calculation of heavy-atom
molecules have been developed, employing the generalized relativistic effective
core potential and relativistic coupled cluster (RCC) methods at the first
step, followed by nonvariational one-center restoration of proper
four-component spinors in the heavy cores. Electron correlation is included for
the first time in an ab initio calculation of the interaction of the permanent
P,T-odd proton electric dipole moment with the internal electromagnetic field
in a molecule. The calculation is performed for the ground state of TlF at the
experimental equilibrium, R_e=2.0844 A, and at R=2.1 A, with spin-orbit and
correlation effects included by RCC. Calculated results with single cluster
amplitudes only are in good agreement (3% and 1%) with recent
Dirac-Hartree-Fock (DHF) values of the magnetic parameter M; the larger
differences occurring between present and DHF volume parameter (X) values, as
well as between the two DHF calculations, are explained. Inclusion of electron
correlation by GRECP/RCC with single and double excitations has a major effect
on the P,T-odd parameters, decreasing M by 17% and X by 22%.Comment: 5 pages, REVTeX4 style Accepted for publication in Phys.Rev.Letter
- …