319 research outputs found
Molecules Near Absolute Zero and External Field Control of Atomic and Molecular Dynamics
This article reviews the current state of the art in the field of cold and
ultracold molecules and demonstrates that chemical reactions, inelastic
collisions and dissociation of molecules at subKelvin temperatures can be
manipulated with external electric or magnetic fields. The creation of
ultracold molecules may allow for spectroscopy measurements with extremely high
precision and tests of fundamental symmetries of nature, quantum computation
with molecules as qubits, and controlled chemistry. The probability of chemical
reactions and collisional energy transfer can be very large at temperatures
near zero Kelvin. The collision energy of ultracold atoms and molecules is much
smaller than perturbations due to interactions with external electric or
magnetic fields available in the laboratory. External fields may therefore be
used to induce dissociation of weakly bound molecules, stimulate forbidden
electronic transitions, suppress the effect of centrifugal barriers in outgoing
reaction channels or tune Feshbach resonances that enhance chemical reactivity
Rotational predissociation of extremely weakly bound atom-molecule complexes produced by Feshbach resonance association
We study the rotational predissociation of atom - molecule complexes with
very small binding energy. Such complexes can be produced by Feshbach resonance
association of ultracold molecules with ultracold atoms. Numerical calculations
of the predissociation lifetimes based on the computation of the energy
dependence of the scattering matrix elements become inaccurate when the binding
energy is smaller than the energy width of the predissociating state. We derive
expressions that represent accurately the predissociation lifetimes in terms of
the real and imaginary parts of the scattering length and effective range for
molecules in an excited rotational state. Our results show that the
predissociation lifetimes are the longest when the binding energy is positive,
i.e. when the predissociating state is just above the excited state threshold.Comment: 17 pages, 5 figure
Entanglement Switch for Dipole Arrays
We propose a new entanglement switch of qubits consisting of electric
dipoles, oriented along or against an external electric field and coupled by
the electric dipole-dipole interaction. The pairwise entanglement can be tuned
and controlled by the ratio of the Rabi frequency and the dipole-dipole
coupling strength. Tuning the entanglement can be achieved for one, two and
three-dimensional arrangements of the qubits. The feasibility of building such
an entanglement switch is also discussed.Comment: 6 pages and 4 figures. To be published on Journal of Chemical Physic
Predicting and verifying transition strengths from weakly bound molecules
We investigated transition strengths from ultracold weakly bound 41K87Rb
molecules produced via the photoassociation of laser-cooled atoms. An accurate
potential energy curve of the excited state (3)1Sigma+ was constructed by
carrying out direct potential fit analysis of rotational spectra obtained via
depletion spectroscopy. Vibrational energies and rotational constants extracted
from the depletion spectra of v'=41-50 levels were combined with the results of
the previous spectroscopic study, and they were used for modifying an ab initio
potential. An accuracy of 0.14% in vibrational level spacing and 0.3% in
rotational constants was sufficient to predict the large observed variation in
transition strengths among the vibrational levels. Our results show that
transition strengths from weakly bound molecules are a good measure of the
accuracy of an excited state potential.Comment: 7 pages, 7 figure
Fear of an examination
Nach einer kurzen Einführung in die Themen Angst und Stress wird der Zusammenhang zwischen Stress und Leistung dargestellt. Abschließend werden Lösungsmöglichkeiten für Personen mit Prüfungsangst vorgestellt.After a brief introduction to fear and stress the correlation between stress and performance is explained. Solutions are proposed
d-Wave Superfluidity in Optical Lattices of Ultracold Polar Molecules
Recent work on ultracold polar molecules, governed by a generalization of the
t-J Hamiltonian, suggests that molecules may be better suited than atoms for
studying d-wave superfluidity due to stronger interactions and larger
tunability of the system. We compute the phase diagram for polar molecules in a
checkerboard lattice consisting of weakly coupled square plaquettes. In the
simplest experimentally realizable case where there is only tunneling and an
XX-type spin-spin interaction, we identify the parameter regime where d-wave
superfluidity occurs. We also find that the inclusion of a density-density
interaction destroys the superfluid phase and that the inclusion of a
spin-density or an Ising-type spin-spin interaction can enhance the superfluid
phase. We also propose schemes for experimentally realizing the perturbative
calculations exhibiting enhanced d-wave superfluidity.Comment: 22 pages, 12 figures; v2: revised discussion
Coherent transfer of photoassociated molecules into the rovibrational ground state
We report on the direct conversion of laser-cooled 41K and 87Rb atoms into
ultracold 41K87Rb molecules in the rovibrational ground state via
photoassociation followed by stimulated Raman adiabatic passage.
High-resolution spectroscopy based on the coherent transfer revealed the
hyperfine structure of weakly bound molecules in an unexplored region. Our
results show that a rovibrationally pure sample of ultracold ground-state
molecules is achieved via the all-optical association of laser-cooled atoms,
opening possibilities to coherently manipulate a wide variety of molecules.Comment: 4 pages, 4 figure
Quantum rainbow scattering at tunable velocities
Elastic scattering cross sections are measured for lithium atoms colliding
with rare gas atoms and SF6 molecules at tunable relative velocities down to
~50 m/s. Our scattering apparatus combines a velocity-tunable molecular beam
with a magneto-optic trap that provides an ultracold cloud of lithium atoms as
a scattering target. Comparison with theory reveals the quantum nature of the
collision dynamics in the studied regime, including both rainbows as well as
orbiting resonances
Dispersion interactions and reactive collisions of ultracold polar molecules
Progress in ultracold experiments with polar molecules requires a clear
understanding of their interactions and reactivity at ultra-low collisional
energies. Two important theoretical steps in this process are the
characterization of interaction potentials between molecules and the modeling
of reactive scattering mechanism. Here, we report on the {\it abinitio}
calculation of isotropic and anisotropic van der Waals interaction potentials
for polar KRb and RbCs colliding with each other or with ultracold atoms. Based
on these potentials and two short-range scattering parameters we then develop a
single-channel scattering model with flexible boundary conditions. Our
calculations show that at low temperatures (and in absence of an external
electric field) the reaction rates between molecules or molecules with atoms
have a resonant character as a function of the short-range parameters. We also
find that both the isotropic and anisotropic van der Waals coefficients have
significant contributions from dipole coupling to excited electronic states.
Their values can differ dramatically from those solely obtained from the
permanent dipole moment. A comparison with recently obtained reaction rates of
fermionic KRb shows that the experimental data can not be
explained by a model where the short-range scattering parameters are
independent of the relative orbital angular momentum or partial wave.Comment: 15 pages, 12 figure
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