188 research outputs found
Tunable Holstein model with cold polar molecules
We show that an ensemble of polar molecules trapped in an optical lattice can
be considered as a controllable open quantum system. The coupling between
collective rotational excitations and the motion of the molecules in the
lattice potential can be controlled by varying the strength and orientation of
an external DC electric field as well as the intensity of the trapping laser.
The system can be described by a generalized Holstein Hamiltonian with tunable
parameters and can be used as a quantum simulator of excitation energy transfer
and polaron phenomena. We show that the character of excitation energy transfer
can be modified by tuning experimental parameters.Comment: 5 pages, 3 figures (accepted in as a Rapid Communication in
Phys.Rev.A
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
Quantum phases of dipolar rotors on two-dimensional lattices
The quantum phase transitions of dipoles confined to the vertices of two
dimensional (2D) lattices of square and triangular geometry is studied using
path integral ground state quantum Monte Carlo (PIGS). We analyze the phase
diagram as a function of the strength of both the dipolar interaction and a
transverse electric field. The study reveals the existence of a class of
orientational phases of quantum dipolar rotors whose properties are determined
by the ratios between the strength anisotropic dipole-dipole interaction, the
strength of the applied transverse field, and the rotational constant. For the
triangular lattice, the generic orientationally disordered phase found at zero
and weak values of both dipolar interaction strength and applied field, is
found to show a transition to a phase characterized by net polarization in the
lattice plane as the strength of the dipole-dipole interaction is increased,
independent of the strength of the applied transverse field, in addition to the
expected transition to a transverse polarized phase as the electric field
strength increases. The square lattice is also found to exhibit a transition
from a disordered phase to an ordered phase as the dipole-dipole interaction
strength is increased, as well as the expected transition to a transverse
polarized phase as the electric field strength increases. In contrast to the
situation with a triangular lattice, on square lattices the ordered phase at
high dipole-dipole interaction strength possesses a striped ordering. The
properties of these quantum dipolar rotor phases are dominated by the
anisotropy of the interaction and provide useful models for developing quantum
phases beyond the well-known paradigms of spin Hamiltonian models, realizing in
particular a novel physical realization of a quantum rotor-like Hamiltonian
that possesses an anisotropic long range interaction.Comment: Updated credit line and changed line spacin
Quo vadis, cold molecules? - Editorial review
We give a snapshot of the rapidly developing field of ultracold polar
molecules abd walk the reader through the papers appearing in this topical
issue
Formation of Ultracold Heteronuclear Dimers in Electric Fields
The formation of ultracold molecules via stimulated emission followed by a
radiative deexcitation cascade in the presence of a static electric field is
investigated. By analyzing the corresponding cross sections, we demonstrate the
possibility to populate the lowest rotational excitations via photoassociation.
The modification of the radiative cascade due to the electric field leads to
narrow rotational state distributions in the vibrational ground state. External
fields might therefore represent an additional valuable tool towards the
ultimate goal of quantum state preparation of molecules
Ultracold polar molecules near quantum degeneracy
We report the creation and characterization of a near quantum-degenerate gas
of polar K-Rb molecules in their absolute rovibrational ground
state. Starting from weakly bound heteronuclear KRb Feshbach molecules, we
implement precise control of the molecular electronic, vibrational, and
rotational degrees of freedom with phase-coherent laser fields. In particular,
we coherently transfer these weakly bound molecules across a 125 THz frequency
gap in a single step into the absolute rovibrational ground state of the
electronic ground potential. Phase coherence between lasers involved in the
transfer process is ensured by referencing the lasers to two single components
of a phase-stabilized optical frequency comb. Using these methods, we prepare a
dense gas of polar molecules at a temperature below 400 nK. This
fermionic molecular ensemble is close to quantum degeneracy and can be
characterized by a degeneracy parameter of . We have measured the
molecular polarizability in an optical dipole trap where the trap lifetime
gives clues to interesting ultracold chemical processes. Given the large
measured dipole moment of the KRb molecules of 0.5 Debye, the study of quantum
degenerate molecular gases interacting via strong dipolar interactions is now
within experimental reach
Improved setup for producing slow beams of cold molecules using a rotating nozzle
Intense beams of cold and slow molecules are produced by supersonic expansion
out of a rapidly rotating nozzle, as first demonstrated by Gupta and
Herschbach. An improved setup is presented that allows to accelerate or
decelerate cold atomic and molecular beams by up to 500 m/s. Technical
improvements are discussed and beam parameters are characterized by detailed
analysis of time of flight density distributions. The possibility of combining
this beam source with electrostatic fields for guiding polar molecules is
demonstrated
Observation of Quantum Effects in sub Kelvin Cold Reactions
There has been a long-standing quest to observe chemical reactions at low
temperatures where reaction rates and pathways are governed by quantum
mechanical effects. So far this field of Quantum Chemistry has been dominated
by theory. The difficulty has been to realize in the laboratory low enough
collisional velocities between neutral reactants, so that the quantum wave
nature could be observed. We report here the first realization of merged
neutral supersonic beams, and the observation of clear quantum effects in the
resulting reactions. We observe orbiting resonances in the Penning ionization
reaction of argon and molecular hydrogen with metastable helium leading to a
sharp increase in the absolute reaction rate in the energy range corresponding
to a few degrees kelvin down to 10 mK. Our method is widely applicable to many
canonical chemical reactions, and will enable a breakthrough in the
experimental study of Quantum Chemistry
Machine-learning-corrected quantum dynamics calculations
Quantum scattering calculations for all but low-dimensional systems at low
energies must rely on approximations. All approximations introduce errors. The
impact of these errors is often difficult to assess because they depend on the
Hamiltonian parameters and the particular observable under study. Here, we
illustrate a general, system and approximation-independent, approach to improve
the accuracy of quantum dynamics approximations. The method is based on a
Bayesian machine learning (BML) algorithm that is trained by a small number of
rigorous results and a large number of approximate calculations, resulting in
ML models that accurately capture the dependence of the dynamics results on the
quantum dynamics parameters. Most importantly, the present work demonstrates
that the BML models can generalize quantum results to different dynamical
processes. Thus, a ML model trained by a combination of approximate and
rigorous results for a certain inelastic transition can make accurate
predictions for different transitions without rigorous calculations. This opens
the possibility of improving the accuracy of approximate calculations for
quantum transitions that are out of reach of rigorous scattering calculations.Comment: 6 pages, 4 figure
Pulsed rotating supersonic source used with merged molecular beams
We describe a pulsed rotating supersonic beam source, evolved from an
ancestral device [M. Gupta and D. Herschbach, J. Phys. Chem. A 105, 1626
(2001)]. The beam emerges from a nozzle near the tip of a hollow rotor which
can be spun at high-speed to shift the molecular velocity distribution downward
or upward over a wide range. Here we consider mostly the slowing mode.
Introducing a pulsed gas inlet system, cryocooling, and a shutter gate
eliminated the main handicap of the original device, in which continuous gas
flow imposed high background pressure. The new version provides intense pulses,
of duration 0.1-0.6 ms (depending on rotor speed) and containing ~10^12
molecules at lab speeds as low as 35 m/s and ~ 10^15 molecules at 400 m/s.
Beams of any molecule available as a gas can be slowed (or speeded); e.g., we
have produced slow and fast beams of rare gases, O2, Cl2, NO2, NH3, and SF6.
For collision experiments, the ability to scan the beam speed by merely
adjusting the rotor is especially advantageous when using two merged beams. By
closely matching the beam speeds, very low relative collision energies can be
attained without making either beam very slow.Comment: 26 pages, 10 figure
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