37,839 research outputs found
Dileptons from transport and hydrodynamical models
Transport and hydrodynamical models used to describe the expansion stage of a
heavy-ion collision at the CERN SPS give different dilepton spectrum even if
they are tuned to reproduce the observed hadron spectra. To understand the
origin of this difference we compare the dilepton emission from transport and
hydrodynamical models using similar initial states in both models. We find that
the requirement of pion number conservation in a hydrodynamical model does not
change the dilepton emission. Also the mass distribution from the transport
model indicates faster cooling and longer lifetime of the fireball.Comment: 5 pages, 2 Postscript figures, contribution to the `International
Workshop XXVIII on Gross Properties of Nuclei and Nuclear Excitations',
Hirschegg, Austria, January 16-22 200
Engineering an all-optical route to ultracold molecules in their vibronic ground state
We propose an improved photoassociation scheme to produce ultracold molecules
in their vibronic ground state for the generic case where non-adiabatic effects
facilitating transfer to deeply bound levels are absent. Formation of molecules
is achieved by short laser pulses in a Raman-like pump-dump process where an
additional near-infrared laser field couples the excited state to an auxiliary
state. The coupling due to the additional field effectively changes the shape
of the excited state potential and allows for efficient population transfer to
low-lying vibrational levels of the electronic ground state. Repetition of many
pump-dump sequences together with collisional relaxation allows for
accumulation of molecules in v=0.Comment: Phys. Rev. A, in pres
Quantum control of molecular rotation
The angular momentum of molecules, or, equivalently, their rotation in
three-dimensional space, is ideally suited for quantum control. Molecular
angular momentum is naturally quantized, time evolution is governed by a
well-known Hamiltonian with only a few accurately known parameters, and
transitions between rotational levels can be driven by external fields from
various parts of the electromagnetic spectrum. Control over the rotational
motion can be exerted in one-, two- and many-body scenarios, thereby allowing
to probe Anderson localization, target stereoselectivity of bimolecular
reactions, or encode quantum information, to name just a few examples. The
corresponding approaches to quantum control are pursued within separate, and
typically disjoint, subfields of physics, including ultrafast science, cold
collisions, ultracold gases, quantum information science, and condensed matter
physics. It is the purpose of this review to present the various control
phenomena, which all rely on the same underlying physics, within a unified
framework. To this end, we recall the Hamiltonian for free rotations, assuming
the rigid rotor approximation to be valid, and summarize the different ways for
a rotor to interact with external electromagnetic fields. These interactions
can be exploited for control --- from achieving alignment, orientation, or
laser cooling in a one-body framework, steering bimolecular collisions, or
realizing a quantum computer or quantum simulator in the many-body setting.Comment: 52 pages, 11 figures, 607 reference
- …