360 research outputs found
Analytical solution of the optimal laser control problem in two-level systems
The optimal control of two-level systems by time-dependent laser fields is
studied using a variational theory. We obtain, for the first time, general
analytical expressions for the optimal pulse shapes leading to global
maximization or minimization of different physical quantities. We present
solutions which reproduce and improve previous numerical results.Comment: 12 pages, 2 figure
Deterministic spatio-temporal control of nano-optical fields in optical antennas and nano transmission lines
We show that pulse shaping techniques can be applied to tailor the ultrafast
temporal response of the strongly confined and enhanced optical near fields in
the feed gap of resonant optical antennas (ROAs). Using finite-difference
time-domain (FDTD) simulations followed by Fourier transformation, we obtain
the impulse response of a nano structure in the frequency domain, which allows
obtaining its temporal response to any arbitrary pulse shape. We apply the
method to achieve deterministic optimal temporal field compression in ROAs with
reduced symmetry and in a two-wire transmission line connected to a symmetric
dipole antenna. The method described here will be of importance for experiments
involving coherent control of field propagation in nanophotonic structures and
of light-induced processes in nanometer scale volumes.Comment: 5 pages, 5 figure
Nine years of comparative effectiveness research education and training: initiative supported by the PhRMA Foundation
The term comparative effectiveness research (CER) took center stage with passage of the American Recovery and Reinvestment Act (2009). The companion US$1.1 billion in funding prompted the launch of initiatives to train the scientific workforce capable of conducting and using CER. Passage of the Patient Protection and Affordable Care Act (2010) focused these initiatives on patients, coining the term ‘patient-centered outcomes research’ (PCOR). Educational and training initiatives were soon launched. This report describes the initiative of the Pharmaceutical Research and Manufacturers Association of America (PhRMA) Foundation. Through provision of grant funding to six academic Centers of Excellence, to spearheading and sponsoring three national conferences, the PhRMA Foundation has made significant contributions to creation of the scientific workforce that conducts and uses CER/PCOR
Navigation between quantum states by quantum mirrors
We introduce a technique that allows one to connect any two arbitrary (pure
or mixed) superposition states of an N-state quantum system. The proposed
solution to this inverse quantum mechanical problem is analytical, exact, and
very compact. The technique uses standard and generalized quantum Householder
reflections (QHRs), which require external pulses of precise areas and
frequencies. We show that any two pure states can be linked by just a single
generalized QHR. The transfer between any two mixed states with the same
dynamic invariants (e.g., the same density matrix eigenvalues) requires in
general N QHRs. Moreover, we propose recipes for synthesis of arbitrary
preselected mixed states using a combination of QHRs and incoherent processes
(pure dephasing or spontaneous emission).Comment: 9 pages, 6 figure
Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse
We present a joint experimental and theoretical study on strong-field
photo-ionization of sodium atoms using chirped femtosecond laser pulses. By
tuning the chirp parameter, selectivity among the population in the highly
excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways
enabling control are identified by simultaneous ionization and measurement of
photoelectron angular distributions employing the velocity map imaging
technique. Free electron wave packets at an energy of around 1 eV are observed.
These photoelectrons originate from two channels. The predominant 2+1+1
Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly
driven two-photon transition , and subsequent
ionization from the states 5p, 6p and 7p whereas the second pathway involves
3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from
two-photon ionization of the non-resonant transiently populated state 3p are
observed close to the ionization threshold. A mainly qualitative five-state
model for the predominant excitation channel is studied theoretically to
provide insights into the physical mechanisms at play. Our analysis shows that
by tuning the chirp parameter the dynamics is effectively controlled by dynamic
Stark-shifts and level crossings. In particular, we show that under the
experimental conditions the passage through an uncommon three-state "bow-tie"
level crossing allows the preparation of coherent superposition states
Flat branches and pressure amorphization
After summarizing the phenomenology of pressure amorphization (PA), we
present a theory of PA based on the notion that one or more branches of the
phonon spectrum soften and flatten with increasing pressure. The theory
expresses the anharmonic dynamics of the flat branches in terms of local modes,
represented by lattice Wannier functions, which are in turn used to construct
an effective Hamiltonian. When the low-pressure structure becomes metastable
with respect to the high-pressure equilibrium phase and the relevant branches
are sufficiently flat, transformation into an amorphous phase is shown to be
kinetically favored because of the exponentially large number of both amorphous
phases and reaction pathways. In effect, the critical-size nucleus for the
first-order phase transition is found to be reduced to a single unit cell, or
nearly so. Random nucleation into symmetrically equivalent local configurations
characteristic of the high-pressure structure is then shown to overwhelm any
possible domain growth, and an ``amorphous'' structure results.Comment: 8 pages with 3 postscript figures embedded; Proceedings of the 4th
International Discussion Meeting on Relaxations in Complex Systems,
Hersonissos, Heraklion, Crete, June 16-23, ed. K. L. Ngai, Special Issues of
the Journal of Non-Crystalline Solids, 200
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