59,672 research outputs found
Modeling laser pulses as -kicks: reevaluating the impulsive limit in molecular rotational dynamics
The impulsive limit (the "sudden approximation") has been widely employed to
describe the interaction between molecules and short, far-off-resonant laser
pulses. This approximation assumes that the timescale of the laser--molecule
interaction is significantly shorter than the internal rotational period of the
molecule, resulting in the rotational motion being instantaneously "frozen"
during the interaction. This simplified description of laser-molecule
interaction is incorporated in various theoretical models predicting rotational
dynamics of molecules driven by short laser pulses. In this theoretical work,
we develop an effective theory for ultrashort laser pulses by examining the
full time-evolution operator and solving the time-dependent Schr\"odinger
equation at the operator level. Our findings reveal a critical angular
momentum, , at which the impulsive limit breaks down. In other
words, the validity of the sudden approximation depends not only on the pulse
duration, but also on its intensity, since the latter determines how many
angular momentum states are populated. We explore both ultrashort multi-cycle
(Gaussian) pulses and the somewhat less studied half-cycle pulses, which
produce distinct effective potentials. We discuss the limitations of the
impulsive limit and propose a new method that rescales the effective matrix
elements, enabling an improved and more accurate description of laser-molecule
interactions.Comment: 10 pages, 11 figure
Nonclassical correlations in damped quantum solitons
Using cumulant expansion in Gaussian approximation, the internal quantum
statistics of damped soliton-like pulses in Kerr media are studied numerically,
considering both narrow and finite bandwidth spectral pulse components. It is
shown that the sub-Poissonian statistics can be enhanced, under certain
circumstances, by absorption, which damps out some destructive interferences.
Further, it is shown that both the photon-number correlation and the
correlation of the photon-number variance between different pulse components
can be highly nonclassical even for an absorbing fiber. Optimum frequency
windows are determined in order to realize strong nonclassical behavior, which
offers novel possibilities of using solitons in optical fibers as a source of
nonclassically correlated light beams.Comment: 15 pages, 11 PS figures (color
Optimum pulse shapes for stimulated Raman adiabatic passage
Stimulated Raman adiabatic passage (STIRAP), driven with pulses of optimum
shape and delay has the potential of reaching fidelities high enough to make it
suitable for fault-tolerant quantum information processing. The optimum pulse
shapes are obtained upon reduction of STIRAP to effective two-state systems. We
use the Dykhne-Davis-Pechukas (DDP) method to minimize nonadiabatic transitions
and to maximize the fidelity of STIRAP. This results in a particular relation
between the pulse shapes of the two fields driving the Raman process. The
DDP-optimized version of STIRAP maintains its robustness against variations in
the pulse intensities and durations, the single-photon detuning and possible
losses from the intermediate state.Comment: 8 pages, 6 figures. submitted to Phys. Rev.
A Fully Differential Digital CMOS Pulse UWB Generator
A new fully-digital CMOS pulse generator for impulse-radio Ultra-Wide-Band (UWB) systems is presented. First, the shape of the pulse which best fits the FCC regulation in the 3.1-5 GHz sub-band of the entire 3.1-10.6 GHz UWB bandwidth is derived and approximated using rectangular digital pulses. In particular, the number and width of pulses that approximate an ideal template is found through an ad-hoc optimization methodology. Then a fully differential digital CMOS circuit that synthesizes the pulse sequence is conceived and its functionality demonstrated through post-layout simulations. The results show a very good agreement with the FCC requirements and a low power consumptio
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