161 research outputs found
A measurement-based approach to quantum arrival times
For a quantum-mechanically spread-out particle we investigate a method for
determining its arrival time at a specific location. The procedure is based on
the emission of a first photon from a two-level system moving into a
laser-illuminated region. The resulting temporal distribution is explicitly
calculated for the one-dimensional case and compared with axiomatically
proposed expressions. As a main result we show that by means of a deconvolution
one obtains the well known quantum mechanical probability flux of the particle
at the location as a limiting distribution.Comment: 11 pages, 4 figures, submitted to Phys. Rev.
Sources of quantum waves
Due to the space and time dependence of the wave function in the time
dependent Schroedinger equation, different boundary conditions are possible.
The equation is usually solved as an ``initial value problem'', by fixing the
value of the wave function in all space at a given instant. We compare this
standard approach to "source boundary conditions'' that fix the wave at all
times in a given region, in particular at a point in one dimension. In contrast
to the well-known physical interpretation of the initial-value-problem
approach, the interpretation of the source approach has remained unclear, since
it introduces negative energy components, even for ``free motion'', and a
time-dependent norm. This work provides physical meaning to the source method
by finding the link with equivalent initial value problems.Comment: 12 pages, 7 inlined figures; typos correcte
Comment on "Measurement of time of arrival in quantum mechanics"
The analysis of the model quantum clocks proposed by Aharonov et al. [Phys.
Rev. A 57 (1998) 4130 - quant-ph/9709031] requires considering evanescent
components, previously ignored. We also clarify the meaning of the operational
time of arrival distribution which had been investigated.Comment: 3 inlined figures; comment on quant-ph/970903
Quantum times of arrival for multiparticle states
Using the concept of crossing state and the formalism of second quantization,
we propose a prescription for computing the density of arrivals of particles
for multiparticle states, both in the free and the interacting case. The
densities thus computed are positive, covariant in time for time independent
hamiltonians, normalized to the total number of arrivals, and related to the
flux. We investigate the behaviour of this prescriptions for bosons and
fermions, finding boson enhancement and fermion depletion of arrivals.Comment: 10 a4 pages, 5 inlined figure
Transient response of a quantum wave to an instantaneous potential step switching
The transient response of a stationary state of a quantum particle in a step
potential to an instantaneous change in the step height (a simplified model for
a sudden bias switch in an electronic semiconductor device) is solved exactly
by means of a semianalytical expression. The characteristic times for the
transient process up to the new stationary state are identified. A comparison
is made between the exact results and an approximate method.Comment: 8 pages, 8 figures, Revtex
Generalizations of Kijowski's time-of-arrival distribution for interaction potentials
Several proposals for a time-of-arrival distribution of ensembles of
independent quantum particles subject to an external interaction potential are
compared making use of the ``crossing state'' concept. It is shown that only
one of them has the properties expected for a classical distribution in the
classical limit. The comparison is illustrated numerically with a collision of
a Gaussian wave packet with an opaque square barrier.Comment: 5 inlined figures: some typo correction
Atomic time-of-arrival measurements with a laser of finite beam width
A natural approach to measure the time of arrival of an atom at a spatial
region is to illuminate this region with a laser and detect the first
fluorescence photons produced by the excitation of the atom and subsequent
decay. We investigate the actual physical content of such a measurement in
terms of atomic dynamical variables, taking into account the finite width of
the laser beam. Different operation regimes are identified, in particular the
ones in which the quantum current density may be obtained.Comment: 7 figure
Towards a time-reversal mirror for quantum systems
The reversion of the time evolution of a quantum state can be achieved by
changing the sign of the Hamiltonian as in the polarization echo experiment in
NMR. In this work we describe an alternative mechanism inspired by the acoustic
time reversal mirror. By solving the inverse time problem in a discrete space
we develop a new procedure, the perfect inverse filter. It achieves the exact
time reversion in a given region by reinjecting a prescribed wave function at
its periphery.Comment: 6 pages, 4 figures. Introduction modified, references added, one
figure added to improve the discussio
RELACIÓN TERAPÉUTICA: EL PILAR DE LA PROFESIÓN ENFERMERA
This revision presents a global vision about the therapeutic relationship and its importance for nursing professionals. With this, we will show the importance of the acquisition of skills and attitudes for initiating the therapeutic relationship. One of the most important characteristic of nursing is the concept of care which is the essence of this profession. Among the most important needs, we found the need to establish a correct therapeutic relationship. If nursing students are coached in these skills, we will be able to achieve better prepared professionals, who have a holistic vision of the patient, they offering an integral attention to the patient. This is a demand to our current society, which assures the improvement of individual healthEn esta revisión se presenta una visión global de la relación terapéutica y su relevancia en la profesión enfermera. A través de ésta, se quiere demostrar la importancia de la adquisición de habilidades y actitudes fundamentales a la hora de iniciar una relación de ayuda. Una de las características de la enfermería es el concepto del cuidado a través del cual damos significado a la profesión. Entre las necesidades de los cuidados encontramos la más importante de ellas que es la necesidad de establecer una correcta relación terapéutica. Si los estudiantes de enfermería están entrenados en estas habilidades, conseguiremos profesionales mejores preparados, que contemplarán a las personas en su globalidad, proporcionando una atención integral. Ésta es una demanda de la sociedad actual, la cual conllevaría a una mejora de la salud individual.
Time-of-arrival distribution for arbitrary potentials and Wigner's time-energy uncertainty relation
A realization of the concept of "crossing state" invoked, but not
implemented, by Wigner, allows to advance in two important aspects of the time
of arrival in quantum mechanics: (i) For free motion, we find that the
limitations described by Aharonov et al. in Phys. Rev. A 57, 4130 (1998) for
the time-of-arrival uncertainty at low energies for certain mesurement models
are in fact already present in the intrinsic time-of-arrival distribution of
Kijowski; (ii) We have also found a covariant generalization of this
distribution for arbitrary potentials and positions.Comment: 4 pages, revtex, 2 eps figures include
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