178 research outputs found
Simple computer model for the quantum Zeno effect
This paper presents a simple model for repeated measurement of a quantum
system: the evolution of a free particle, simulated by discretising the
particle's position. This model is easily simulated by computer and provides a
useful arena to investigate the effects of measurement upon dynamics, in
particular the slowing of evolution due to measurement (the `quantum Zeno
effect'). The results of this simulation are discussed for two rather different
sorts of measurement process, both of which are (simplified forms of)
measurements used in previous simulations of position measurement. A number of
interesting results due to measurement are found, and the investigation casts
some light on previous disagreements about the presence or absence of the Zeno
effect.Comment: REVTeX; 12 pages including 11 figures; figures reformatted to be more
readable; some small changes made to the description of the mode
Quantum jumps in hydrogen-like systems
In this paper it is shown that the Lyman- transition of a single
hydrogen-like system driven by a laser exhibits macroscopic dark periods,
provided there exists an additional constant electric field. We describe the
photon-counting process under the condition that the polarization of the laser
coincides with the direction of the constant electric field. The theoretical
results are given for the example of . We show that the emission
behavior depends sensitively on the Lamb shift (W.E. Lamb, R.C. Retherford,
Phys. Rev. 72, 241 (1947)) between the and energy levels.
A possibly realizable measurement of the mean duration of the dark periods
should give quantitative information about the above energy difference by using
the proposed photon-counting process.Comment: 7 pages RevTeX + 2 figures Phys. Rev A accepte
Cavity-induced coherence effects in spontaneous emission from pre-Selection of polarization
Spontaneous emission can create coherences in a multilevel atom having close
lying levels, subject to the condition that the atomic dipole matrix elements
are non-orthogonal. This condition is rarely met in atomic systems. We report
the possibility of bypassing this condition and thereby creating coherences by
letting the atom with orthogonal dipoles to interact with the vacuum of a
pre-selected polarized cavity mode rather than the free space vacuum. We derive
a master equation for the reduced density operator of a model four level atomic
system, and obtain its analytical solution to describe the interference
effects. We report the quantum beat structure in the populations.Comment: 6 pages in REVTEX multicolumn format, 5 figures, new references
added, journal reference adde
Projection Postulate and Atomic Quantum Zeno Effect
The projection postulate has been used to predict a slow-down of the time
evolution of the state of a system under rapidly repeated measurements, and
ultimately a freezing of the state. To test this so-called quantum Zeno effect
an experiment was performed by Itano et al. (Phys. Rev. A 41, 2295 (1990)) in
which an atomic-level measurement was realized by means of a short laser pulse.
The relevance of the results has given rise to controversies in the literature.
In particular the projection postulate and its applicability in this experiment
have been cast into doubt. In this paper we show analytically that for a wide
range of parameters such a short laser pulse acts as an effective level
measurement to which the usual projection postulate applies with high accuracy.
The corrections to the ideal reductions and their accumulation over n pulses
are calculated. Our conclusion is that the projection postulate is an excellent
pragmatic tool for a quick and simple understanding of the slow-down of time
evolution in experiments of this type. However, corrections have to be
included, and an actual freezing does not seem possible because of the finite
duration of measurements.Comment: 25 pages, LaTeX, no figures; to appear in Phys. Rev.
Zeno dynamics yields ordinary constraints
The dynamics of a quantum system undergoing frequent measurements (quantum
Zeno effect) is investigated. Using asymptotic analysis, the system is found to
evolve unitarily in a proper subspace of the total Hilbert space. For spatial
projections, the generator of the "Zeno dynamics" is the Hamiltonian with
Dirichlet boundary conditions.Comment: 6 page
Vacuum Induced Coherences in Radiatively Coupled Multilevel Systems
We show that radiative coupling between two multilevel atoms having
near-degenerate states can produce new interference effects in spontaneous
emission. We explicitly demonstrate this possibility by considering two
identical V systems each having a pair of transition dipole matrix elements
which are orthogonal to each other. We discuss in detail the origin of the new
interference terms and their consequences. Such terms lead to the evolution of
certain coherences and excitations which would not occur otherwise. The special
choice of the orientation of the transition dipole matrix elements enables us
to illustrate the significance of vacuum induced coherence in multi-atom
multilevel systems. These coherences can be significant in energy transfer
studies.Comment: 13 pages including 8 figures in Revtex; submitted to PR
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Improved Upper Limit on the Neutrino Mass from a Direct Kinematic Method by KATRIN.
We report on the neutrino mass measurement result from the first four-week science run of the Karlsruhe Tritium Neutrino experiment KATRIN in spring 2019. Beta-decay electrons from a high-purity gaseous molecular tritium source are energy analyzed by a high-resolution MAC-E filter. A fit of the integrated electron spectrum over a narrow interval around the kinematic end point at 18.57 keV gives an effective neutrino mass square value of (-1.0_{-1.1}^{+0.9}) eV^{2}. From this, we derive an upper limit of 1.1 eV (90% confidence level) on the absolute mass scale of neutrinos. This value coincides with the KATRIN sensitivity. It improves upon previous mass limits from kinematic measurements by almost a factor of 2 and provides model-independent input to cosmological studies of structure formation
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