37 research outputs found
Stroboscopic Laser Diagnostics for Detection of Ordering in One-Dimensional Ion beam
A novel diagnostic method for detecting ordering in one-dimensional ion beams
is presented. The ions are excited by a pulsed laser at two different positions
along the beam and fluorescence is observed by a group of four
photomultipliers. Correlation in fluorescence signals is firm indication that
the ion beam has an ordered structure.Comment: 7 pages, REVTEX, fig3 uuencoded, figs 1-2 available upon request from
[email protected], to appear in Phys. Rev.
Quantum damping of position due to energy measurements
Quantum theory for measurements of energy is introduced and its consequences
for the average position of monitored dynamical systems are analyzed. It turns
out that energy measurements lead to a localization of the expectation values
of other observables. This is manifested, in the case of position, as a damping
of the motion without classical analogue. Quantum damping of position for an
atom bouncing on a reflecting surface in presence of a homogeneous
gravitational field is dealt in detail and the connection with an experiment
already performed in the classical regime is studied. We show that quantum
damping is testable provided that the same measurement strength obtained in the
experimental verification of the quantum Zeno effect in atomic spectroscopy [W.
M. Itano et al., Phys. Rev. A {\bf 41}, 2295 (1990)] is made available.Comment: 19 pages + 4 figures available upon request; Plain REVTeX; To be
published in Phys. Rev.
Continuous Fuzzy Measurement of Energy for a Two-Level System
A continuous measurement of energy which is sharp (perfect) leads to the
quantum Zeno effect (freezing of the state). Only if the quantum measurement is
fuzzy, continuous monitoring gives a readout E(t) from which information about
the dynamical development of the state vector of the system may be obtained in
certain cases. This is studied in detail. Fuzziness is thereby introduced with
the help of restricted path integrals equivalent to non-Hermitian Hamiltonians.
For an otherwise undisturbed multilevel system it is shown that this
measurement represents a model of decoherence. If it lasts long enough, the
measurement readout discriminates between the energy levels and the von Neumann
state reduction is obtained. For a two-level system under resonance influence
(which undergoes in absence of measurement Rabi oscillations between the
levels) different regimes of measurement are specified depending on its
duration and fuzziness: 1) the Zeno regime where the measurement results in a
freezing of the transitions between the levels and 2) the Rabi regime when the
transitions maintain. It is shown that in the Rabi regime at the border to the
Zeno regime a correlation exists between the time dependent measurement readout
and the modified Rabi oscillations of the state of the measured system.
Possible realizations of continuous fuzzy measurements of energy are sketched.Comment: 29 pages in LATEX, 1 figure in EPS, to be published in Physical
Review
Children’s rights online: challenges, dilemmas and emerging directions
In debates over internet governance, the interests of children figure unevenly, and only partial progress has been made in supporting children’s rights online globally. This chapter examines how the UN Convention on the Rights of the Child is helpful in mapping children’s rights to provision, protection and participation as they apply online as well as offline. However, challenges remain. First, opportunities and risks are positively linked, policy approaches are needed to resolve the potential conflict between protection on the one hand, and provision and participation on the other. Second, while parents may be relied on to some degree to balance their child’s rights and needs, the evidence suggests that a minority of parents are ill-equipped to manage this. Third, resolution is needed regarding the responsibility for implementing digital rights, since many governments prefer self-regulation in relation to internet governance. The chapter concludes by calling for a global governance body charged with ensuring the delivery of children’s rights
Autobiographical thinking interferes with episodic memory consolidation
New episodic memories are retained better if learning is followed by a few minutes of wakeful rest than by the encoding of novel external information. Novel encoding is said to interfere with the consolidation of recently acquired episodic memories. Here we report four experiments in which we examined whether autobiographical thinking, i.e. an 'internal' memory activity, also interferes with episodic memory consolidation. Participants were presented with three wordlists consisting of common nouns; one list was followed by wakeful rest, one by novel picture encoding and one by autobiographical retrieval/future imagination, cued by concrete sounds. Both novel encoding and autobiographical retrieval/future imagination lowered wordlist retention significantly. Follow-up experiments demonstrated that the interference by our cued autobiographical retrieval/future imagination delay condition could not be accounted for by the sound cues alone or by executive retrieval processes. Moreover, our results demonstrated evidence of a temporal gradient of interference across experiments. Thus, we propose that rich autobiographical retrieval/future imagination hampers the consolidation of recently acquired episodic memories and that such interference is particularly likely in the presence of external concrete cues
Continuous measurements in quantum systems
During a continuous measurement, quantum systems can be described by a
stochastic Schr\"odinger equation which, in the appropriate limit, reproduces
the von Neumann wave-function collapse. The average behavior on the ensemble of
all measurement results is described by a master equation obtained from a
general model of measurement apparatus consisting of an infinite set of degrees
of freedom linearly interacting with the measured system and in contact with a
reservoir at high temperature.Comment: 4 pages, presented at the 3rd Euroconference on Atomic Physics with
Stored Highly Charged Ions, Ferrara 22-26 September 199
Massive quarks in neutron stars
We study various neutron star properties using the Color-Dielectric model to describe quark matter. For the baryon sector at low densities we employ the Walecka model. Applying Gibbs criteria to this composite system, we find that, for matter at β-equilibrium, the pure hadronic phase ends at 0.11 fm−3 and that the mixed quark and hadronic phase extends to 0.31 fm−3. The resulting equation of state yields a maximum neutron star mass of 1.59M⊙. A neutron star with total mass of 1.4M⊙ will consist of a crust made of hadronic matter only, a ∼ 1 km thick region of mixed phase and a core composed of pure quark matter. Implications for the cooling of neutron stars are discussed
Laser diagnostics of a one-dimensional ordered ion beam
A novel method to detect ordering within a one-
dimensional ion beam is proposed. The method exploits
detection of fluorescence induced by two laser beams
which simultaneously cross the ion beam. Appearance of
correlation in fluorescence signal while moving the
distance between the laser beams is indication of ordering
in the beam
Ultrashort laser-pulse diagnostics for detection of ordering within an ion beam
A novel diagnostic method to detect ordering within one-dimensional ion beams
in a storage ring is presented. The ions are simultaneously excited by a
ultrashort pulsed laser ( ps) at two different locations along the
beam and fluorescence is detected by a group of four photo-multipliers.
Correlation in fluorescence signals is a firm indication that the ion beam has
an ordered structure.Comment: 10 pages, REVTEX, figs 3-5 uuencoded, figs 1-2 available upon request
from [email protected]. To be published in Hyperfine In