13,537 research outputs found
Magnetic imaging with an ensemble of Nitrogen Vacancy centers in diamond
The nitrogen-vacancy (NV) color center in diamond is an atom-like system in
the solid-state which specific spin properties can be efficiently used as a
sensitive magnetic sensor. An external magnetic field induces Zeeman shifts of
the NV center levels which can be measured using Optically Detected Magnetic
Resonance (ODMR). In this work, we exploit the ODMR signal of an ensemble of NV
centers in order to quantitatively map the vectorial structure of a magnetic
field produced by a sample close to the surface of a CVD diamond hosting a thin
layer of NV centers. The reconstruction of the magnetic field is based on a
maximum-likelihood technique which exploits the response of the four intrinsic
orientations of the NV center inside the diamond lattice. The sensitivity
associated to a 1 {\mu}m^2 area of the doped layer, equivalent to a sensor
consisting of approximately 10^4 NV centers, is of the order of 2
{\mu}T/sqrt{Hz}. The spatial resolution of the imaging device is 400 nm,
limited by the numerical aperture of the optical microscope which is used to
collect the photoluminescence of the NV layer. The versatility of the sensor is
illustrated by the accurate reconstruction of the magnetic field created by a
DC current inside a copper wire deposited on the diamond sample.Comment: 11 pages, 5 figures, figure 4 added, results unchange
Privacy in (mobile) telecommunications services
Telecommunications services are for long subject to privacy regulations. At stake are traditionally: privacy of the communication and the protection of traffic data. Privacy of the communication is legally founded. Traffic data subsume under the notion of data protection and are central in the discussion.
The telecommunications environment is profoundly changing. The traditionally closed markets with closed networks change into an open market with open networks. Within these open networks more privacy sensitive data are generated and have to be exchanged between growing numbers of parties. Also telecommunications and computer networks are rapidly being integrated and thus the distinction between telephony and computing disappears. Traditional telecommunications privacy regulations are revised to cover internet applications.
In this paper telecommunications issues are recalled to aid the on-going debate.
Cellular mobile phones have recently be introduced. Cellular networks process a particular category of traffic data namely location data, thereby introducing the issue of territorial privacy into the telecommunications domain. Location data are bound to be used for pervasive future services. Designs for future services are discussed and evaluated for their impact on privacy protection.</p
Quantitative stray field imaging of a magnetic vortex core
Thin-film ferromagnetic disks present a vortex spin structure whose dynamics,
added to the small size (~10 nm) of their core, earned them intensive study.
Here we use a scanning nitrogen-vacancy (NV) center microscope to
quantitatively map the stray magnetic field above a 1 micron-diameter disk of
permalloy, unambiguously revealing the vortex core. Analysis of both
probe-to-sample distance and tip motion effects through stroboscopic
measurements, allows us to compare directly our quantitative images to
micromagnetic simulations of an ideal structure. Slight perturbations with
respect to the perfect vortex structure are clearly detected either due to an
applied in-plane magnetic field or imperfections of the magnetic structures.
This work demonstrates the potential of scanning NV microscopy to map tiny
stray field variations from nanostructures, providing a nanoscale,
non-perturbative detection of their magnetic texture.Comment: 5 pages, 4 figure
âFixed-axisâ magnetic orientation by an amphibian: non-shoreward-directed compass orientation, misdirected homing or positioning a magnetite-based map detector in a consistent alignment relative to the magnetic field?
Experiments were carried out to investigate the earlier prediction that prolonged exposure to long-wavelength (>500 nm) light would eliminate homing orientation by male Eastern red-spotted newts Notophthalmus viridescens. As in previous experiments, controls held in outdoor tanks under natural lighting conditions and tested in a visually uniform indoor arena under full-spectrum light were homeward oriented. As predicted, however, newts held under long-wavelength light and tested under either full-spectrum or long-wavelength light (>500 nm) failed to show consistent homeward orientation. The newts also did not orient with respect to the shore directions in the outdoor tanks in which they were held prior to testing. Unexpectedly, however, the newts exhibited bimodal orientation along a more-or-less `fixed' north-northeastâsouth-southwest magnetic axis. The orientation exhibited by newts tested under full-spectrum light was indistinguishable from that of newts tested under long-wavelength light, although these two wavelength conditions have previously been shown to differentially affect both shoreward compass orientation and homing orientation. To investigate the possibility that the `fixed-axis' response of the newts was mediated by a magnetoreception mechanism involving single-domain particles of magnetite, natural remanent magnetism (NRM) was measured from a subset of the newts. The distribution of NRM alignments with respect to the headâbody axis of the newts was indistinguishable from random. Furthermore, there was no consistent relationship between the NRM of individual newts and their directional response in the overall sample. However, under full-spectrum, but not long-wavelength, light, the alignment of the NRM when the newts reached the 20 cm radius criterion circle in the indoor testing arena (estimated by adding the NRM alignment measured from each newt to its magnetic bearing) was non-randomly distributed. These findings are consistent with the earlier suggestion that homing newts use the light-dependent magnetic compass to align a magnetite-based `map detector' when obtaining the precise measurements necessary to derive map information from the magnetic field. However, aligning the putative map detector does not explain the fixed-axis response of newts tested under long-wavelength light. Preliminary evidence suggests that, in the absence of reliable directional information from the magnetic compass (caused by the 90° rotation of the response of the magnetic compass under long-wavelength light), newts may resort to a systematic sampling strategy to identify alignment(s) of the map detector that yields reliable magnetic field measurements
Stability of the self-phase-locked pump-enhanced singly resonant parametric oscillator
Steady-state and dynamics of the self-phase-locked (3\omega ==> 2\omega,
\omega) subharmonic optical parametric oscillator are analyzed in the
pump-and-signal resonant configuration, using an approximate analytical model
and a full propagation model. The upper branch solutions are found always
stable, regardless of the degree of pump enhancement. The domain of existence
of stationary states is found to critically depend on the phase-mismatch of the
competing second-harmonic process.Comment: LateX2e/RevteX4, 4 pages, 5 figures. Submitted to Phys. Rev. A
(accepted on Jan. 17, 2003
Skyrmion morphology in ultrathin magnetic films
Nitrogen-vacancy magnetic microscopy is employed in quenching mode as a
non-invasive, high resolution tool to investigate the morphology of isolated
skyrmions in ultrathin magnetic films. The skyrmion size and shape are found to
be strongly affected by local pinning effects and magnetic field history.
Micromagnetic simulations including static disorder, based on a physical model
of grain-to-grain thickness variations, reproduce all experimental observations
and reveal the key role of disorder and magnetic history in the stabilization
of skyrmions in ultrathin magnetic films. This work opens the way to an
in-depth understanding of skyrmion dynamics in real, disordered media.Comment: 9 pages, 8 figures, including supplementary information
Synchronization and Redundancy: Implications for Robustness of Neural Learning and Decision Making
Learning and decision making in the brain are key processes critical to
survival, and yet are processes implemented by non-ideal biological building
blocks which can impose significant error. We explore quantitatively how the
brain might cope with this inherent source of error by taking advantage of two
ubiquitous mechanisms, redundancy and synchronization. In particular we
consider a neural process whose goal is to learn a decision function by
implementing a nonlinear gradient dynamics. The dynamics, however, are assumed
to be corrupted by perturbations modeling the error which might be incurred due
to limitations of the biology, intrinsic neuronal noise, and imperfect
measurements. We show that error, and the associated uncertainty surrounding a
learned solution, can be controlled in large part by trading off
synchronization strength among multiple redundant neural systems against the
noise amplitude. The impact of the coupling between such redundant systems is
quantified by the spectrum of the network Laplacian, and we discuss the role of
network topology in synchronization and in reducing the effect of noise. A
range of situations in which the mechanisms we model arise in brain science are
discussed, and we draw attention to experimental evidence suggesting that
cortical circuits capable of implementing the computations of interest here can
be found on several scales. Finally, simulations comparing theoretical bounds
to the relevant empirical quantities show that the theoretical estimates we
derive can be tight.Comment: Preprint, accepted for publication in Neural Computatio
Observation of correlations up to the micrometer scale in sliding charge-density waves
High-resolution coherent x-ray diffraction experiment has been performed on
the charge density wave (CDW) system KMoO. The satellite
reflection associated with the CDW has been measured with respect to external
dc currents. In the sliding regime, the satellite reflection displays
secondary satellites along the chain axis which corresponds to correlations up
to the micrometer scale. This super long range order is 1500 times larger than
the CDW period itself. This new type of electronic correlation seems inherent
to the collective dynamics of electrons in charge density wave systems. Several
scenarios are discussed.Comment: 4 pages, 3 figures Typos added, references remove
Singe ferroelectric and chiral magnetic domain of single-crystalline BiFeO in an electric field
We report polarized neutron scattering and piezoresponse force microscopy
studies of millimeter-sized single crystals of multiferroic BiFeO. The
crystals, grown below the Curie temperature, consist of a single ferroelectric
domain. Two unique electric polarization directions, as well as the populations
of equivalent spiral magnetic domains, can be switched reversibly by an
electric field. A ferroelectric monodomain with a single- single-helicity
spin spiral can be obtained. This level of control, so far unachievable in thin
films, makes single-crystal BiFeO a promising object for multiferroics
research.Comment: 4 figures in separate jpg file
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