98 research outputs found
Testing Electron Boost Invariance with 2S-1S Hydrogen Spectroscopy
There are few good direct laboratory tests of boost invariance for electrons,
because the experiments required often involve repeated precision measurements
performed at different times of year. However, existing measurements and
remeasurements of the 2S-1S two-photon transition frequency in H--which were
done to search for a time variation in the fine structure constant--also
constitute a measurement of the boost symmetry violation parameter 0.83c_(TX) +
0.51c_(TY) + 0.22c_(TZ) = (4 +/- 8) x 10^(-11). This is an eight order of
magnitude improvement over preexisting laboratory bounds, and with only one
additional measurements, this system could yield a second comparable
constraint.Comment: 8 page
Disentangling Forms of Lorentz Violation With Complementary Clock Comparison Experiments
Atomic clock comparisons provide some of the most precise tests of Lorentz
and CPT symmetries in the laboratory. With data from multiple such experiments
using different nuclei, it is possible to constrain new regions of the
parameter space for Lorentz violation. Relativistic effects in the nuclei allow
us to disentangle forms of Lorentz violation which could not be separately
measured in purely nonrelativistic experiments. The disentangled bounds in the
neutron sectors are at the 10^(-28) GeV level, far better than could be
obtained with any other current technique.Comment: 9 page
A low-noise ferrite magnetic shield
Ferrite materials provide magnetic shielding performance similar to commonly
used high permeability metals but have lower intrinsic magnetic noise generated
by thermal Johnson currents due to their high electrical resistivity.
Measurements inside a ferrite shield with a spin-exchange relaxation-free
atomic magnetometer reveal a noise level of 0.75 fT Hz^(-1/2), 25 times lower
than what would be expected in a comparable mu-metal shield. The authors
identify a 1/f component of the magnetic noise due to magnetization
fluctuations and derive general relationships for the Johnson current noise and
magnetization noise in cylindrical ferromagnetic shields in terms of their
conductivity and complex magnetic permeability.Comment: 4 pages, 3 figures. Published in Appl. Phys. Lett.; replacement
reflects published wor
Limits on new long range nuclear spin-dependent forces set with a K-3He co-magnetometer
A magnetometer using spin-polarized K and He atoms occupying the same
volume is used to search for anomalous nuclear spin-dependent forces generated
by a separate He spin source. We measure changes in the He spin
precession frequency with a resolution of 18 pHz and constrain anomalous spin
forces between neutrons to be less than of their magnetic or
less than of their gravitational interactions on a length
scale of 50 cm. We present new limits on neutron coupling to light pseudoscalar
and vector particles, including torsion, and constraints on recently proposed
models involving unparticles and spontaneous breaking of Lorentz symmetry.Comment: 4 pages, 4 figures, latest version as appeared in PR
Dipolar and scalar He and Xe frequency shifts in mm-sized cells
We describe a He-Xe comagnetometer operating in stemless
anodically bonded cells with a 6 mm volume and a Xe spin coherence
time of 300 sec. We use a Rb pulse-train magnetometer with co-linear
pump and probe beams to study the nuclear spin frequency shifts caused by spin
polarization of He. By systematically varying the cell geometry in a
batch cell fabrication process we can separately measure the cell shape
dependent and independent frequency shifts. We find that a certain aspect ratio
of the cylindrical cell can cancel the effects of He magnetization that
limit the stability of vapor-cell comagnetometers. Using this control we also
observe for the first time a scalar He-Xe collisional frequency
shift characterized by an enhancement factor .Comment: 4 pages, 4 figure
Heading errors in all-optical alkali-vapor magnetometers in geomagnetic fields
Alkali-metal atomic magnetometers suffer from heading errors in geomagnetic
fields as the measured magnetic field depends on the orientation of the sensor
with respect to the field. In addition to the nonlinear Zeeman splitting, the
difference between Zeeman resonances in the two hyperfine ground states can
also generate heading errors depending on initial spin polarization. We examine
heading errors in an all-optical scalar magnetometer that uses free precession
of polarized atoms by varying the direction and magnitude of
the magnetic field at different spin polarization regimes. In the high
polarization limit where the lower hyperfine ground state is almost
depopulated, we show that heading errors can be corrected with an analytical
expression, reducing the errors by two orders of magnitude in Earth's field. We
also verify the linearity of the measured Zeeman precession frequency with the
magnetic field. With lower spin polarization, we find that the splitting of the
Zeeman resonances for the two hyperfine states causes beating in the precession
signals and nonlinearity of the measured precession frequency with the magnetic
field. We correct for the frequency shifts by using the unique probe geometry
where two orthogonal probe beams measure opposite relative phases between the
two hyperfine states during the spin precession
Portable magnetometry for detection of biomagnetism in ambient environments
We present a method of optical magnetometry with parts-per-billion resolution
that is able to detect biomagnetic signals generated from the human brain and
heart in Earth's ambient environment. Our magnetically silent sensors measure
the total magnetic field by detecting the free-precession frequency of highly
spin-polarized alkali metal vapor. A first-order gradiometer is formed from two
magnetometers that are separated by a 3 cm baseline. Our gradiometer operates
from a laptop consuming 5 W over a USB port, enabled by state-of-the-art
micro-fabricated alkali vapor cells, advanced thermal insulation, custom
electronics, and laser packages within the sensor head. The gradiometer obtains
a sensitivity of 16 fT/cm/Hz outdoors, which we use to detect neuronal
electrical currents and magnetic cardiography signals. Recording of neuronal
magnetic fields is one of a few available methods for non-invasive functional
brain imaging that usually requires extensive magnetic shielding and other
infractructure. This work demonstrates the possibility of a dense array of
portable biomagnetic sensors that are deployable in a variety of natural
environments
Atom interferometry tests of local Lorentz invariance in gravity and electrodynamics
We present atom-interferometer tests of the local Lorentz invariance of
post-Newtonian gravity. An experiment probing for anomalous vertical gravity on
Earth, which has already been performed by us, uses the highest-resolution
atomic gravimeter so far. The influence of Lorentz violation in electrodynamics
is also taken into account, resulting in combined bounds on Lorentz violation
in gravity and electrodynamics. Expressed within the standard model extension
or Nordtvedt's anisotropic universe model, we limit twelve linear combinations
of seven coefficients for Lorentz violation at the part per billion level, from
which we derive limits on six coefficients (and seven when taking into account
additional data from lunar laser ranging). We also discuss the use of
horizontal interferometers, including atom-chip or guided-atom devices, which
potentially allow the use of longer coherence times in order to achieve higher
sensitivity.Comment: Reference added; corrected factor of 2 in Tab. IV and V. 12 pages, 4
figures, 6 table
Human hippocampal neurogenesis drops sharply in children to undetectable levels in adults.
New neurons continue to be generated in the subgranular zone of the dentate gyrus of the adult mammalian hippocampus. This process has been linked to learning and memory, stress and exercise, and is thought to be altered in neurological disease. In humans, some studies have suggested that hundreds of new neurons are added to the adult dentate gyrus every day, whereas other studies find many fewer putative new neurons. Despite these discrepancies, it is generally believed that the adult human hippocampus continues to generate new neurons. Here we show that a defined population of progenitor cells does not coalesce in the subgranular zone during human fetal or postnatal development. We also find that the number of proliferating progenitors and young neurons in the dentate gyrus declines sharply during the first year of life and only a few isolated young neurons are observed by 7 and 13 years of age. In adult patients with epilepsy and healthy adults (18-77 years; n = 17 post-mortem samples from controls; n = 12 surgical resection samples from patients with epilepsy), young neurons were not detected in the dentate gyrus. In the monkey (Macaca mulatta) hippocampus, proliferation of neurons in the subgranular zone was found in early postnatal life, but this diminished during juvenile development as neurogenesis decreased. We conclude that recruitment of young neurons to the primate hippocampus decreases rapidly during the first years of life, and that neurogenesis in the dentate gyrus does not continue, or is extremely rare, in adult humans. The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved
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