335 research outputs found
Correlation function of spin noise due to atomic diffusion
We use paramagnetic Faraday rotation to study spin noise spectrum from
unpolarized Rb vapor in a tightly focused probe beam in the presence of N
buffer gas. We derive an analytical form for the diffusion component of the
spin noise time-correlation function in a Gaussian probe beam. We also obtain
analytical forms for the frequency spectrum of the spin noise in the limit of a
tightly focused or a collimated Gaussian beam in the presence of diffusion. In
particular, we find that in a tightly focused probe beam the spectral lineshape
can be independent of the buffer gas pressure. Experimentally, we find good
agreement between the calculated and measured spin noise spectra for N gas
pressures ranging from 56 to 820 torr.Comment: 6 pages, 4 figure
A macroscopic quantum state analysed particle by particle
Explaining how microscopic entities collectively produce macroscopic
phenomena is a fundamental goal of many-body physics. Theory predicts that
large-scale entanglement is responsible for exotic macroscopic phenomena, but
observation of entangled particles in naturally occurring systems is extremely
challenging. Synthetic quantum systems made of atoms in optical lattices have
been con- structed with the goal of observing macroscopic quantum phenomena
with single-atom resolution. Serious challenges remain in producing and
detecting long-range quantum correlations in these systems, however. Here we
exploit the strengths of photonic technology, including high coherence and
efficient single-particle detection, to study the predicted large-scale
entanglement underlying the macroscopic quantum phenomenon of polarization
squeezing. We generate a polarization-squeezed beam, extract photon pairs at
random, and make a tomographic reconstruction of their joint quantum state. We
present experimental evidence showing that all photons arriving within the
squeezing coherence time are entangled, that entanglement monogamy dilutes
entanglement with increasing photon density and that, counterintuitively,
increased squeezing can reduce bipartite entanglement. The results provide
direct evidence for entanglement of macroscopic numbers of particles and
introduce micro-analysis to the study of macroscopic quantum phenomena
Femtotesla direct magnetic gradiometer using a single multipass cell
We describe a direct gradiometer using optical pumping with opposite circular
polarization in two Rb atomic ensembles within a single multipass cell.
A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation
due to the intrinsic subtraction of two contributions exceeding 3.5 rad from
the highly-polarized ensembles. We develop analysis methods for the direct
gradiometer signal and measure a gradiometer sensitivity of
fT/cm. We also demonstrate that our multipass design, in
addition to increasing the optical depth, provides a fundamental advantage due
to the significantly reduced effect of atomic diffusion on the spin noise
time-correlation, in excellent agreement with theoretical estimate.Comment: 5 pages, 4 figure
COVID-19 contact tracing apps: UK public perceptions
In order to combat the COVID-19 pandemic, policymakers around the globe have increasingly invested in digital health technologies to support the ‘test, track and trace’ approach of containing the spread of the novel coronavirus. These technologies include mobile ‘contact tracing’ applications (apps), which can trace individuals likely to have come into contact with those who have reported symptoms or tested positive for the virus and request that they self-isolate. This paper takes a critical public health perspective that advocates for ‘genuine participation’ in public health interventions and emphasises the need to take citizen’s knowledge into account during public health decision-making. In doing so, it presents and discusses the findings of a UK interview study that explored public views on the possibility of using a COVID-19 contact-tracing app public health intervention at the time the United Kingdom (UK) Government announced their decision to develop such a technology. Findings illustrated interviewees’ range and degree of understandings, misconceptions, and concerns about the possibility of using an app. In particular, concerns about privacy and surveillance predominated. Interviewees associated these concerns much more broadly than health by identifying with pre-existent British national narratives associated with individual liberty and autonomy. In extending and contributing to ongoing sociological research with public health, we argue that understanding and responding to these matters is vital, and that our findings demonstrate the need for a forward-looking, anticipatory strategy for public engagement as part of the responsible innovation of the COVID-19 contact-tracing app in the UK
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
Miniature biplanar coils for alkali-metal-vapor magnetometry
Atomic spin sensors offer precision measurements using compact,
microfabricated packages, placing them in a competitive position for both
market and research applications. Performance of these sensors such as dynamic
range may be enhanced through magnetic field control. In this work, we discuss
the design of miniature coils for three-dimensional, localized field control by
direct placement around the sensor, as a flexible and compact alternative to
global approaches used previously. Coils are designed on biplanar surfaces
using a stream-function approach and then fabricated using standard
printed-circuit techniques. Application to a laboratory-scale optically pumped
magnetometer of sensitivity 20 fT/Hz is shown. We also
demonstrate the performance of a coil set measuring
mm that is optimized specifically for magnetoencephalography, where
multiple sensors are operated in proximity to one another. Characterization of
the field profile using Rb free-induction spectroscopy and other
techniques show 96% field homogeneity over the target volume of a MEMS vapor
cell and a compact stray field contour of 1% at 20 mm from the center of
the cell
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
Assessing Expectations: Towards a Toolbox for an Ethics of Emerging Technologies
In recent years, several authors have argued that the desirability of novel technologies should be assessed early, when they are still emerging. Such an ethical assessment of emerging technologies is by definition focused on an elusive object. Usually promises, expectations, and visions of the technology are taken as a starting point. As Nordmann and Rip have pointed out in a recent article, however, ethicists should not take for granted the plausibility of such expectations and visions. In this paper, we explore how the quality of expectations on emerging technologies might be assessed when engaging in a reflection on the desirability of emerging technologies. We propose that an assessment of expectations’ plausibility should focus on statements on technological feasibility, societal usability, and desirability of the expected technology. Whereas the feasibility statement and, to a lesser extent, the usability statements are frequently quite futuristic, the claims on desirability, by contrast, often display a conservative stance towards the future. Assessing the quality of expectations and visions on behalf of emerging technologies requires, then, a careful and well-directed use of both skepticism and imagination. We conclude with a brief overview of the tools and methods ethicists could use to assess claims made on behalf of emerging technologies and improve the ethical reflection on them
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