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$_2$ 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$_2$ 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 $^{87}$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 $10.1$ fT/cm$\sqrt{\mathrm{Hz}}$. 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 $^{87}\text{Rb}$ 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 $F = 1$ 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 $\sim$20 fT/Hz$^{1/2}$ is shown. We also demonstrate the performance of a coil set measuring $7 \times 17 \times 17$ mm$^3$ that is optimized specifically for magnetoencephalography, where multiple sensors are operated in proximity to one another. Characterization of the field profile using $^{87}$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 $\sim$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$^{1/2}$ 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