Microfabricated optically-pumped magnetometers (OPMs) are advancing magnetic sensing and imaging for applications in space, defense, geophysics, industrial, and biomedical applications. OPM sensors have been developed in academia and national laboratories over the last 20 years, demonstrating the capabilities of small uncooled magnetometers with performance rivaling those of low-temperature superconductors. Many cross-validation demonstrations enabled the adoption of these novel quantum magnetometers in new applications. Translating this technology into industry poses many new challenges, but also opens the door for faster adoption by putting them into the hands of the users. Two example applications are discussed: microfabricated zero-field OPMs for non-invasive functional brain imaging and microfabricated Mz scalar-vector magnetometers for integration into Cubesats for geomagnetic surveying and monitoring. Both applications pose unique challenges and take advantage of unique features of these quantum sensors.
Sensors based on optically-pumped magnetometers allow the development of room-temperature, wearable imaging systems for biomagnetism detection due to their excellent sensitivity, with applications such as Magnetoencephalography and Brain-Computer Interfaces. The small size of sensors based on microfabricated vapor cell technology promises high spatial resolution. The high sensitivity also opens up the possibility to use OPM sensors in other applications such as Very Low Frequency communications and ultrasensitive microwave detection.
Optically-pumped magnetometers have demonstrated magnetic field measurements as precise as the best superconducting quantum interference device magnetometers. Our group develops miniature alkali atom-based magnetic sensors using microfabrication technology. Our sensors do not require cryogenic cooling, and can be positioned very close to the sample, making these sensors an attractive option for development in the medical community. We will present our latest chip-scale optically-pumped gradiometer developed for array applications to image magnetic fields from the brain noninvasively. These developments should lead to improved spatial resolution, and potentially sensitive measurements in unshielded environments.
Coherent population trapping (CPT) resonances usually exhibit contrasts below 10% when interrogated
with frequency modulated lasers. We discuss a relatively simple way to increase the resonance contrast to
nearly 100% generating an additional light field through a nonlinear four-wave mixing interaction in the
atomic vapor.1 A similar method can also be used to create a beat signal at the CPT resonance frequency
that can injection-lock a low-power microwave oscillator at 3.4 GHz directly to the atomic resonance.2 This
could lead to chip-scale atomic clocks (CSACs) with improved performance. Furthermore, we introduce a
miniature microfabricated saturated absorption spectrometer3 that produces a signal for locking a laser
frequency to optical transitions in alkali atoms. The Rb absorption spectra are comparable to signals
obtained with standard table-top setups, although the rubidium vapor cell has an interior volume of only 1
mm3 and the volume of the entire spectrometer is around 0.1 cm3.
Svenja Knappe, Peter Schwindt, Vladislav Gerginov, Vishal Shah, Alan Brannon, Brad Lindseth, Li-Anne Liew, Hugh Robinson, John Moreland, Zoya Popovic, Leo Hollberg, John Kitching
We provide an overview of our research on chip-scale atomic devices. By miniaturizing optical setups based on precision spectroscopy, we have developed small atomic sensors and atomic references such as atomic clocks, atomic magnetometers, and optical wavelength references. We have integrated microfabricated alkali vapor cells with small low-power lasers, micro-optics, and low-power microwave oscillators. As a result, we anticipate that atomic stability can be achieved with small size, low cost, battery-operated devices. Advances in fabrication methods and performance are presented.
We present preliminary results showing that some noise sources in vapor cell atomic clocks based on coherent
population trapping (CPT) can be suppressed with differential detection. The scheme we propose differs from more
conventional differential detection in that both optical fields pass through the alkali vapor cell but have different
polarizations, one circular and one linear. Because CPT resonances are only excited by the circularly polarized beam, the
linearly polarized beam can be used to reduce several important sources of noise. With this technique, we demonstrate
reduction of the short-term frequency instability of a CPT atomic frequency reference by a factor of about 1.5.
We discuss the long-term stability of the NIST chip-scale atomic clock (CSAC) physics packages. We
identify the major factors that currently limit the frequency stability of our CSAC packages after 100 s. The
requirements for the stability of the vapor cell and laser temperature, local magnetic field, and local
oscillator output power are evaluated. Due to the small size of CSAC physics package assemblies, advances
MEMS packaging techniques for vacuum sealing and thermal isolation can be used to achieve the
temperature stability goals. We discuss various ideas on how to aid temperature control solutions over wide
variations in ambient temperature by implementing atom-based stabilization schemes. Control of
environment-related frequency instabilities will be critical for successful insertion of CSACs into portable
instruments in the areas of navigation and communication.
We have experimentally investigated the potential of narrow coherent population trapping (CPT) resonances for precision applications like magnetometry or atomic frequency standards, using the D lines in thermal Cs or Rb vapor. The magnetometer operates by monitoring the position of the Zeeman-shifted outermost resonance component. The central Zeeman component is well suited for frequency standard applications because its position is shifted by magnetic fields only in second order. We derive the required pair of laser fields form a special diode laser by direct modulation of the injection current. Magneto metric sensitivity down to a picotesla and 10-12 relative instability for a finger-sized clock have been achieved in this way. Use of a magnetic gradiometer allows to cancel fluctuations of ambient magnetic fields to a large degree, making possible sensitive measurements even outside magnetically shielded rooms.
Radiation properties of an atom, moving in a magneto-optical trap are subject to change along its trajectory, due to variations of intensity and polarization. The emphasis of this report is to explicit simple models for the influence of diffusive atomic motion on its radiative properties. Various extensions for improved models are discussed.
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