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Broadband Self-calibrated Rydberg-based RF Electric Field and Power Sensor


There is a critical need for capabilities that will enable the DoD to have self-calibrated electric field and power sensors in the RF, microwave, and millimeter-wavelength regimes. This topic seeks the demonstration of a portable broadband (1 GHz – 1 THz) electric field, power sensor, or key components towards a device. The sensor should be capable of operating in greater than 1 kV/m electric fields as to be usable for high-energy DoD applications. The electric field and power measurements must be SI traceable to remove the need for the recalibration process. Furthermore, the electric field-sensing device should be capable of sub-wavelength imaging of RF electric fields with spatial resolutions exceeding 10 μm. Many DoD and commercial applications critically rely on using calibrated electric field and power sensors in the RF, microwave, and millimeter-wavelength regimes. Currently no self-calibrated sensor exists in the 100 GHz – 1 THz frequency band. Typical detectors in the sub-THz frequency range are antennas which inherently perturb the field they are trying to sense, resulting in greater than 5% measurement errors. Antennas have the further limitation that they are narrow-band detectors. A SI-traceable sensor in the 1 GHz – 1 THz range would remove the need for costly recalibration of older devices and would replace many narrow-band antennas with a single low-SWaP device in a handheld package. Quantum sensors based upon Rydberg atoms offer the potential of traceable calibration, high sensitivity, wide spectral coverage, and high power capability. In addition to DoD applications, a Rydberg field and power sensor would have numerous commercial applications: circuit design [1, 2], biological sensing [3], aeronautics applications [4], and mobile communication [5]. This technology would not only verify circuit design but inform it by employing sub-wavelength RF field imaging of the complicated electronic fields from various dense circuits and metamaterials [1, 2]. Current technology employing electromagnetically induced transparency (EIT) in Rydberg atoms in an atomic vapor cell is a promising route but requires further development in order to achieve DoD functionality. These devices function by converting an electric field amplitude into a measurable frequency splitting [6] that is SI-traceable [7]. The electric field magnitude E is given by |E|=ℏΔf/P, where ℏ is Planck’s constant divided by 2π, Δf is the measured frequency splitting, and P is the transition dipole moment. Current work has demonstrated sensitivities of 3 μV/sqrt(Hz) measuring electric fields as low as 7.3 μV/cm [8] and up to 40 V/m [9] in a 1-130 GHz frequency range. These results are the first calibrated field measurements in the 100 GHz – 1 THz frequency band to date. Employing this technique to image RF electric fields resulted in sub-100 μm spatial resolutions [1] for electric fields with frequencies up to 104 GHz [2, 10]. The fabrication of micrometer-sized vapor cells is one of the more challenging technological developments necessary for these sensors. The size of these vapor cells must be reduced to at least one quarter of the length of the minimum wavelength of interest in order to prevent variations in the measured RF fields produced by standing waves. These cells must be all dielectric, made of quartz or Pyrex for example, and must be filled with alkali atoms such as Rb and Cs or a mixture of atomic species. The fabrication of micrometer-sized vapor cells suffers from atomic adsorption to the cell walls. These vapor cells must employ a mitigation technique for the reduced vapor pressure such as novel coatings or materials, bonded infrared absorption glass to the outside of the cell for IR heating or optical coupling mirrors bonded to the cell to form optical resonators for enhanced atom-light interaction. Such vapor cell production would not only benefit electric field sensing but atomic vapor-based magnetometry. Atomic vapor magnetometry currently provides the most sensitive magnetic field measurements [11] but it does not have high spatial resolution because it is limited to integration over the vapor cell length. Commercially available micrometer-sized atomic vapor cells would allow for the extension of atom-based magnetometry into a different spatial resolution regime [12, 13]. PHASE I: Demonstrate the operation of key components towards the electric field or power sensor in a laboratory setting such as: broadband measurements (100-250 GHz), electric field sensitivities better than 100 μV/cm, circuitry imaging with better than 50 μm spatial resolution, or fabrication of an alkali vapor cell with sub-mm length scales, and the development of a technique to mitigate reduced vapor pressures. Phase I deliverables include a final report that documents the results of each demonstration and design concepts to extend the measurement space to 1 GHz - 1 THz, improve the spatial resolution, and detail an experimental method to use the device in a high electric field environment (greater than 1 kV/m). PHASE II: Construct and demonstrate a breadboard system with a path towards a portable device. If the performer is developing components, fabricate the miniaturized alkali vapor cell to less than a 100 μm length. Phase II deliverables: 1) a demonstration in a simulated or relevant environment achieving broadband measurement (1 GHz – 1 THz), detection of less than 1 μV/cm electric fields, and sub-wavelength imaging with better than 10 μm spatial resolution. 2) a final report that documents the results of the demonstration and specifications of the fabricated alkali vapor cell 3) Completed designs for a portable prototype. This phase is expected to reach TRL 5. PHASE III: If successful this technology could transition to multiple DoD offices and could eventually replace current 1 GHz – 1 THz based electric field and power sensors, removing the need for recalibration against standards. This device could also be commercially viable to examine densely packed microwave circuit designs imaging the electric fields with sub-100 μm resolution to strongly inform and guide circuit design. Development of the micrometer-sized alkali-based vapor cells would be commercially usable for atomic vapor-based magnetometry opening new realms of spatial resolution for the highest magnetic field sensitive magnetometers. Such vapor cells could also have potential use in the timing community.
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