OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Quantum Science OBJECTIVE: Demonstrate atomic magnetometry in air using radar Resonantly Enhanced Multiphoton Ionization (REMPI) readout of 129Xe quantum states. DESCRIPTION: Remote magnetometry has recently been demonstrated at 100+ kilometers using sodium atoms in the mesosphere [Refs 1,2]. At this high altitude, the naturally occurring alkali atoms are at low pressure making the environment similar to atomic measurements inside of traditional vapor cells. As in conventional atomic magnetometers [Refs 3,4,5], first pumping with circularly polarized light depopulates all but one hyperfine sublevel of the ground state creating a net polarization. Next, the polarized spins precess at the Larmor frequency that is directly proportional to the strength of the magnetic field. The constant of proportionality is the gyromagnetic ratio, which is a known atomic physics constant for a specific atomic isotope. Finally, the atoms fluoresce producing a readout signal. This STTR topic seeks to demonstrate atomic magnetometry with atoms in the air at sea level. Historic work has identified 129Xenon as a naturally occurring species in air with a long nuclear spin lifetime that could be used for magnetometry [Ref 6]. Like other species in the air, the transition energy to the first excited state in xenon is a vacuum ultraviolet (VUV) transition [Refs 7,8,9] but commercially available ultrashort pulse lasers can generate intensities above 1 GW/cm^2 for efficient multi-photon excitation using visible blue light. While the ~100 picosecond time between collisions in air is too short for fluorescence and the xenon concentration is too low for stimulated emission, recent work has used ultrashort pulse lasers to demonstrate Resonance Enhanced Multi-Photon Ionization (REMPI) in air. Thomson scattering of radar off the resulting low-density plasma (i.e., radar REMPI) could remotely readout the ionized electrons for a standoff magnetometry signal [Refs 10,11]. In this method, the laser wavelength provides the selectivity, and the radar intensity determines the sensitivity. Initial proposals should present a specific plan for reading the spin of a hyperfine 129Xe ground state, including the planned energy levels that will be utilized for three-photon radar REMPI. PHASE I: Demonstrate radar REMPI detection of 129Xe using three-photon excitation in a vapor cell, ideally down to a pressure of 1e11 atoms/cc or show how this sensitivity could be achieved. Assess the time for spin to transfer from an excited electron to the nuclear spin during REMPI readout. Present a plan to demonstrate an all-optical standoff magnetometry measurement in a vapor cell including optical polarization and radar REMPI readout of the 129Xe. PHASE II: Demonstrate remote magnetometry using radar REMPI readout with 129Xe in a vapor cell [Ref 12]. Publish a journal article ideally demonstrating the measurement in air (with the naturally occurring 129Xe concentration at 23 ppb) or clearly present how a measurement in air could be accomplished. PHASE III DUAL USE APPLICATIONS: Explore methods to improve the measurement sensitivity, study schemes to increase the pumping efficiency and polarization with pressure broadened lines at atmospheric pressure, theoretically understand the spin transfer time relative to quenching from collisions, and advance methods to decrease the laser energy requirements. Commercial applications include trace gas detection for air quality monitoring, combustion characterization, and magnetic mapping for geophysical prospecting. REFERENCES: Kane et al. Laser Remote Magnetometry Using Mesospheric Sodium. JGR Space Physics 123, 8, 2018. Bustos et al. Remote sensing of geomagnetic fields and atomic collisions in the mesosphere, Nature Communications 9, 3981, 2018. Sheng et al. Subfemtotesla Scalar Atomic Magnetometry Using Multipass Cells., PRL 110, 160802, 2013. Patton, B. et al. A remotely interrogated all-optical 87Rb magnetometer. APL 101, 083502. 2012. Degenkolb, Skyler M. Optical Magnetometry Using Multiphoton Transitions. University of Michigan Dissertation 2016. https://deepblue.lib.umich.edu/handle/2027.42/135807 Happer, William. Laser Remote Sensing of Magnetic Fields in the Atmosphere by Two-Photon Optical Pumping of Xe129. (1978), https://physics.princeton.edu/atomic/happer/Publications.html Saloman, E.B. Energy Levels and Observed Spectral Lines of Xenon, Xe I through Xe LIV. Journal of Physical and Chemical Reference Data 33, 765. 2004. D'Amico, G. et al. Isotope-shift and hyperfine-constant measurements of near-infrared xenon transitions in glow discharges and on a metastable Xe(3P2) beam. PRA 60, 6, 1999. G. Grynberg, G. Three-photon absorption: selection rules and line intensities. Journal de Physique, 40 (10), 1979, pp. 965-968. Zhang, Zhili et al. Coherent microwave scattering from resonance enhanced multi-photon ionization (radar REMPI): a review. Plasma Sources Sci. Technol. 30, 103001, 2021. Galea, Christopher A. Coherent Microwave Scattering from Laser-Generated Plasma in External Magnetic Field and Weakly Ionized Plasma Environments. Dissertation Princeton University, 2021. Breeze, Stephen et al. Coatings for optical pumping cells and short-term storage of hyperpolarized xenon., JAP 87, 8013, 2000. KEYWORDS: Quantum; atomic; magnetometry; radar; multi-photon; ultrashort pulse laser; trace gas detection; plasma; ionization; remote sensing
