Rydberg-Atom-Compatible Alkali-Metal Vapor Cells with Nontraditional Geometries

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Integrated Sensing and Cyber; Integrated Network Systems-of-Systems; Quantum Science OBJECTIVE: Develop rubidium and cesium vapor cells with a thin rectangular geometry that are compatible with the excitation of Rydberg atomic states. DESCRIPTION: Vapor cells containing dilute gaseous samples of alkali metals (particularly cesium and rubidium) are a critical component in many quantum technologies relevant to the DoD mission. In particular, quantum electric field sensors based on highly excited Rydberg atomic states in thermal vapors are an emerging platform for receiving radio-frequency communications, calibrating antennas, and imaging teraHertz-frequency (THz) electromagnetic sources. Cell geometries beyond traditional cubic and cylindrical designs will be advantageous for optimizing sensor performance and extending the technology's applications. Reliably obtaining vapor cells with nontraditional geometries that are also capable of supporting excitation to Rydberg states is an ongoing challenge for DoD researchers. Producing vapor cells with consistent alkali vapor pressures, low permeability, and appropriate optical coatings is already something of an art form. Highly reactive rubidium precludes using many materials for enclosures, coatings, and integrated structures. Moreover, alkali metal tends to deposit on inner surfaces, reducing transmission of probing laser beams and creating a Faraday cage that shields low-frequency electromagnetic fields. The ability to excite Rydberg levels is a further challenge due to the various optical wavelengths involved and their propensity to induce unwanted charging on cell surfaces, which perturbs the atoms. Even in cells that support stable ground-state populations, collisions of Rydberg atoms with background gasses, interactions with surface charges, and other chemical reactions can suppress excitations to Rydberg states. The goal of this SBIR is to foster reliable development and delivery of rubidium and cesium vapor cells with rectangular geometry that are compatible with the excitation of Rydberg atomic states. Accomplishing this will require investments in cell design, bonding and fabrication techniques, sample filling process development, and quality assurance testing to verify Rydberg state excitation capability. Many existing research efforts have focused on creating compact cells, however, maintaining the capability to excite atoms to Rydberg states introduces additional constraints in the fabrication process and materials. A thin rectangular form factor with thin (6 cm^3) must be transparent to electromagnetic waves with frequencies of 0.1 3.0 terahertz (THz). The cells will be tested by the government to determine adequate Rydberg sensor performance based on measurement of narrow unperturbed atomic resonances and transparency to agreed electromagnetic frequencies. Optional deliverable requirements include: Minimum alkali optical thickness or vapor pressure, wall material choice (e.g. borosilicate glass with sapphire coating), optical access, window transparency and optical quality. Vapor density control that is RF transparent, waveguide and/or electrode integration, surface charge mitigation, integrated micro-optics or fiber coupled cells, aging tests. Phase II Base amount must not exceed $1,000,000 for a 12-month period of performance and the Option amount must not exceed $700,000 for a 12-month period of performance. PHASE III DUAL USE APPLICATIONS: Creating a reliable process for filling alkali vapor cells that support Rydberg state excitation and have non-traditional geometries will have applications for quantum sensing in a variety of military and commercial spaces. Rydberg-atom electrometers are an emerging technology for creating communication receivers simultaneously operating at frequencies spanning many tens of GHz in a single device. This could have applications in the defense and commercial telecommunication industries, to include 5G technology. Rydberg-atom electrometers can also be used to calibrate antenna emissions in a way that does not perturb the radiation pattern and that is ambivalent to strong fields that could damage traditional technologies. REFERENCES: 1. Phys. Rev. Applied 13, 054034 (2020) - Vapor-Cell-Based Atomic Electrometry for Detection Frequencies below 1 kHz (aps.org) 2. Phys. Rev. X 10, 011027 (2020) - Full-Field Terahertz Imaging at Kilohertz Frame Rates Using Atomic Vapor (aps.org) KEYWORDS: Vapor; cell; rubidium; cesium; Rydberg; quantum; atomic; teraHertz