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IEEE Transactions on Microwave Theory and Techniques

Resonant Structures for Sensitivity Enhancement of Rydberg-Atom Microwave Receivers

Georgia Sandidge, Gabriel Santamaría-Botello, Eric Bottomley, Haoquan Fan, Zoya B. Popovic

Published Feb 6, 2024• 44 citations
Publisher

Abstract

Rydberg atoms at room temperature have been demonstrated as narrowband-sensitive receivers of incident microwave electric fields across MHz to THz frequencies. A comparison to conventional microwave receiver noise temperature suggests that some type of microwave field enhancement is required to make these atomic receivers competitive with or better than their conventional room-temperature electronic counterparts. This article presents a sensitivity analysis appropriate for the comparison of the two fundamentally different types of receivers, which relates the noise-equivalent temperature (NET), noise-equivalent field (NEF), and enhancement factor (F). Several methods for achieving enhancement are presented, and the design and performance of a resonant waveguide cavity method are presented in further detail. The experimental validation is performed at 10.22 GHz with a Rydberg-atom vapor cell enclosed in a$X$-band resonant cavity designed for field uniformity. The Rydberg-atom receiver with no enhancement has a sensitivity of$23~\mu \text{V}$/m$(\mathrm {Hz})^{1/2}$at room temperature. When the resonant cavity is introduced, the sensitivity improves to$1.58~\mu \text{V}$/m$(\mathrm {Hz})^{1/2}$, as predicted by the analysis. This corresponds to an LNA with a noise figure of 42.44 dB before enhancement and 19.23 dB after the enhancement is implemented.

DOI
10.1109/tmtt.2024.3355763

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Rydberg atoms at room temperature have been demonstrated as narrowband-sensitive receivers of incident microwave electric fields across MHz to THz frequencies. A comparison to conventional microwave receiver noise temperature suggests that some type of microwave field enhancement is required to make these atomic receivers competitive with or better than their conventional room-temperature electronic counterparts. This article presents a sensitivity analysis appropriate for the comparison of the two fundamentally different types of receivers, which relates the noise-equivalent temperature (NET), noise-equivalent field (NEF), and enhancement factor (F). Several methods for achieving enhancement are presented, and the design and performance of a resonant waveguide cavity method are presented in further detail. The experimental validation is performed at 10.22 GHz with a Rydberg-atom vapor cell enclosed in aXXX-band resonant cavity designed for field uniformity. The Rydberg-atom receiver with no enhancement has a sensitivity of23 μV23~\mu \text{V}23 μV/m(Hz)1/2(\mathrm {Hz})^{1/2}(Hz)1/2at room temperature. When the resonant cavity is introduced, the sensitivity improves to1.58 μV1.58~\mu \text{V}1.58 μV/m(Hz)1/2(\mathrm {Hz})^{1/2}(Hz)1/2, as predicted by the analysis. This corresponds to an LNA with a noise figure of 42.44 dB before enhancement and 19.23 dB after the enhancement is implemented.
Microwave receivers using electromagnetically-induced transparency (EIT) in Rydberg atoms have recently demonstrated improved sensitivities. It is not evident how their state-of-the-art electric field sensitivities compare to those achieved using standard electronic receivers consisting of low-noise amplifiers (LNAs) and mixers. In this paper, we show that conventional room-temperature electronic receivers greatly outperform the best demonstrated sensitivities of room-temperature Rydberg electrometers in standard free-space coupled configurations. However, Rydberg-atom receivers can surpass the sensitivity of conventional receivers if resonant or confining microwave structures are designed to enhance the electric fields sensed by the atoms. For a given microwave resonator, the external (coupling) quality factor must be carefully chosen to minimize their thermal and quantum noise contributions. Closed-form expressions for these optimal design points are found, and compared in terms of noise temperature with conventional LNAs reported in the literature from 600 MHz to 330 GHz.
This paper presents a microwave cavity for sensitivity enhancement in room-temperature Rydberg atom electric field sensors. It can be shown theoretically that without enhancement, such sensors currently do not compete with conventional low-noise amplifier based receivers. We designed a 15GHz resonator for a specific set of transitions in Rubidium atoms. Owing to its high Q and finesse, the resonator enhances the electric field magnitude sensed by the atoms by a factor F with respect to an external field coupled through an antenna. The resonator is designed to fit a 4mm×4mm×25mm fused silica vapor cell. The simulated and measured Q-factors are 1900 and 1100, respectively, which corresponds with field enhancement factors of 20 and 14, respectively. The resonator is mechanically tunable over 216MHz to ensure overlap with the atomic resonance.
Rydberg atom-based microwave electric field sensing has attracted growing interest owing to its inherent advantages, such as absolute calibration, wideband operability, and compatibility with room-temperature devices. A critical bottleneck that limits sensitivity is the inefficient coupling between the Rydberg atoms and the incident microwave field, particularly when detecting weak signals propagating in free space. Here we propose and experimentally validate a scheme that integrates a horn antenna with a resonant microwave cavity to significantly improve this coupling for free-space signal reception in the S-band. Using a two-photon excitation scheme in a cesium vapor cell, we systematically characterize the sensing performance under three configurations: a bare cell, direct cavity injection, and a cavity coupled to a horn antenna that captures free-space microwave signals over a 1 m distance. In the antenna-coupled cavity configuration, we achieve an optimal sensitivity of 2.33 nV/cm/Hz\sqrt{\text{Hz}}Hz​ at the receiving antenna, which corresponds to an enhancement of approximately 17.9 dB compared to the optimized bare vapor cell configuration. Our findings offer a practical and effective route to boost the sensitivity of Rydberg atomic sensors, facilitating their adoption in real-world microwave metrology and wireless communication applications where weak free-space electric fields must be reliably measured.
Rydberg atom-based microwave (MW) atomic receiver is regarded as a promising atomic receiver to surpass the sensitivity of conventional receivers. This article proposes a tunable Fabry-Perot (FP) cavity that locally enhances the MW electric (E) field to improve measurement sensitivities. This FP cavity includes a 0.9947 reflection coefficient periodic metal slit structure and a reflective plate. In our experiment, 21-dB signal-to-noise ratio (SNR) enhancement is achieved by utilizing the FP cavity. Ultimately, the atomic receiver loaded with a FP cavity provides a sensitivity of$ \mathbf {5 \mathbf {nv/cm/\sqrt {Hz}}}@8.57GHzthatisthemostadvancedsensitivityknownbasedon@8.57 GHz that is the most advanced sensitivity known based on@8.57GHzthatisthemostadvancedsensitivityknownbasedon\mathbf {^{133}\textrm {Cs}}resonanttransitionresonant transitionresonanttransition\mathbf {|44D_{5/2}\rangle \rightarrow |45P_{3/2}\rangle }$. Since the FP cavity is different from the traditional closed cavity and split-ring resonance mechanism, a large enhancement tunable frequency bandwidth of 4.8–12.2 GHz can be achieved by adjusting the plate spacing. In addition, the FP cavity solves the direct coupling between the electromagnetic wave and the resonator in the X-band, which provides mobility and flexibility. We take all the vapor cell wall’s refractive index and metal reflections into account as the resonance-based system is more sensitive to errors.
Abstract Developing microwave electric field sensing based on Rydberg atoms has received significant attention due to its unique advantages. However, achieving effective coupling between Rydberg atoms and the microwave electric field in the sensing process is a challenging problem that greatly impacts the sensitivity. To address this, we propose using a microwave resonant cavity to enhance the effective coupling between the Rydberg atoms and the microwave electric field. In our experiment, Rydberg atoms are prepared via a three-photon excitation scheme, and the electric fields are measured without and with a microwave cavity in which the vapor cell is placed inside, respectively. As a result, we achieved an 18 dB enhancement of power sensitivity by adding the cavity, which is an effective enhancement in electric field pulse signal detection. This experimental testing provides a promising direction for enhancing the sensitivity of Rydberg atomic electric field sensors and paves the way for their application in precision electric field measurements.
High-sensitivity amplitude and frequency detection of microwave electric fields is of significant importance in various fields, including communications, microwave quantum technology, aerospace, medicine, and meteorology. Rydberg atom-based sensors promise traceable standards and high sensitivity for detecting weak electric fields and enhancing sensing sensitivity, mainly by using superheterodyne technology and resonant structures. Detecting high-frequency (HF) band fields with high sensitivity using Rydberg atom sensors is a significant challenge due to the requirement of exciting atoms to extremely high energy states for resonant detection. We demonstrate the off-resonant Rydberg atomic superheterodyne architecture with additional resonators for greatly enhancing the sensing sensitivity to HF band fields. In this approach, we propose a miniaturized square spiral resonator with an electrical size of$0.011~\lambda $at 15.54 MHz. To reduce the electric-field-screening effect of the adsorption of alkali metal atoms, we use a monocrystalline sapphire cesium (Cs) vapor cell. By utilizing the off-resonant alternating current (ac) Stark effect, the initial sensitivity of the Rydberg atom sensor without the resonator is measured to be10.39 μV/cmHz10.39~\mu \text {V/cm}\sqrt {\text {Hz}}10.39 μV/cmHz​at room temperature. With the addition of the miniaturized square spiral resonator, the sensitivity significantly improved to2.60 nV/cmHz2.60~\text {nV/cm}\sqrt {\text {Hz}}2.60 nV/cmHz​, which has been inaccessible so far with atomic sensors. Here, we provide an enhancement of 72 dB, which is three orders of magnitude improvement in sensitivity than when the resonator is not used. This enhancement resonator extends Rydberg atomic high-sensitivity sensing to the HF band and shows great potential, especially in shortwave radio reception.