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IEEE Transactions on Antennas and Propagation

Enhancing Sensitivity of an Atomic Microwave Receiver via a Fabry-Perot Cavity

Bo Wu, Dunwei Liao, Di Sang, Yi Liu, Yunqi Fu

Published Oct 21, 2024• 34 citations
Publisher

Abstract

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}}}$@8.57 GHz that is the most advanced sensitivity known based on$\mathbf {^{133}\textrm {Cs}}$resonant transition$\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.

DOI
10.1109/tap.2024.3480459

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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 of5nv/cm/Hz \mathbf {5 \mathbf {nv/cm/\sqrt {Hz}}}5nv/cm/Hz​@8.57 GHz that is the most advanced sensitivity known based on133Cs\mathbf {^{133}\textrm {Cs}}133Csresonant transition∣44D5/2⟩→∣45P3/2⟩\mathbf {|44D_{5/2}\rangle \rightarrow |45P_{3/2}\rangle }∣44D5/2​⟩→∣45P3/2​⟩. 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.
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.
High-sensitivity measurements of the microwave electric field are important in applications of communication and metrology. The sensitivity of traditional Rydberg superheterodyne receivers in free space is effectively determined by the signal-to-noise ratio (SNR), which is often considered equivalent to sensitivity in practical sensing applications. In this work, we demonstrate a cavity-enhanced receiver, where an optical cavity significantly amplifies the interaction between the probe light and cesium atoms, which substantially improves the signal-to-noise ratio via enhancing the expansion coefficient κ . Here, κ is the edge slope of the single peak obtained by fitting the double-peak EIT-AT spectrum, characterizing the response of the probe light to the frequency detuning of the coupling laser. The sensitivity is thus boosted by a factor of approximately 19 dB. This study highlights the pivotal role of optical cavities in advancing Rydberg-based detection systems, offering a promising approach for high-sensitivity microwave electric field measurements.
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, which exhibits a strong response to weak electric fields, is regarded as a promising atomic receiver to surpass the sensitivity of conventional receivers. However, its sensitivity is strongly limited by the noise coming from both classical and quantum levels, and how to enhance it significantly remains challenging. In theory, we aim to optimize the laser array, highlighting the importance of coherence among microwave signals and the uniformity of laser beam power within the array, to enhance the signal-to-noise ratio (SNR) and elevate sensitivity to a new standard. To enhance practical application mobility and integration, we propose two optimization schemes: a six-port fiber-coupled vapor cell probe array and cascaded diffraction gratings. We maintain steady MW signal coherence in the laser array by using identical vapor cells and positioning the probe lasers on the incident wave’s equal phase plane. Our findings reveal that the wavefront phase difference for the six-port fiber-coupled vapor cell array is 10°, while the cascaded diffraction gratings show a 16° difference, with laser intensity uniformity reaching 90.4%. In our experiment, a 4.6-dB SNR enhancement was achieved by utilizing the cascaded diffraction gratings to generate2×22 \times 22×2probe laser arrays compared to the performance of a laser beam, which can be enhanced further just by adding more laser beams. In particular, a2×22 \times 22×2laser array experimental verification yielded19 nv/cm/Hz19~\text {nv/cm}/\sqrt {\textrm {Hz}}19 nv/cm/Hz​@8.57 GHz. More importantly, the intermediate signals of the laser array maintain coherence. Therefore, SNR improvement is allowed even in detecting weak target signals amid strong noise and clutter. The results could offer an avenue for the design and optimization of ultrahigh-sensitivity Rydberg atomic receivers and promote applications in cosmology, meteorology, communication, and MW quantum technology.
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.
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.