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Applied Optics and Photonics China (AOPC)

Impact of beam diameter on sensitivity in Rydberg atomic microwave receiver

Fengting Zhao, Qiang An, Jinhao Zhang, Fu Yunqi

Published Oct 28, 2025• 0 citations
Publisher

Abstract

Quantum microwave measurement techniques based on Rydberg atoms exhibit exceptional potential in microwave communication, sub-wavelength imaging and quantum metrology, due to the inherent advantages of self-calibration, minimal interference with external electric fields, and a broadband range of measurable frequencies. However, the current measurement sensitivity is not yet comparable to that of classical receivers. One contributing factor is the low utilization of atoms within the vapor cell, which remains a significant challenge to enhance substantially. To address this problem, scholars have proposed a variety of optical domain methods to enhance sensitivity. For example, increasing vapor cell length, utilizing multi-photons excitation or repump lasers. Despite these advancements, the study of the influence of laser beam diameter on the sensitivity is insufficient, and the lack of a comprehensive experimental investigation is not conducive to the practical application and optimization of the receiver. In this study, we investigate the impact of varying probe laser diameters on receiver sensitivity using a beam expander to adjust the diameter from 600 μm to 3 mm. Modifying the beam diameter could alter the light-atom interaction area in the vapor cell, offering a feasible approach to adjust the utilization of the atoms. Our findings indicate that under the experimental conditions, the sensitivity could be enhanced by expanding the beam diameter, where the sensitivity is changed from 299 nV cm-1 Hz-1/2 to 127 nV cm-1 Hz-1/2. These results could provide valuable insights for designing and optimizing ultrahigh-sensitivity Rydberg atomic receivers and facilitating applications in meteorology, communication, and quantum precision measurement.

DOI
10.1117/12.3082281

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Quantum microwave measurement techniques based on Rydberg atoms exhibit exceptional potential in microwave communication, sub-wavelength imaging and quantum metrology, due to the inherent advantages of self-calibration, minimal interference with external electric fields, and a broadband range of measurable frequencies. However, the current measurement sensitivity is not yet comparable to that of classical receivers. One contributing factor is the low utilization of atoms within the vapor cell, which remains a significant challenge to enhance substantially. To address this problem, scholars have proposed a variety of optical domain methods to enhance sensitivity. For example, increasing vapor cell length, utilizing multi-photons excitation or repump lasers. Despite these advancements, the study of the influence of laser beam diameter on the sensitivity is insufficient, and the lack of a comprehensive experimental investigation is not conducive to the practical application and optimization of the receiver. In this study, we investigate the impact of varying probe laser diameters on receiver sensitivity using a beam expander to adjust the diameter from 600 μm to 3 mm. Modifying the beam diameter could alter the light-atom interaction area in the vapor cell, offering a feasible approach to adjust the utilization of the atoms. Our findings indicate that under the experimental conditions, the sensitivity could be enhanced by expanding the beam diameter, where the sensitivity is changed from 299 nV cm-1 Hz-1/2 to 127 nV cm-1 Hz-1/2. These results could provide valuable insights for designing and optimizing ultrahigh-sensitivity Rydberg atomic receivers and facilitating applications in meteorology, communication, and quantum precision measurement.
Rydberg atom,which exhibits a strong response to weak electric(E) fields,is regarded as a promising atomic receiver to surpass 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. Here we experimentally prove that the sensitivity of Rydberg atomic receiver can be increased to 23 nV/cm/Hz1/2 by combining laser arrays. Theoretically, we demonstrate that multiple beams illuminating on a PD perform better than multiple PDs for laser arrays.In our experiment,10 dB SNR enhancement is achieved by utilizing 2 * 2 probe beam arrays, compared to the performance of a laser beam,and it can be enhanced further just by adding a resonator. 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 microwave quantum technology.
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.
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.
Measurement sensitivity is crucial for the practical application of Rydberg atomic microwave superheterodyne receiver systems in communication, radar, and navigation. Based on laser-atom-microwave interaction mechanisms, the influence of laser powers and local-oscillator (LO) microwave field intensity on measurement sensitivity is systematically studied in this paper. Measurement sensitivity is analyzed through electromagnetically induced transparency (EIT) spectral characteristics, specifically EIT amplitude and full-width-half-maximum (FWHM),larger amplitudes and narrower FWHM indicate improved signal-to-noise ratios. For a 60D-state Rydberg atomic system with fixed probe and coupling laser beam waists (800 µm and 875 µm), optimal parameters are determined: probe laser power of 60 µW(Rabi frequency: 2π×7.02 MHz), coupling laser power of 40 mW (Rabi frequency: 2π×0.66 MHz), and LO microwave Rabi frequency of 2π×8.34 MHz. Experimental results demonstrate a 4.6 dB sensitivity enhancement, advancing from −137.5dBm/Hz to −142.1 dBm/Hz at 3.21 GHz microwave field. This improvement is attributed to precise alignment of laser-atom and microwave-atom interactions, bridging the gap toward quantum sensitivity limits.
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.
Since its theoretical sensitivity is limited by quantum noise, radio wave sensing based on Rydberg atoms has the potential to replace its traditional counterparts with higher sensitivity and has developed rapidly in recent years. However, as the most sensitive atomic radio wave sensor, the atomic superheterodyne receiver lacks a detailed noise analysis to pave its way to achieve theoretical sensitivity. In this work, we quantitatively study the noise power spectrum of the atomic receiver versus the number of atoms, where the number of atoms is precisely controlled by changing the diameters of flat-top excitation laser beams. The results show that under the experimental conditions that the diameters of excitation beams are less than or equal to 2 mm and the read-out frequency is larger than 70 kHz, the sensitivity of the atomic receiver is limited only by the quantum noise and, in the other conditions, limited by classical noise. However, the experimental quantum-projection-noise-limited sensitivity this atomic receiver reaches is far from the theoretical sensitivity. This is because all atoms involved in light-atom interaction will contribute to noise, but only a fraction of them participating in the radio wave transition can provide valuable signals. At the same time, the calculation of the theoretical sensitivity considers both the noise and signal are contributed by the same amount of atoms. This work is essential in making the sensitivity of the atomic receiver reach its ultimate limit and is significant in quantum precision measurement.