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arXiv:2306.14791 · Review of Scientific Instruments

A planar cloverleaf antenna for circularly polarized microwave fields in atomic and molecular physics experiments

Weijun Yuan, Siwei Zhang, Niccolò Bigagli, Claire Warner, Ian Stevenson, Sebastian Will

Published Dec 1, 2023• 8 citations
arXivHTMLPDF

Abstract

We report on the design and characterization of a compact microwave antenna for atomic and molecular physics experiments. The antenna is comprised of four loop antennas arranged in a cloverleaf shape, allowing for precise adjustment of polarization by tuning the relative phase of the loops. We optimize the antenna for left-circularly polarized microwaves at 3.5 GHz and characterize its near-field performance using ultracold NaCs molecules as a precise quantum sensor. Observing an unusually high Rabi frequency of 2π × 46.1(2) MHz, we extract an electric field amplitude of 33(2) V/cm at 22 mm distance from the antenna. The polarization ellipticity is 2.3(4)°, corresponding to a 24 dB suppression of right-circular polarization. The cloverleaf antenna is planar and provides large optical access, making it highly suitable for quantum control of atoms and molecules and potentially other quantum systems that operate in the microwave regime.

DOI
10.1063/5.0167572

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We report on the design and characterization of a compact microwave antenna for atomic and molecular physics experiments. The antenna is comprised of four loop antennas arranged in a cloverleaf shape, allowing for precise adjustment of polarization by tuning the relative phase of the loops. We optimize the antenna for left-circularly polarized microwaves at 3.5 GHz and characterize its near-field performance using ultracold NaCs molecules as a precise quantum sensor. Observing an unusually high Rabi frequency of 2π × 46.1(2) MHz, we extract an electric field amplitude of 33(2) V/cm at 22 mm distance from the antenna. The polarization ellipticity is 2.3(4)°, corresponding to a 24 dB suppression of right-circular polarization. The cloverleaf antenna is planar and provides large optical access, making it highly suitable for quantum control of atoms and molecules and potentially other quantum systems that operate in the microwave regime.
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Nitrogen-vacancy (NV) centers in diamond are promising quantum sensors, where microwave antennas play a crucial role in manipulating the spin states accurately. Conventional microwave antennas often struggle to balance radiation efficiency and bandwidth. To address this challenge, we design a planar double-turn spiral antenna (PDTSA), based on the ring microstrip antenna (RMA). PDTSA demonstrates an ∼4.5-fold increase in radiation efficiency compared to RMA. In addition, the PDTSA allows linear tunability of the resonance frequency up to 500 MHz by adjusting the spiral input length. This feature addresses the limitations of a narrow working frequency range, which are typically caused by the narrowband in high-radiation-efficiency antennas. The experimental results show that at an absolute input power of 1 W, the PDTSA increases the Rabi frequency from 1.72 to 8.06 MHz compared to the RMA. This enhancement accelerates quantum state manipulation and reduces phase accumulation errors. These characteristics make PDTSA suitable for applications in quantum sensing and precision measurements using NV centers.
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Control of the polarization of microwave fields is a key experimental capability for a number of atomic physics platforms. However, producing high-fidelity microwaves requires a well-controlled microwave environment, where reflections that distort the polarization must be avoided or well characterized, a constraint that often conflicts with other experimental design considerations. Here we demonstrate a microwave control system capable of producing high-fidelity microwave polarizations in a Rydberg-ensemble experiment. We use three in-vacuum DC electrodes, repurposed as microwave antennae, to produce imperfect and initially unknown polarizations. Each source is driven with independent phase and amplitude control to generate the desired microwave fields. We probe the fields produced at the position of the atoms using Rydberg-EIT spectroscopy of the microwave-induced avoided crossings. We produce σ−σ_-σ−​, πππ, and σ+σ_+σ+​ polarized microwaves with > 99 % fidelity and generate their combinations. We extend our purification techniques to frequencies away from Rydberg resonances by utilizing an auxiliary microwave field, generating two-photon microwave resonances. The techniques developed here will facilitate the engineering of dipolar interactions in atomic and molecular physics experiments.