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Review of Scientific Instruments

A compact gain-enhanced microwave helical antenna for 87Rb atomic experiments

Yong-Guang Zheng, Lei Jiang, Zi-Hang Zhu, Wei-Yong Zhang, Zhaoyu Zhou, Bo Xiao, Zhen-Sheng Yuan

Published Jun 1, 2022• 3 citations
Publisher

Abstract

We present a compact and gain-enhanced microwave helical antenna for manipulating ultracold 87Rb atoms coherently. By replacing the reflecting plate with an enhancing cup, the voltage standing wave ratio is reduced by 0.5 in the frequency range of 6.73–6.93 GHz, which covers the resonant frequency between the ground-state hyperfine levels of the 87Rb atom. The gain of the helical antenna is increased by 1.25–1.63 dBi, whose length is 89 mm. Applying the antenna to ultracold 87Rb atomic experiments, we achieve a Rabi frequency of 60(1) ×2π kHz of the oscillation between the hyperfine levels.

DOI
10.1063/5.0088161

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We present a compact and gain-enhanced microwave helical antenna for manipulating ultracold 87Rb atoms coherently. By replacing the reflecting plate with an enhancing cup, the voltage standing wave ratio is reduced by 0.5 in the frequency range of 6.73–6.93 GHz, which covers the resonant frequency between the ground-state hyperfine levels of the 87Rb atom. The gain of the helical antenna is increased by 1.25–1.63 dBi, whose length is 89 mm. Applying the antenna to ultracold 87Rb atomic experiments, we achieve a Rabi frequency of 60(1) ×2π kHz of the oscillation between the hyperfine levels.
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.
Antenna gain reflects the ability to concentrate input power and radiate electromagnetic waves in a specific direction, and is a key indicator for evaluating antenna performance. In this paper, a high-accuracy Rydberg-atom-assisted gain-transfer measurement method is proposed for Ku -band microwave antenna gain calibration. The microwave electric field (E-field) intensity is precisely measured via electromagnetically induced transparency (EIT) and Autler-Townes (AT) splitting effects. Under the same experimental conditions, the microwave intensity distributions of a standard gain antenna (SGA) and the antenna under test (AUT) are systematically compared, enabling gain calibration of the AUT at 12.007 GHz through the proposed method. Furthermore, a Rydberg atom microwave sensor with multi-level transitions ( nD5/2nD_{5/2}nD5/2​ - (n+1)P3/2(n+1)P_{3/2}(n+1)P3/2​ , n =50 ∼\sim∼ 55), is employed to extend the calibration over a wide frequency range within 12.694 ∼\sim∼ 17.041 GHz, achieving a measurement uncertainty below 0.38 dB. This work establishes a systematic framework for microwave device calibration, providing both theoretical support and experimental validation for precision metrology of high-performance microwave components.
Abstract We have observed the Rabi oscillations of the population of 87 Rb atoms in the hyperfine ground state, when a cold atom cloud prepared in the hyperfine ground state (F = 1) was exposed to a microwave (MW) radiation field resonant to the |F = 1, m F = 0 > → |F = 2, m F = 0 > transition. After exposure to MW field, the atom cloud was imaged using the absorption probe method, and the cloud images have revealed that the population excited to the hyperfine state (F = 2) varied with the position in the atom cloud. At a given position in the atom cloud, the excited population shows Rabi oscillations between two hyperfine states with an increase in the MW field exposure time. Using these Rabi oscillations, the amplitude of the microwave magnetic field was estimated, which showed good agreement with the calculated field values. PACS numbers: 32.80.Pj, 33.55.Be, 67.85.-d.
Recently, the thulium atom was cooled down to the Bose-Einstein condensation temperature, thus opening a pathway to quantum simulation with this atom. However, successful simulations require instruments to control and read out states of the atom as well as the ability to control the interaction between either different species or different states of the same type of species. In this paper, we provide an experimental demonstration of high-fidelity (over 93%) manipulation of the ground-state magnetic levels of thulium, which utilizes a simple and efficient design of a microwave (MW) antenna. The coherence time and dephasing rate of the energetically highest hyperfine level of the ground state were also examined.
This thesis describes the use of microwave frequency sweeps across the resonance of a magnetic dipole transition to induce adiabatic rapid passage of an ultracold (<1 μK) atomic ensemble between Zeeman sub-levels of the 87Rb ground state. Atoms begin in the |F = 2, mF = 2> quantum state, and efficient preparation of the sample in the |F = 1, mF = 1> state is demonstrated.\nA dispersive probing system is developed using the technique of frequency modulation spectroscopy, which involves a heterodyne detection scheme. Dispersive probing is implemented in parallel to the state preparation, such that the real-time evolution of coherent transfer processes can be monitored.\nThe dispersive probe is used as a tool to quickly and accurately determine the microwave resonance frequency to within a 30 kHz range. Resonant coherent manipulations (Rabi oscillations) between the |F = 1, mF = 1> and |F = 2, mF = 2> states are then demonstrated and probed dispersively.\nA proof of concept application of the dispersive probing system is also presented. It is used to monitor an RF evaporative cooling sequence, providing a precursor measurement on a trapped atomic cloud. This leads to the ability to terminate the experimental sequence in the case of an unfavourable starting point, and thus a reduction of classical number fluctuations in the final atom number.
The generation and manipulation of ultracold atomic ensembles in the quantum regime require the application of dynamically controllable microwave fields with ultra-low noise performance. Here, we present a low-phase-noise microwave source with two independently controllable output paths. Both paths generate frequencies in the range of 6.835 GHz ± 25 MHz for hyperfine transitions in 87Rb. The presented microwave source combines two commercially available frequency synthesizers: an ultra-low-noise oscillator at 7 GHz and a direct digital synthesizer for radio frequencies. We demonstrate a low integrated phase noise of 480 µrad in the range of 10 Hz to 100 kHz and fast updates of frequency, amplitude, and phase in sub-µs time scales. The highly dynamic control enables the generation of shaped pulse forms and the deployment of composite pulses to suppress the influence of various noise sources.