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arXiv:2502.11549 · Review of Scientific Instruments

A radio frequency emitter design for the low-frequency regime in atomic experiments

Yicheng Wei, Zhongshu Hu, Yajing Guo, Zhentian Qian, Shengjie Jin, Xuzong Chen, Xiong-Jun Liu

Published Sep 1, 2025• 1 citation
arXivHTMLPDF

Abstract

Radio frequency (RF) control is a key technique in cold atom experiments. We present a compact and efficient RF circuit based on a capacitive transformer network, where a low-frequency coil operating up to 30 MHz serves as both an intrinsic inductor and a power-sharing element. The design enables high current delivery and flexible impedance matching across a wide frequency range. We integrate both broadband and narrowband RF networks into a unified configuration that overcomes the geometric constraints imposed by the metallic chamber. In evaporative cooling, the broadband network allows a reduction in the applied RF input power from 14.7 dBW to -3.5 dBW, owing to its non-zero coil current even at ultra-low frequencies. This feature enables the Bose-Fermi mixture to be cooled below 10 μK. In a Landau-Zener protocol, the coil driven by the narrowband network transfers 80% of rubidium atoms from |F = 2, mF = 2⟩ to |2, -2⟩ in 1 millisecond, achieving a Rabi frequency of ∼9 kHz at an input power of 0.1 dBW.

DOI
10.1063/5.0265742

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Radio frequency (RF) control is a key technique in cold atom experiments. We present a compact and efficient RF circuit based on a capacitive transformer network, where a low-frequency coil operating up to 30 MHz serves as both an intrinsic inductor and a power-sharing element. The design enables high current delivery and flexible impedance matching across a wide frequency range. We integrate both broadband and narrowband RF networks into a unified configuration that overcomes the geometric constraints imposed by the metallic chamber. In evaporative cooling, the broadband network allows a reduction in the applied RF input power from 14.7 dBW to -3.5 dBW, owing to its non-zero coil current even at ultra-low frequencies. This feature enables the Bose-Fermi mixture to be cooled below 10 μK. In a Landau-Zener protocol, the coil driven by the narrowband network transfers 80% of rubidium atoms from |F = 2, mF = 2⟩ to |2, -2⟩ in 1 millisecond, achieving a Rabi frequency of ∼9 kHz at an input power of 0.1 dBW.
We design and implement a low-impedance, high-current radio-frequency (RF) circuit, enabling fast coherent coupling between magnetic levels in cold alkali atomic samples. It is based on a compact, shape-optimized coil that maximizes the RF field coupling with the atomic magnetic dipole, and on coaxial transmission-line transformers that step up the field-generating current flowing in the coil by a factor ∼4 to about 7.5 A for 100 W of RF driving. This allows us to obtain a RF coupling field of about 0.035G/W at the atomic sample location. The system is robust and versatile, as it generates a large RF field without compromising the available optical access, and its central resonant frequency can be adjusted in situ. Our approach provides a cost-effective, reliable solution, featuring a negligible level of interference with surrounding electronic equipment thanks to its symmetric layout. We test the circuit performance using a maximum RF power of 80 W at a frequency around 82 MHz, which corresponds to a measured Rabi frequency ΩR/2π ≃ 18.5 kHz, that is, a π-pulse duration of about 27 μs, between two of the lowest states of 6Li at an offset magnetic field of 770 G. Our solution can be readily adapted to other atomic species and vacuum chamber designs, in view of an increasing modularity of cold atom experiments.
Cold atom experiments utilize magnetic fields, commonly generated by coils, as an essential tool to control and manipulate atomic samples. In these experiments, it is often necessary to rapidly switch the magnetic field between two values. However, typical power supplies have a limited switching time for the current flowing through the coil. We present a control scheme implemented as an electronic circuit that overcomes this limitation. A faster control is achieved by momentarily applying an on-demand high voltage when the control signal variation surpasses the limits that the conventional power supply can follow, allowing a faster current flow into the magnetic coil. In our specific application, a coil with an inductance of 491 μH and a resistance of 0.26 Ω, corresponding to a time constant of ∼1.9 ms, is driven so that its current faithfully follows the control signals from -1 to 1 A. It is capable of completing a full-scale transition in just ∼31 μs, with an effective bandwidth of 15.2 kHz. This corresponds to an improvement by a factor of more than 20 in both bandwidth and switching speed over a conventional power supply. By appropriately selecting the components of the circuit, both the bandwidth and switching time can be tuned to match specific needs within a wide range of inductive and power requirements.
A radio-frequency (RF) source designed for cold atom experiments is presented. The source uses AD9858, a direct digital synthesizer, to generate the sine wave directly, up to 400 MHz, with sub-Hz resolution. An amplitude control circuit consisting of wideband variable gain amplifier and high speed digital to analog converter is integrated into the source, capable of 70 dB off isolation and 4 ns on-off keying. A field programmable gate array is used to implement a versatile frequency and amplitude co-sweep logic. Owing to modular design, the RF sources have been used on many cold atom experiments to generate various complicated RF sequences, enriching the operation schemes of cold atoms, which cannot be done by standard RF source instruments.
We construct a compact experimental apparatus for producing high-repetition-rate ultra cold 87Rb atom based on an external atom chip. Initially, we produce an atomic flux from a 2D+ MOT (magneto-optical trap) in a nearly isolated 2D+ MOT chamber and improve the flux by adjusting the cooling beam detuning of the 2D+ MOT. The flux is trapped in the 3D MOT for 1.2 s and is further cooled by performing compressing the MOT for 20 ms and using polarization gradient cooling for 6.2 ms. After optical pumping into the 5S F = 2 (m F = 2) ground state, we transport atoms toward the chip surface by performing external Z coil trapping. At 1 mm below the chip surface, the external Z coil trap is completely switched to an atom chip trap by using a Z wire and a dimple wire on the chip with external bias fields. The transport and the switching take totally 520 ms. The number of atoms trapped on the atom chip is measured to be about 2 × 107. After the atoms has been compressed for 100 ms to increase trap frequencies, we perform RF evaporative cooling in the RF frequency range from 32 MHz to 6.82 MHz for 2.4 seconds. The final number of atoms in the atomic cloud after RF cooling is 1.2 × 105 atoms, and we are able to observe some Bose-Einstein condensation. The repetition rate for producing the condensation is 0.234 Hz. In this paper, we describe our experimental apparatus and processes.
The present Diploma thesis was motivated by the need for a versatile radio frequency source to be used in experiments with cold atoms. The working principles and the design of said source are presented including its characterization and an example of its evaluation in an experiment. The desired experimental uses impose a set of strict requirements on the design such as highly stable output phase and frequency as well as reproducible and precisely controllable profiles of phase, frequency and output power. The device developed in this thesis features output frequencies from 5MHz up to 500MHz, phasecontinuous operation, fast linear frequency sweeps with very small frequency jumps, phase synchronized output of two frequencies and arbitrary sweep profiles with update rates of up to 8 kHz. All parameter and profile controls can be are accessed and adjusted using a graphical user interface that is integrated into the experiment control framework. Validating the features experimentally, RubidiumBose-Einstein condensates were transported over distances of a few centimeters in a moving interference pattern of two counterpropagating laser beams. The frequency difference of the laser beams was controlled by the device, permitting precise and reproducible positioning of the atoms. Further possible applications include generation of radio frequency sweeps to drive Landau-Zener transitions or generation of Ramsey type pulses. The required precise control of timing and phase of multiple pulses is achievable with the present design approach.
We report the experimental achievement of 87Rb Bose-Einstein condensation in a magnetic trap with microwave and radio frequency (RF) induced evaporation. Evaporative cooling is realized by using 6.8 GHz microwave radiation driving the 87Rb atoms to transit from the ground-state hyperfine state |F = 2,mF =2> to |F = 1,mF = 1>. Compared with RF-induced evaporation, 87Rb atoms are hardly to achieve pure condensate by microwave evaporation cooling due to the effect of atoms in the |F = 1,mF = 1> state being pumped back into the trapping |F = 2,mF = 1> state.