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arXiv:2104.12730 · Review of Scientific Instruments

A low-impedance radio-frequency circuit for fast spin manipulations in cold alkali atoms

Francesco Scazza, Giulia Del Pace, L. Pieri, Roberto Concas, Woo Jin Kwon, G. Roati

Published Oct 1, 2025• 2 citations

Abstract

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.

DOI: 10.1063/5.0276530

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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.

Cold atom experiments commonly use broad magnetic Feshbach resonances to manipulate the interaction between atoms. In order to induce quantum dynamics by a change in the interaction strength, rapid (∼μs) magnetic field changes over several tens of Gauss are required. Here, we present a compact design of a coil and its control circuit for a change in the magnetic field up to 36 G in 3 µs. The setup comprises two concentric solenoids with minimal space requirements, which can be readily added to existing apparatuses. This design makes the observation of non-equilibrium physics with broad Feshbach resonances accessible.

Abstract Well controlled and highly stable magnetic fields are desired for a wide range of applications in physical research, including quantum metrology, sensing, information processing, and simulation. Here we introduce a low-cost hybrid assembly of rare-earth magnets and magnetic field coils to generate a field strength of

\simeq

≃ 10.9 mT with a calculated spatial variation of less than 10−6 within a diameter of spherical volume of 150 μm. We characterise its tuneability and stability performance using a single Mg+ atom confined in a radio-frequency surface-electrode trap under ultra-high vacuum conditions. The strength of the field can be tuned with a relative precision of ≤2 × 10−5 and we find a passive temporal stability of our setup of better than 1.0 × 10−4 over the course of one hour. Slow drifts on time scales of a few minutes are actively stabilised by adjusting electric currents in the magnetic field coils. In this way, we observe coherence times of electronic superposition states of greater than six seconds using a first-order field insensitive (clock) transition. In a first application, we demonstrate sensing of magnetic fields with amplitudes of ≥0.2 μT oscillating at

\simeq

≃ 2π × 60 MHz. Our approach can be implemented in compact and robust applications with strict power and load requirements.

This work presents the design and implementation of an RF system for the manipulation of the Zeeman sublevels of the87Rb atomic ground-state. This system allows efficient generation, control and transmission of the RF field that interacts with the atoms, facilitating the preparation of quantum states on demand via Rabi oscillations, while fine-tuning experimental parameters such as signal intensity and frequency. As Zeeman levels are highly sensitive to external magnetic fields, the system facilitates radio frequency spectroscopy, enabling precise measurement of such fields.