An efficient high-current circuit for fast radio-frequency spectroscopy in cold atomic gases

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 $\sim\,4$ to about $7.5\,$A for $100\,$W of RF driving. This allows to obtain a RF coupling field of about $0.035\,\text{G}/\sqrt{\text{W}}$ at the atomic sample location. The system is robust and versatile, as it generates a large RF field without compromising on 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 $\Omega_R/2\pi \simeq 18.5\,$kHz, i.e. a $\pi$-pulse duration of about $27\,\mu$s, between two of the lowest states of ${}^6$Li 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.