Ultrasensitive single-ion electrometry in a magnetic field gradient

Hyperfine energy levels in trapped ions offer long-lived spin states. In addition, the motion of these charged particles couples strongly to external electric field perturbations. These characteristics make trapped ions attractive platforms for the quantum sensing of electric fields. However, the spin states do not exhibit a strong intrinsic coupling to electric fields. This limits the achievable sensitivities. Here, we amplify the coupling between electric field perturbations and the spin states by using a static magnetic field gradient. Displacements of the trapped ion resulting from the forces experienced by an applied external electric field perturbation are thereby mapped to an instantaneous change in the energy level splitting of the internal spin states. This gradient mediated coupling of the electric field to the spin enables the use of a range of well-established magnetometry protocols for electrometry. Using our quantum sensor, we demonstrate AC sensitivities of $\mathrm{S^{AC}_{min}=960(10)\times 10^{-6}~V m^{-1}Hz^{-\frac{1}{2}}}$ at a signal frequency of $\omega_{\epsilon}/2\pi=5.82~\mathrm{Hz}$, and DC sensitivities of $\mathrm{S^{DC}_{min}=1.97(3)\times 10^{-3} ~V m^{-1}Hz^{-\frac{1}{2}}}$ with a Hahn-echo type sensing sequence. We also employ a rotating frame relaxometry technique, with which our quantum sensor can be utilised as an electric field noise spectrum analyser. We measure electric field signals down to a noise floor of $\mathrm{S_{E}(\omega)=6.2(5)\times 10^{-12}~V^2 m^{-2}Hz^{-1}}$ at a frequency of $\mathrm{30.0(3)~kHz}$. We therefore demonstrate unprecedented electric field sensitivities for the measurement of both DC signals and AC signals across a frequency range of sub-Hz to $\sim\mathrm{500~kHz}$. Finally, we describe a set of hardware modifications that are capable of achieving a further improvement in sensitivity by up to six orders of magnitude.