Since the advent of Newton’s mechanics, force has been at the forefront of our quantitative description of physical interactions. Ultrasensitive force measurements are an important tool for investigating the fundamental physics of magnetic (1, 2), atomic (3), quantum (4, 5), and surface (6–8) phenomena. Force measurement is also an implicitly important quantity in the systematics of precision measurement of other physical quantities such as the effect of gravity on time in general relativity (9). The development of high-resolution imaging of laser-cooled trapped ions (10–12) opened the possibility for the realization of an ion-based sensor that could resolve an external force in all three directions using a single atomic ion. In a harmonic potential, Hooke’s law Fi = kiDxi allows us to convert displacement measurements Dxi into a force measurement Fi through the associated spring constants ki. Forces parallel to the image plane are detected by measuring the dis-placement of the ion’s centroid, whereas forces applied orthogonal to that plane in the focusing direction, parallel to the optical axis of the imaging apparatus, are measured from a change in the width of a slightly defocused ion image. Although the resolution of a fluorescence image is limited by the wavelength of light, the exact centroid location and width can be determined to a much greater precision through super-resolution imaging techniques (13, 14). Doppler velocimetry–based force sensing in Penning (15, 16)and Paul(17) traps has demonstrated sensitivities down to 28 yN/ion/pHz; however, this is limited to one dimension and a narrow frequency band around a driven oscillation. Force measurement through imaging is a broadband rather than a narrowband sensing technique, with no fundamental lower frequency limit, and upper limit set by the speed and efficiency of the detection apparatus and the ion’s scattering rate.