Long ion chains as a sensitive spatial probe of stray field

Many ions trapped in an equally spaced linear chain can serve as a register for quantum computation with all-to-all connectivity. A significant, little researched barrier to controlling the inter-ion spacing and avoiding a transition to the zigzag state is an axial stray field pointing in the direction of the ion chain. In an equidistant trapping potential, even a constant axial stray field causes bunching of the ions and local zigzag segments. On the flip-side, long ion chains serve as a sensitive spatial probe of both the intentional potentials induced by trapping electrodes and the stray electric field and allow us to separate between the effects of the two. We present a successful effort to trap a 200 ion chain, as well as the axial stray field measured in this configuration at a spatial resolution of the inter-ion spacing of approximately 5 microns. In addition we use this method to explore the dynamics of charging in an ion trap and employ an axial field to drive the linear-to-zigzag transition in a controlled manner.