Three-dimensional nanoscale superconducting quantum interference device pickup loops

Nanoscale superconducting quantum interference devices (SQUIDs) have sensitivities approaching that required for single-spin detection, but they only measure fields perpendicular to their plane and can be difficult to tightly couple to magnetic sources on the same chip. To remove these limitations we used focused-ion-beam-induced chemical vapor deposition to directly write a SQUID structure with three-dimensional, freestanding pickup loops using superconducting tungsten nanowires. By applying a localized field, we investigated the pickup loop response, and found that it exhibits Meissner screening corresponding to a penetration depth λ(T) consistent with BCS theory in the dirty limit and λ(0)=330 nm.