rf SQUID detector for single-ion trapping experiments

We have designed and constructed a novel superconducting detector sensitive enough to detect the axial motion of a single, trapped ion. This detector employs a tuned, superconducting transformer matched to a rf SQUID (superconducting quantum interference device) to yield an effective current sensitivity of 2.7×10−15A/(Hz)1/2 at 160 kHz. To optimize the detection system, we have used existing small-signal equivalent circuit models, and thus have tested these models in a new regime. We include details of our superconducting circuitry and of the modifications required to stabilize the commerical SQUID controls at 160 kHz. Finally, we present typical detected signals from trapped ions.