Electron-beam lithography and reactive ion etching have been used to fabricate thin-film Au/Nb bridges with widths ∼50 nm. The Au layer was used as both a mask for etching the Nb superconducting bridge and as a resistive shunt in the completed devices. Using these junctions, a dc superconducting quantum interference device (SQUID) design with a hole size of 200 nm×200 nm (nano-SQUID) has also been fabricated and characterized. A flux noise of approximately 7×10−6 Φ0/Hz1/2 at 4.2 K has been achieved, from which a calculated spin sensitivity of 250 spin/Hz1/2 is predicted.

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The sensitivity of nanoscale SQUIDs (nanoSQUIDs; SQUID: superconducting quantum interference device) to single and small populations of magnetic dipoles is considered. The simple estimate given previously for the atomic spin sensitivity of a nanoSQUID coupled to an isolated magnetic dipole at its centre is confirmed. It is demonstrated that the sensitivity is constrained in most practical situations by the finite size of the SQUID loop and nanobridges. An exact analytic result is obtained for a nanoSQUID composed of an idealized filamentary circular loop. The issue of optimum placement and orientation of the dipole with respect to the nanoSQUID hole is also considered, and it is shown that the optimum position for the dipole depends upon the height of the dipole above the plane of the nanoSQUID. It is pointed out that the conclusion quoted by previous authors, that the optimum position is above one of the Josephson junctions or Dayem bridges, although true in the limit of very narrow bridges with tight magnetic coupling, is not true in general, and estimates of the potential sensitivity when the dipole is placed in this region based on simple filamentary models are likely to overestimate the sensitivity achievable in practice.