Low-noise modular microsusceptometer using nearly quantum limited dc SQUIDs

A flexible combination of superconducting integrated circuits was used to construct a low-temperature magneto-optic microsusceptometer utilizing a dc superconducting quantum inteference device (SQUID) detector operating near the quantum limit (coupled energy sensitivity of 1.7ℏ). Miniature pick-up loop assemblies on transparent substrates were joined by superconducting interchip connections to a thin-film dc SQUID, which is in turn read out by a second dc SQUID connected to room-temperature electronics. Measurements on an 8.5-μm-diam titanium dot evaporated directly into the pick-up loop demonstrate a spin sensitivity of ∼103 spins/(Hz)1/2 at T=290 mK.