Noise Measurements of Series SQUID Arrays

We have fabricated series arrays of 50 gradiometer SQUIDs and 100 magnetometer SQUIDs using Nb/AlOx/Nb junctions with a Planarized All Refractory Technology for Superconductivity (PARTS) [1] and have measured the white noise of these arrays. The individual devices are 50 pH, 2-hole and 100 pH, 1-hole SQUIDs with integrated single turn input coils, 1 mu m(2) and 0.5 mu m(2) junctions and PtRh shunt resistors. The input coil inductance L-i of 10 nII will effectively match with a wide bandwidth to miniature pick-up loop structures for various experiments. Ideally, the coupled energy sensitivity should remain constant as devices arc added in series, while the output impedance rises to a level practical for direct coupling to room temperature electronics. The output impedance of the arrays is 250-1000 Omega. The white noise was measured directly with an ultra low noise preamplifier at room temperature. The best result was Phi(N) = 0.12 mu Phi(0)/root HZ with a corresponding coupled energy sensitvity of epsilon(c) = 56 h.