Development of series SQUID array with on-chip filter for TES detector

A cold preamplifier based on superconducting quantum interference devices（SQUIDs） is currently the preferred readout technology for the low-noise transition edge sensor（TES）.In this work,we have designed and fabricated a series SQUID array（SSA） amplifier for the TES detector readout circuit.In this SSA amplifier,each SQUID cell is composed of a first-order gradiometer formed using two equally large square washers,and an on-chip low pass filter（LPF） as a radiofrequency（RF） choke has been developed to reduce the Josephson oscillation interference between individual SQUID cells.In addition,a highly symmetric layout has been designed carefully to provide a fully consistent embedded electromagnetic environment and achieve coherent flux operation.The measured results show smooth V-Φ characteristics and a swing voltage that increases linearly with increasing SQUID cell number N.A white flux noise level as low as 0.28 μφ;/Hz;is achieved at 0.1 K,corresponding to a low current noise level of 7 pA/Hz;.We analyze the measured noise contribution at mK-scale temperatures and find that the dominant noise derives from a combination of the SSA intrinsic noise and the equivalent current noise of the room temperature electronics.