Two-Stage SQUID Amplifier With Bias Current Re-Use

Biasing arrangements in multi-channel multi-stage SQUID amplifier systems, such as Transition Edge Sensor matrices for astronomical observation (Barrett et al., 2023) or quantum science (Hummatov et al., 2023), typically require a large number of wires. This is due to the need for two or more cascaded SQUID stages to obtain sufficiently large power gain over a sufficient bandwidth, and due to moderate obtainable multiplexing factors, which forces implementation of many parallel readout chains to serve all the sensor pixels. We suggest an arrangement where one bias line and one flux setpoint line are shared by two cascaded SQUID stages on a single chip, halving the number of lines two cascaded stages would ordinarily require. The stages are connected in series, sharing a single supply <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">current</i>, dual to ordinary integrated transistor circuits in which many transistor stages are connected in parallel and share a single supply <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">voltage</i>. We show experimental results at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> = 4.2 K for a proof-of-concept amplifier chip, fabricated in the VTT Micronova foundry, using SWAPS Josephson junctions (Grönberg et al., 2017) at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_C</i> = 20 μA/(μm)<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> critical current density. The device shows larger than 3 kΩ transresistance, when operating from <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L_IN</i> = 29 nH input inductance to <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R_D</i> < 150 Ω output dynamic resistance.