Microwave SQUID Multiplexer for Cosmic Microwave Background Imagers
Bradley J. Dober, Daniel T. Becker, Douglas A. Bennett, Sean A. Bryan, Shannon M. Duff, Johnathon D. Gard, James P. Hays-Wehle, Johannes Hubmayr, John A. Mates, Carl D. Reintsema, Leila R. Vale
Key performance characteristics are demonstrated for the microwave SQUID multiplexer (µmux) coupled to transition edge sensor (TES) bolometers that have been optimized for cosmic microwave background (CMB) observations. In a 64-channel demonstration, we show that the µmux produces a white, input referred current noise level of 29 pA/√Hz at -77 dB microwave probe tone power, which is well below expected fundamental detector and photon noise sources for a ground-based CMB-optimized bolometer. Operated with negligible photon loading, we measure 98 pA/√Hz in the TES-coupled channels biased at 65% of the sensor normal resistance. This noise level is consistent with that predicted from bolometer thermal fluctuation (i.e. phonon) noise. Furthermore, the power spectral density exhibits a white spectrum at low frequencies (∼ 100 mHz), which enables CMB mapping on large angular scales that constrain the physics of inflation. Additionally, we report cross-talk measurements that indicate a level below 0.3%, which is less than the level of cross-talk from multiplexed readout systems in deployed CMB imagers. These measurements demonstrate the µmux as a viable readout technique for future CMB imaging instruments.