Microwave SQUID Multiplexer demonstration for Cosmic Microwave Background Imagers

Abstract

Key performance characteristics are demonstrated for the microwave SQUID multiplexer ($\mu$MUX) 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 $\mu$MUX produces a white, input referred current noise level of 29~pA$/\sqrt{\mathrm{Hz}}$ 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$/\sqrt{\mathrm{Hz}}$ 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 ($\sim$~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 $\mu$MUX as a viable readout technique for future CMB imaging instruments.

6 pages, 4 figures

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• Advanced ACTPol Cryogenic Detector Arrays and Readout
• SPT-3G: A Next-Generation Cosmic Microwave Background Polarization Experiment on the South Pole Telescope
• The Atacama Cosmology Telescope: The polarization-sensitive ACTPol instrument
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• Characterising the optical response of ultra-low-noise far-infrared 60-110 $\mu$m transition edge sensors
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