Microwave SQUID Multiplexer demonstration for Cosmic Microwave Background Imagers

TL;DR

This paper demonstrates the effectiveness of the microwave SQUID multiplexer ($$MUX) for reading out TES bolometers in CMB experiments, showing low noise, minimal cross-talk, and suitability for large-scale CMB mapping.

Contribution

The study presents the first detailed characterization of the $$MUX with 64 channels, confirming its low noise, low cross-talk, and potential for future CMB imaging instruments.

Findings

Achieved a white, input-referred current noise of 29 pA/√Hz at -77 dB probe tone power.

Measured TES noise consistent with thermal fluctuation predictions.

Cross-talk below 0.3%, lower than existing readout systems.

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.