Development of readout electronics for POLARBEAR-2 Cosmic Microwave Background experiment

TL;DR

This paper discusses the development of high-bandwidth cryogenic readout electronics for the POLARBEAR-2 CMB experiment, focusing on reducing cross talk and non-linearities in large TES bolometer arrays.

Contribution

It introduces a cryogenic readout system with increased bandwidth and low impedance, enabling multiplexing of 40 bolometers and improved control of systematic errors.

Findings

Achieved a multiplexing factor of 40 in the frequency domain.

Developed a cryogenic readout with bandwidth above 4 MHz.

Reduced cross talk and non-linear response in TES bolometers.

Abstract

The readout of transition-edge sensor (TES) bolometers with a large multiplexing factor is key for the next generation Cosmic Microwave Background (CMB) experiment, Polarbear-2, having 7,588 TES bolometers. To enable the large arrays, we have been developing a readout system with a multiplexing factor of 40 in the frequency domain. Extending that architecture to 40 bolometers requires an increase in the bandwidth of the SQUID electronics above 4 MHz. This paper focuses on cryogenic readout and shows how it affects cross talk and the responsivity of the TES bolometers. A series resistance, such as equivalent series resistance (ESR) of capacitors for LC filters, leads to non-linear response of the bolometers. A wiring inductance modulates a voltage across the bolometers and causes cross talk. They should be controlled well to reduce systematic errors in CMB observations. We have been developing a cryogenic readout with a low series impedance and have tuned bolometers in the middle of their transition at a high frequency (> 3 MHz).