Microwave multiplexing on the Keck Array

TL;DR

This paper reports on the on-sky demonstration of a microwave-multiplexing readout system for the Keck Array, enabling efficient observation of the cosmic microwave background with 528 channels over a single coaxial cable.

Contribution

We developed and tested a microwave-multiplexing readout system for the Keck Array, replacing traditional methods with superconducting resonators and RF systems for improved channel capacity.

Findings

Successfully read out 528 channels over 5-6 GHz range

Demonstrated initial system performance in on-sky observations

Integrated microwave multiplexing with existing CMB instrumentation

Abstract

We describe an on-sky demonstration of a microwave-multiplexing readout system in one of the receivers of the Keck Array, a polarimetry experiment observing the cosmic microwave background at the South Pole. During the austral summer of 2018-2019, we replaced the time-division multiplexing readout system with microwave-multiplexing components including superconducting microwave resonators coupled to radio-frequency superconducting quantum interference devices at the sub-Kelvin focal plane, coaxial-cable plumbing and amplification between room temperature and the cold stages, and a SLAC Microresonator Radio Frequency system for the warm electronics. In the range 5-6 GHz, a single coaxial cable reads out 528 channels. The readout system is coupled to transition-edge sensors, which are in turn coupled to 150-GHz slot-dipole phased-array antennas. Observations began in April 2019, and we report here on an initial characterization of the system performance.