At the heart of The Simons Observatory is a new generation of superconducting detectors. Operating at extremely low temperatures (0.1 Kelvin), these detectors employ exquisitely sensitive devices known as transition edge sensors (TESs) which can record the minute signals sourced by the CMB anisotropies. Four of these detectors are combined to make a single SO pixel which can simultaneously detect two directions of linear polarization of light at two frequencies. SO will deploy an unprecedented number detectors on its telescopes, totaling more than 60,000.

Two types of detectors have been developed by members of the SO collaboration, and in particular by the two detector fabrication locations, the National Institute of Standards and Technology (NIST) and UC Berkeley. Both institutions are employing state-of-the-art nano-fabrication facilities.

TES detector design
The TESs are AlMn alloys with a critical temperature target of 160 mK. The sensor will be designed to be low impedance for microwave multiplexing, having a normal resistance of 8 mΩ.


NIST style detector wafer

NIST is fabricating detector arrays which radiatively couple light from the sky to a monolithic detector array via feedhorns with mm-scale diameter (very tiny!). Each spatial pixel in the array consists of a planar orthomode transducer (OMT) which splits the signal into orthogonal polarizations. A superconducting coplanar waveguide then routes the signal to microstrip transitions which couple to diplexed stub filters. The filters define the frequency bands and allow for each pixel to measure two frequencies of light in each linear polarization for a total of four detectors per pixel. After passing the filters the signal is absorbed by a weakly thermally coupled TES bolometer to measure the signal.


UC Berkeley is fabricating detector arrays which radiatively couple light from the sky to a monolithic detector array via mm-scale hemispherical lenses. The anti-reflection-coated silicon lenslet focuses the beam onto a sinuous antenna, which collects photons over a wide frequency range and splits the signal into the two linear polarization directions. The signal is routed through on-chip filters, similar to the NIST pixels (note however that NIST pixels are coupled to stub filters, while UC Berkeley pixels are coupled to lumped-element filters). The filters diplex into two frequency bands. Finally, the superconducting TES bolometer detects the signal.

UC Berkeley style detector module
UC Berkeley style detector module with silicon lenslets that couple incoming light to the detector array.