Cosmic Microwave Background Polarization Detector with High Efficiency, Broad Bandwidth, and Highly Symmetric Coupling to Transition Edge Sensor Bolometers

We describe a prototype detector system designed for precise measurements of Cosmic Microwave Background polarization. The design combines a quasi-optical polarization modulator, a metal feedhorn, a superconducting planar microwave circuit, and a pair of transition-edge sensor (TES) bolometers operating at <100 mK. The circular feedhorn produces highly symmetric beams with very low cross-polarization. The planar circuit preserves symmetry in coupling to bolometers measuring orthogonal polarizations. We implement the circuit with superconducting niobium transmission lines. Three-dimensional interfaces between the planar circuit and waveguides leading to feedhorn and backshort have been carefully designed with electromagnetic simulations. Power is thermalized in resistors and conducted to bolometers via normal electrons. Our system is designed for a 29–43 GHz signal band. We have tested individual circuit elements in this frequency range. Fabrication of a full single-pixel system is underway.