Development of a microwave SQUID multiplexer for superconducting transition-edge sensor bolometer arrays

Abstract

A microwave SQUID multiplexer (μMUX) with a high multiplexing factor is a promising readout approach for large-scale superconducting transition-edge sensor (TES) bolometer arrays. This paper reports on the characteristics of 16-channel μMUX prototype chips and a 60-channel readout module developed to read out TES bolometers for the Ali cosmic microwave background polarization telescope (AliCPT). We designed two different coupling structures between the resonator and the radio-frequency superconducting interference device: design A with a mutual inductance ( MT) of 2.3 pH and design B with an MT of 6.5 pH. For design A, the mean resonator bandwidth ( ΔfBW) and peak-to-peak frequency shift ( Δfpp) are approximately 610 kHz and 290 kHz, respectively. The measured mean open-loop current noise is less than 40 pA/Hz. Preliminary results indicate that crosstalk between adjacent channels is below 1% in our design A μMUX chip. A measured channel with design B shows a ΔfBW of 586 kHz and a current noise of 33 pA/Hz, similar to the design A channels. In comparison, the Δfpp of 803 kHz is much larger than the design A channels due to a larger MT. The readout module, consisting of two design A chips and two design B chips, shows 60 resonances, corresponding to a yield of 94%. In the future, we will place 32 design A channels within the same chip dimension to increase the channel density and build a larger readout module suitable for AliCPT TES arrays.