Investigation of Large Coupling Between TES X-Ray Microcalorimeter and Microwave Multiplexer Based on Microstrip SQUID

We report on the study of a microwave superconducting quantum interference device (SQUID) multiplexer (MW-Mux) with current noise lower than that of our typical transition-edge sensor (TES) X-ray microcalorimeter. A large mutual inductance, between SQUID loop and input inductor connected to a TES X-ray microcalorimeter in series, realizes such low current noise referred to the SQUID input, but also can cause undesirable resonances which affect the SQUID performances. In this paper, we have developed the MW-Mux with mutual inductance of about <inline-formula><tex-math notation="LaTeX">$\text{370 pH}$</tex-math></inline-formula>, which is six times larger than that of the previous one, by adopting a spiral input coil longer than a half wavelength at the readout microwave frequency. Its characteristics are evaluated, and the results have shown that the distortion of the SQUID characteristics and the degradation of its noise performance are not clearly observed. Finally, we have developed an MW-Mux microstrip SQUID with a low current noise of <inline-formula><tex-math notation="LaTeX">$<\text{20 pA}/\sqrt{{\rm Hz}}$</tex-math></inline-formula> which is below the typical noise of our TES X-ray microcalorimeter, <inline-formula><tex-math notation="LaTeX">$\text{30 pA}/\sqrt{{\rm Hz}}$</tex-math></inline-formula>.

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