Adjustable SQUID-resonator direct coupling in microwave SQUID multiplexer for TES microcalorimeter array

We have succeeded in the first demonstration of a simple and accurate resonator-superconducting quantum interference device (SQUID) coupling for microwave SQUID multiplexers. A simple theory shows our direct coupling with adjustable fractional inductance in the SQUID loop can decrease the deviation of resonance frequencies from designed values in contrast to a conventional inductive coupling. Our direct coupling provides the individual coupling that can be optimized with keeping identical structure, shape, and dimension of the SQUID among all pixels on the same chip. It covers experimentally three or potentially more factors of a frequency band that is larger than that of cryogenic high electron mobility transistor amplifiers. The deviation of experimental fractional inductance from the designed one is less than −3/+10%.