The microwave superconducting quantum interference device (SQUID) multiplexer (MW-Mux) is a promising readout circuit for transition-edge sensor (TES) microcalorimeters and metallic magnetic calorimeters (MMCs). Owing to its gigahertz bandwidth, it can provide a much larger number of multiplexed TES pixels U per readout wire than three conventional methods with megahertz bandwidth, namely, time-division multiplexing (TDM), code-division multiplexing (CDM), and low-frequency frequency-division multiplexing (FDM). The MW-Mux consists of quarter-wavelength resonators terminated by dissipationless radio frequency (RF) SQUIDs; the resonance frequencies f_r are modulated by the output of the corresponding pixel of the TES. To linearize the SQUID response, flux-ramp modulation (FRM) is adopted. This involves ramping the flux in RF-SQUID at a fundamental frequency f_ramp resulting in a relative phase shift of the modulated signal averaged over the period of the flux ramp. Therefore, the TES output is measured at sampling frequency f_ramp. Over the past decade, the MW-Mux has been developed to achieve a larger U and higher f_ramp, with S_I « δI_ph² + δIJ2, where SI denotes the spectral densities of the current noise referred to the SQUID input originating from MW-Mux, and δIph2 and δIJ2 denote the phonon and Johnson noise of TES, respectively. For 4000-pixel millimeter-wave TES bolometers, Henderson et al. reported √SI = 32 pA/√Hz; this was followed by a TES readout with U = 120 and f_ramp = 4 kHz by Cukierman et al. For 512-pixel γ-ray calorimeters, Mates et al. demonstrated U = 128, f_ramp = 62.5 kHz, and √SI = 19 pA/√Hz, corresponding to a flux noise of 2μΦ₀/√Hz, where Φ0 is the flux quantum. In contrast to these studies with f_ramp < 100 kHz, multiplexed readouts of x-ray TES calorimeters with MW-Mux have been reported only for U = 28 with f_ramp = 160 kHz by Yoon et al., and U = 2 with f_ramp = 500 kHz by Alpert et al. In this work, we first demonstrate a simultaneous readout of 38 x-ray TESs using our 40-channel MW-Mux, with framp = 500 kHz and an unprecedented noise level of √SI = 19 pA/√Hz.¹⁾ We will present the demonstration of simultaneous readout of 38 pixels from an array of x-ray TES microcalorimeters as well as development of a 40-channels microwave SQUID multiplexer.

1) Y. Nakashima et al., ``Low-noise microwave SQUID multiplexed readout of 38 x-ray transition-edge sensor microcalorimeters,’’ Appl. Phys. Lett., 2020.

Keywords: Multiplex, Superconducting microwave resonator, SQUID, Transition-edge sensor