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IEEE Transactions on Applied Superconductivity

Nearly Quantum Limited Two-Stage SQUID Amplifiers for the Frequency Domain Multiplexing of TES Based X-ray and Infrared Detectors

Luciano G. Gottardi, M. Kiviranta, Jan van der Kuur, Hiroki Akamatsu, M. P. Bruijn, R. den Hartog

Published Nov 10, 2014• 14 citations
Publisher

Abstract

The frequency domain multiplexing (FDM) readout of arrays of transition edges sensors for the X-IFU/Athena and SAFARI/SPICA instruments requires SQUID amplifiers with low input inductance, low flux noise, high-dynamic range and low power consumption. We report the performance of two-stage SQUID amplifiers developed at VTT coupled to multiple high-Q LC resonators of the SRON FDM readout operating in the frequency range from 2 to 5 MHz. The SQUIDs have been tested in the following configurations: 1) the two SQUIDs at base temperature with open input; 2) the amplifier SQUID at 1K temperature and the front-end SQUID at base temperature in the FDM readout of low-G TES bolometers; and 3) and in the FDM readout of X-ray microcalorimeter arrays with both SQUIDS at base temperature. We measured an input flux noise equal to 0.18 μΦ0/Hz1/2at 20 mK, equivalent to an energy resolution of 9 b. The measured current noise at the SQUID input is ~1 pA/Hz1/2at 20 mK, which corresponds to a coupled energy resolution of about 15 b.

DOI
10.1109/tasc.2014.2369234

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The frequency domain multiplexing (FDM) readout of arrays of transition edges sensors for the X-IFU/Athena and SAFARI/SPICA instruments requires SQUID amplifiers with low input inductance, low flux noise, high-dynamic range and low power consumption. We report the performance of two-stage SQUID amplifiers developed at VTT coupled to multiple high-Q LC resonators of the SRON FDM readout operating in the frequency range from 2 to 5 MHz. The SQUIDs have been tested in the following configurations: 1) the two SQUIDs at base temperature with open input; 2) the amplifier SQUID at 1K temperature and the front-end SQUID at base temperature in the FDM readout of low-G TES bolometers; and 3) and in the FDM readout of X-ray microcalorimeter arrays with both SQUIDS at base temperature. We measured an input flux noise equal to 0.18 μΦ0/Hz1/2at 20 mK, equivalent to an energy resolution of 9 b. The measured current noise at the SQUID input is ~1 pA/Hz1/2at 20 mK, which corresponds to a coupled energy resolution of about 15 b.
A SQUID tandem is a two-stage amplifier consisting of the front end SQUID atTT∼ 4 K. We give an overview of the dc SQUID design for the tandem used in the recent 37-pixel frequency domain multiplexed (FDM) readout demonstration of X-ray calorimeters. The SQUID devices were fabricated with pillar-type Josephson junctions at the nominalJC= 500 A/cm2critical current density. We further discuss possible improvements to the FDM tandem configuration, including (i) the class-B operation for better power efficiency, (ii) the input power combiner as an improvement over the simple T-junction summing, with the promise for larger multiplexing factors, and (iii) the new booster SQUID device. The new improved booster is a 8-parallel 66-series SQUID array fabricated with SWAPS Josephson junctions at the nominal JC= 1.5 kA/cm2 critical current density, and it shows dV/dΦ > 30 mV/Φ0gain in the symmetric-slope operation, as well as ΦN≈ 0.05 μΦ0/ Hz1/2flux noise.
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SRON is developing the Frequency Domain Multiplexing (FDM) read-out and the ultra low NEP TES bolometers array for the infra-red spectrometer SAFARI on board of the Japanese space mission SPICA. The FDM prototype of the instrument requires critical and complex optimizations. For single pixel characterization under AC bias we are developing a simple FDM system working in the frequency range from 1 to 5 MHz, based on the open loop read-out of a linearized two-stage SQUID amplifier and high Q lithographic LC resonators. We describe the details of the experimental set-up required to achieve low power loading (<1 fW) and low noise ( NEP∼10−19 W/Hz\mathrm{NEP}\sim 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}NEP∼10−19 W/Hz​ ) in the TES bolometers. We conclude the paper by comparing the performance of a 4×10−19 W/Hz4\times 10^{-19}~\mathrm{W}/\sqrt{\mathrm{Hz}}4×10−19 W/Hz​ TES bolometer measured under DC and AC bias.
We demonstrate multiplexing readout of 60 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing readout chain with bias frequencies ranging from 1 to 3.5 MHz and with a typical frequency spacing of 32 kHz. The readout chain starts with a two-stage SQUID amplifier and has a noise level of 9.5 pA/Hz. We compare current–voltage curves and noise spectra of TESs measured in a single-pixel mode and in a multiplexing mode. We also map the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where there are 43 pixels that do not show more than 10% difference in NEP and 5% in saturation power when measured in single pixel and multiplex modes. We have read out a TES with an NEP of 0.45 aW/Hz in the multiplexing-mode, which demonstrates the capability of reading out ultra-low noise TES bolometer arrays for space applications.
We report very-low-noise, fast-response, middle-scale multiplexing in a microwave superconducting quantum interference device multiplexer (MW-Mux) as a transition-edge sensor (TES) readout. Our MW-Mux is able to read 40 channels with 500 kHz sampling and has a low readout noise of 0.9 μΦ0/Hz (where Φ0 is the magnetic flux quantum), equivalent to 9 pA/Hz. By contrast, a multiplexer of less than 10 pixels with 500 kHz sampling and ∼2 μΦ0/Hz readout noise has so far been reported in the literature. Owing to the 500 kHz sampling, our MW-Mux exhibits a fast response to detect a TES pulse with a rise time around 12 μs. We demonstrated simultaneous readout of 38 pixels from an array of x-ray TES microcalorimeters. The measured full-width values at half-maximum spectral resolution ranged from 2.79 to 4.56 eV, with a median value of 3.30 eV at 5.9 keV, including a ∼10% contribution of readout noise, i.e., 0.9–1.7 eV.
Direct-current superconducting quantum interference devices (dc-SQUIDs) are one of the most sensitive magnetic detectors. These sensors are extensively used in the readout of superconducting transition edge sensors (TESs), which are used for the detection of weak signals. A cosmic microwave background (CMB) polarization telescope operating in 22-48 GHz is currently under developing. The TESs calorimeter of the telescope will be readout by a time-division multiplexer (TDM) SQUID readout system. We develop a two-stage dc-SQUID amplifier circuit, comprising an input-stage SQUID with 4 SQUID cells and a series SQUID array (SSA) with 100 SQUID cells. This configuration has been shown to achieve extremely high signal gain while effectively controlling system noise. We assess the system noise at 300300300 mKmKmK in an adiabatic demagnetisation refrigerator (ADR). The the measured magnetic flux noise of the two-stage SQUID circuit system is approximately 0.30.30.3 μΦ0/HzμΦ_{0}/\sqrt{Hz}μΦ0​/Hz​ at 101010 kHzkHzkHz. The current noise equivalent to the input coil of input SQUID is about 2.42.42.4 pA/HzpA/\sqrt{Hz}pA/Hz​. This result meets the low-noise readout requirements of the CMB TES and other applications with TES detectors.