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International Journal of Modern Physics B

DC-Squid Readout for STJ Astronomical Detectors

G. Torrioli, Maria Gabriella Castellano, Roberto Leoni, F. V. Greco, R. Buonanno, Fernando Pedichini, P. Carelli

Published Apr 20, 1999• 1 citation
Publisher

Abstract

The new generation of astronomical detectors based on Superconducting Tunnel Junctions (STJs) is capable of detecting photons in a wide range of wavelengths, and, in some cases, even of counting single photons. Since these detectors operate at very low temperature (down to 100 mK), a natural candidate for the readout electronics is the dc-SQUID, a superconducting device that can work as a current preamplifier, reaching very good noise performance. In this paper we present measurements on a homemade dc-SQUID, showing the response of the device to small current pulses, which simulate the real signals from an STJ. An optimal filtering processing of the data is performed, in order to extract from the noisy data the best estimate for the pulse height, which, in a real experiment with single optical photons, would be the parameter that carries the spectroscopic information.

DOI
10.1142/s0217979299001405

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Superconducting Tunnel Junctions (STJs) have traditionally been read-out using low noise JFET based charge sensitive amplifiers. In some applications, low temperature SQUIDs could be used instead. The advantages are that the first amplification stage is also cooled to cryogenic temperatures and can be physically closer to the STJ, the detector can be voltage biased by means of a small fixed resistor and the detected current pulse shapes can be measured more directly. This work will outline the actual SQUID amplifier configuration and explain its virtues as well as its limitations.
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We report on fabrication and photon detection experiments of Nb/Al and Ta/Al superconducting tunnel junctions (STJs). 5-layer STJ thin-films were fabricated using UV photolithography, DC magnetron sputtering, reactive ion etching, and chemical vapor deposition techniques. STJs with 4 different sizes (20, 40, 60 and 80 μm) were deposited on sapphire substrates and tested in a two stage adiabatic demagnetization refrigerator with an operating temperature ~ 50 mK. Photons from different light sources are injected into the junctions via an optical fiber in combination with a monochromator which can produce photons from 30 nm to 550 nm with 0.1 nm resolution. The junction is read out through a charge-sensitive preamplifier followed by a shaping stage. We have measured some performance indicators and quality factors of the junctions from resultant I-V curves.
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Abstract 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 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 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 300 mK in an adiabatic demagnetization refrigerator. The measured magnetic flux noise of the two-stage SQUID circuit system is approximately 0.3 μ Φ 0 / Hz at 10 kHz. The current noise equivalent to the input coil of input SQUID is about 2.4 pA / Hz . This result meets the low-noise readout requirements of the CMB TES and other applications with TES detectors.