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IEEE Transactions on Applied Superconductivity

Characterization of SQUID-Based Null Detector for a Quantum Metrology Triangle Experiment

Juho Luomahaara, A. Kemppinen, P. Helistö, Juha Hassel

Published Dec 31, 2012• 5 citations
Publisher

Abstract

We present characterization of a null detector designed for an experiment aimed to close the quantum metrological triangle (QMT). The device is a SQUID amplifier equipped with a large winding-ratio planar input transformer improving the current resolution. We demonstrate equivalent input current noise down to 14 fA/Hz1/2. Furthermore, operation in a realistic environment of a dry dilution refrigerator hosting the QMT experiment is studied. We discuss particular challenges set by such system as the cryogenics introduces relatively large magnetic interference. We present data from measurements simulating the final QMT experiment and introduce the requirements for the averaging and bias reversal.

DOI
10.1109/tasc.2012.2237212

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We present characterization of a null detector designed for an experiment aimed to close the quantum metrological triangle (QMT). The device is a SQUID amplifier equipped with a large winding-ratio planar input transformer improving the current resolution. We demonstrate equivalent input current noise down to 14 fA/Hz1/2. Furthermore, operation in a realistic environment of a dry dilution refrigerator hosting the QMT experiment is studied. We discuss particular challenges set by such system as the cryogenics introduces relatively large magnetic interference. We present data from measurements simulating the final QMT experiment and introduce the requirements for the averaging and bias reversal.
We present a dc Superconducting QUantum Interference Device (SQUID)-based current amplifier with an estimated input referred noise of only 2.3 fA/Hz. Because of such a low amplifier noise, the circuit is useful for Johnson noise thermometry of quantum resistors in the kΩ range down to mK temperatures. In particular, we demonstrate that our circuit does not contribute appreciable noise to the Johnson noise of a 3.25 kΩ resistor down to 16 mK. Our circuit is a useful alternative to the commonly used High Electron Mobility Transistor-based amplifiers, but in contrast to the latter, it offers a much reduced 1/f noise. In comparison to SQUIDs interfaced with cryogenic current comparators, our circuit has similar low noise levels, but it is easier to build and to shield from magnetic pickup.
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We describe the experimental set-up used at LNE for closing the quantum metrological triangle (QMT) experiment and also testing the quantization of a current generated by a single electron device. Our present device is an R-electron pump composed of 3 tunnel junctions. The influence of the pumping frequency on the noise and on the current stability is firstly investigated. Then preliminary results are given about the quantized current measured with the complete QMT set-up. A relative random uncertainty of a few parts in 106has been found for a 100 MHz pumping frequency.
We have fabricated and tested thin film, niobium edge junction, double transformer, dc superconducting quantum interference devices (SQUID’s) that were stable under room-temperature storage and thermal cycling and that had very good noise performance. The input inductance, approximately 1.7 μH, was large enough to facilitate good matching to many experiments. When the SQUID was operated as a small-signal amplifier, the minimum detectable energy per unit bandwidth (Se) was 5×10−33 J/Hz at 100 kHz, referred to the SQUID loop (uncoupled). The minimum detectable energy per unit bandwidth was 1.8×10−31 J/Hz at 100 kHz, referred to the input coil. The SQUID’s had good characteristics for flux-locked operation since the small signal Se was low over a substantial range of bias current and magnetic flux. For operation in a flux-locked feedback circuit, Se was 6×10−32 J/Hz at 1 kHz.
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.