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Applied Physics Letters

Superconducting quantum interference device as a near-quantum-limited amplifier for the axion dark-matter experiment

D. Kinion, John L. Clarke

Published May 16, 2011• 30 citations
Publisher

Abstract

We characterized a microstrip superconducting quantum interference device amplifier (MSA) as a function of frequency and temperature. At 40 mK, with optimal current and flux biases, the maximum gain of 20.4 dB was at 620 MHz; the bandwidth was 21 MHz. The minimum noise temperature TN=48±5 mK occurred at 612 MHz, slightly below the frequency of maximum gain. This value of TN is within a factor of 1.6 of the quantum limit TQ=hf/kB (f is frequency) for a linear, phase-preserving amplifier. Incorporating the MSA into the axion detector at the University of Washington will increase the scan rate by two to three orders of magnitude.

DOI
10.1063/1.3583380

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We characterized a microstrip superconducting quantum interference device amplifier (MSA) as a function of frequency and temperature. At 40 mK, with optimal current and flux biases, the maximum gain of 20.4 dB was at 620 MHz; the bandwidth was 21 MHz. The minimum noise temperature TN=48±5 mK occurred at 612 MHz, slightly below the frequency of maximum gain. This value of TN is within a factor of 1.6 of the quantum limit TQ=hf/kB (f is frequency) for a linear, phase-preserving amplifier. Incorporating the MSA into the axion detector at the University of Washington will increase the scan rate by two to three orders of magnitude.
A series of about twenty superconducting quantum interference devices (SQUIDs) has been operated as microstrip-SQUID amplifiers (MSAs) at frequencies ranging from 100 MHz to 2 GHz to study the dependence of their gain and noise temperature on bias current and flux. The measured values were in good agreement with theory. The observed dependence of MSA gain and noise temperature on bias current and flux resembled the static transfer function of the SQUIDs. The gains are relatively insensitive to changes in bias current and bias flux; the noise temperature is strongly dependent on the bias flux.
A dc superconducting quantum interference device (SQUID) with a resonant microstrip input is operated as an amplifier at temperatures down to 20 mK. A second SQUID is used as a postamplifier. Below about 100 mK, the noise temperature is 52±20 mK at 538 MHz, estimated from measurements of signal-to-noise ratio, and 47±10 mK at 519 MHz, estimated from the noise generated by a resonant circuit coupled to the input. The quantum-limited noise temperatures are 26 and 25 mK, respectively. The measured noise temperature is limited by hot electrons generated by the bias current.
We report results of our development of two types of microwave amplifiers based on Superconducting quantum interference devices (SQUIDs) for CAPP (Center for Axion and Precision Physics Research) axion search experiments. The first amplifier, Microstrip SQUID Amplifier (MSA) has a wide bandwidth. Power dissipation in resistive shunts results in a device overheating and limits the MSA performances. A thermal models of shunt cooling processes is developed and recommendations for a future design is provided. The second amplifier, Josephson Parametric Amplifier (JPA), at limited bandwidth showed a low noise near the standard quantum limit (SQL) of 196 mK. Implementation of a JPA in CULTASK (CAPP's Ultra-Low Temperature Axion Search in Korea) experiment is planned in the end of 2019.
A series of five dc superconducting quantum interference devices (SQUIDs) have been operated as microstrip amplifiers at frequencies ranging from 2.2 to 7.4 GHz. In these devices, the signal is connected between the SQUID washer and coil, which acts as a microstrip resonator. The gain measured at 4.2 K ranged from 12±1 to 6±1 dB. The noise temperature of three devices at 4.2 K in the frequency range 2.2–4 GHz was between 1 and 2 K, and the saturation temperature was between 150 and 250 K. Applications of these devices include readout for axion detectors, and intermediate-frequency amplifiers for superconductor–insulator–superconductor and hot-electron bolometer mixers.
The microwave cavity experiment for the dark matter axion search is based on the theoretical prediction that axions could convert into microwave photons inside a High-Q cavity immersed in a strong magnetic field. In such experiments a weak, of order 10-22 W, quasi-monochromatic microwave signal should be detected by scanning it in a very wide frequency range. The best cryogenic semiconductor amplifiers have the lowest noise temperature plateau of about 1.0 K even at significantly lower ambient temperature. Superconducting quantum interference devices, or SQUIDs, can work as microwave amplifiers with temperature noise close to the standard quantum limit that is about 50 mK at 1 GHz [1]. Previously designed SQUID-based high frequency amplifiers have narrow bandwidth due to a microstrip resonant input coil. It requires serial replacements of SQUID preamplifiers in order to scan wide frequency range. This procedure is complex and time consuming because of a large mass of hardware should be cooled down below 100 mK. For suitable amplification at high frequency a SQUID should be designed with the smallest possible Josephson junction capacitance, reasonably low SQUID loop inductance, and maximal steepness of the transfer function. Sub-micron size cross-type Josephson junctions have a very low capacitance of about 0.04 pF that is one order smaller than a capacitance of Josephson junctions fabricated using conventional window-type technology. Such junctions initially were designed and used at IPHT for low-frequency SQUID current sensors [2]. Slightly modified current sensors were tested as amplifiers at frequency range 0.5 – 5 GHz. They showed high gain and both wideband and resonant performance. In this paper we describe wideband SQUID microwave amplifiers fabricated at IPHT using sub-micron size Josephson junctions with very low capacitance.
A dc superconducting quantum interference device (SQUID) was used to amplify radio-frequency signals that were coupled to one end of the microstrip formed by the input coil and the SQUID washer. For one device, the resonant frequency of the microstrip was increased from about 200 to 620 MHz by progressively shortening the length of the coil. At an operating temperature of 4.2 K, the gain was typically 18 dB, and the system noise temperature ranged from 0.5±0.3 K at 80 MHz to 3.0±0.7 K at 500 MHz.