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Progress in Superconductivity and Cryogenics

Gain characteristics of SQUID-based RF amplifiers depending on device parameters

Young Ho Lee, KK Yu, J-M Kim, Seung‐Kyun Lee, Yonuk Chong, S. J. Oh, Yannis Kyriakos Semertzidis

Published Mar 1, 2019• 1 citation
Publisher

Abstract

Radio-frequency (RF) amplifiers based on direct current (DC) superconducting quantum interference device (SQUID) have low-noise performance for precision physics experiments. Gain curves of SQUID RF amplifiers depend on several parameters of the SQUID and operation conditions. We are developing SQUID RF amplifiers for application to measure very weak RF signals from ultra-low-temperature high-magnetic-field microwave cavity in axion search experiments. In this study, we designed, fabricated and characterized SQUID RF amplifiers with different SQUID parameters, such as number of input coil turn, shunt resistance value of the junction and coupling capacitance in the input coil, and compared the results.

DOI
10.9714/psac.2019.21.1.010

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Radio-frequency (RF) amplifiers based on direct current (DC) superconducting quantum interference device (SQUID) have low-noise performance for precision physics experiments. Gain curves of SQUID RF amplifiers depend on several parameters of the SQUID and operation conditions. We are developing SQUID RF amplifiers for application to measure very weak RF signals from ultra-low-temperature high-magnetic-field microwave cavity in axion search experiments. In this study, we designed, fabricated and characterized SQUID RF amplifiers with different SQUID parameters, such as number of input coil turn, shunt resistance value of the junction and coupling capacitance in the input coil, and compared the results.
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.
The noise and signal parameters of several types of RF amplifiers based on different SQUIDs with integrated and hybrid input coils were studied. A new type of multiloop DC SQUID with an integrated input coil and extremely low stray capacitances was designed. The inductance of a four-loop SQUID was 100 pH, the input coil inductance 1.3 nH, and mutual inductance 300 pH. The tuned integrated four-loop amplifier at 420 MHz had a noise temperature lower 0.5 K and a power gain of nearly 20 dB in a 60-MHz bandwidth. For the noise calibration of such amplifiers, SIS junctions were used as a shot noise source, or a cooled attenuator and a room temperature semiconductor noise source were used.>
Superconducting quantum interference devices (SQUIDs) are promising for detection of extremely weak power regimes for the Internet of Things and long-distance satellite networks. In low-frequency magnetic signal measurements, it has demonstrated superior performance over the traditional systems. We review the resistively and capacitively shunted junction model of dc SQUIDs and investigate its transfer function to magnetic flux signals while taking thermal fluctuation into account. We numerically evaluate the influence of bias current and bias flux signal to determine the operating parameters of the device. The performances related to SQUID ring inductance operating at 4.2 K and 77 K are demonstrated. Combined with common radio frequency front-end modules, including low-noise amplifier and low-pass filter, we evaluate the bit-error-rate performance of the system under different inductance values and inferred that the sensitivity of the system can reach −99 dBm at 100 MHz bandwidth.
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.
Superconducting quantum interference device (SQUID) is a sensitive detector of magnetic flux signals. Up to now, the main application of SQUIDs has been measurements of magnetic flux signals in the frequency range from near DC to several MHz. Recently, cryogenic low-noise radio-frequency (RF) amplifiers based on DC SQUID are under development aiming to detect RF signals with sensitivity approaching quantum limit. In this paper, we review the recent progress of cryogenic low-noise RF amplifiers based on SQUID technology.
Arrays of dc-superconducting quantum interference devices (dc-SQUIDs) were investigated, in which each dc-SQUID element has a multiturn input coil. The dependence of flux noise on the number of turns of the input coil was studied for current amplifiers. SQUID arrays were fabricated by a Nb/AlO x /Nb Josephson tunnel junction fabrication process. Results show that the flux noise of the SQUID array amplifier does not depend on the turn number of the input coil. A SQUID array amplifier consisting of 128 dc-SQUID elements was fabricated, in which each element had a 7-turn input coil and a mutual inductance between the input coil and the SQUID loop of 460 pH. This amplifier showed an ultrahigh current resolution of 1 pA/Hz0.5 and a maximum input current of 11 µA for signals of 50 kHz, which is sufficient bandwidth for X-ray detection applications. These values correspond to a large dynamic range of 87 dB. The flux-voltage characteristics of this SQUID array, however, showed asymmetric behavior, which originated from electronic resonance in stray capacitance and the mutual inductance between the SQUID loop and the multiturn input coil.