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Physica C Superconductivity

Low-frequency noise characterization and mechanisms in high-swing multi-loop SQUID magnetometers with sub-micron Josephson junctions

Shumin Yu, Guofeng Zhang, Jun Wu, Jiawei Luo, Yongliang Wang, Xiaojun Dai, Longqing Qiu, Liangliang Rong

Published Aug 12, 2026• 0 citations
Publisher
DOI
10.1016/j.physc.2026.1354929

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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.
We report on the development of highly sensitive SQUIDs featuring sub-micrometer loop dimensions. The integration of high quality and low capacitance SIS Nb/AlO x /Nb cross-type Josephson tunnel junctions results in white flux noise levels as low as 66 n Φ 0 Hz −1/2 , well below state-of-the-art values of their Nb-based counterparts based on constriction type junctions. Estimation of the spin sensitivity of the best SQUIDs yield S μ 1/2 < 7 μ B Hz −1/2 in the white noise region, suitable for the investigation of small spin systems. We discuss fabrication challenges, show results on the electrical characterization of devices with various pickup loops, and describe options for further improvement, which may push the sensitivity of such devices even to single spin resolution.
Abstract We report on the development of nearly quantum limited SQUIDs with miniature pickup loop dimensions. The implemented high quality and low capacitance cross-type Nb/AlO x /Nb Josephson junctions offer large I C R N -products and therefore enable an exceptional low noise level of the SQUIDs. Devices with loop dimensions of 1 μ m exhibit white flux noise levels as low as 45 nΦ 0 Hz −1/2 corresponding to an energy resolution ε of about 1 h at 4.2 K, with h being Planck’s constant. Moreover, the large usable voltage swings of the devices of about 300 μ V allow highly sensitive and easy single-stage operation while exploring nearly the intrinsic noise of the SQUIDs, beneficial e.g. for sensor arrays in SQUID microscopy.