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IEEE Transactions on Applied Superconductivity

Realization of a Monolithic, Planar SLUG Amplifier for the Quantum Electronics Toolbox

Gemma Chapman, Tom Godfrey, Jamie Potter, Laith Meti, Parth Bhandari, Edward J. Romans, John C. Gallop, Ling Hao

Published Mar 24, 2025• 2 citations
Publisher

Abstract

The Superconducting Low-inductance Undulatory Galvanometer (SLUG) amplifier is an ultra-low noise rf amplifier that is simple to operate and can be designed for a wide range of microwave frequencies. A variant of the dc SQUID rf amplifier, the SLUG differs in that the rf signal is directly injected in the SQUID loop, allowing high frequency operation. Here, we realize a planar SLUG amplifier constructed with nanobridge weak-link junctions that is fabricated from a single layer of niobium. Impedance models for the SLUG gain element are presented in conjunction with a discussion of appropriate impedance matching techniques. Experimental characterization of a proof-of-concept SLUG amplifier designed with a single-stub tuning circuit show modulation of the SLUG operation frequency in line with biasing of the SLUG, a well-matched input network and forward gain of 8 dB.

DOI
10.1109/tasc.2025.3549394

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The Superconducting Low-inductance Undulatory Galvanometer (SLUG) amplifier is an ultra-low noise rf amplifier that is simple to operate and can be designed for a wide range of microwave frequencies. A variant of the dc SQUID rf amplifier, the SLUG differs in that the rf signal is directly injected in the SQUID loop, allowing high frequency operation. Here, we realize a planar SLUG amplifier constructed with nanobridge weak-link junctions that is fabricated from a single layer of niobium. Impedance models for the SLUG gain element are presented in conjunction with a discussion of appropriate impedance matching techniques. Experimental characterization of a proof-of-concept SLUG amplifier designed with a single-stub tuning circuit show modulation of the SLUG operation frequency in line with biasing of the SLUG, a well-matched input network and forward gain of 8 dB.
The Superconducting Low-inductance Undulatory Galvanometer (SLUG) is a linear, phase-sensitive, quantumlimited amplifier that can amplify ultra-weak microwave signals at frequencies up to 10 GHz or potentially higher, such as the output from cavity-based searches for the axion, a dark matter candidate. The SLUG amplifier differs from the closely related Superconducting Quantum Interference Device (SQUID) amplifier in that the input signal is directly injected into the SLUG rather than being inductively coupled through a multiturn input coil. The direct injection greatly simplifies the SLUG's microwave circuit integration and modelling, whilst enabling potentially higher frequency operation. Here we present the design and modelling of a SLUG amplifier that can be realised from a single-layer of Nb, with nanobridge weak-links fabricated using either electron-beam (EBL) or focussed ion beam (FIB) lithography as the Josephson elements. We discuss the input and output impedance, the forward transimpedance, and the potential power gain assuming a matched source and load. We also discuss the necessary impedance matching of the SLUG to the characteristic 50 Ω on-chip microwave environment.
The Superconducting Low-inductance Undulatory Galvonometer (SLUG) microwave amplifier is distinct from a conventional SQUID amplifier in that the signal to be amplified is directly injected into the SQUID loop, allowing high frequency operation. Here, we discuss a planar SLUG amplifier constructed with nanobridge weak-links fabricated from a single niobium layer. DC characterisation of the SLUG gain element is presented for two nanobridge fabrication techniques. Measurements show that junctions fabricated from Ne focused-ion-beam, and from 100 keV electron-beam lithography, have significantly different DC characteristics with critical currents on the order of 100 μA and 1mA respectively. Both variants exhibit maximum forward transfer functions greater than 1.5 mV/Φ0, which is larger than those reported in Nb SIS-junction SLUGs. Theoretical modelling has shown that an amplifier containing a planar, nanobridge SLUG as a gain element is expected to exhibit gain exceeding 15 dB with large instantaneous bandwidth, and can be designed to operate well in excess of 20 GHz.
We describe a novel scheme for low-noise phase-insensitive linear amplification at microwave frequencies based on the superconducting low-inductance undulatory galvanometer (SLUG). Direct integration of the junction equations of motion provides access to the full scattering matrix of the SLUG. We discuss the optimization of SLUG amplifiers and calculate amplifier gain and noise temperature in both the thermal and quantum regimes. Loading of the SLUG element by the finite input admittance is taken into account, and strategies for decoupling the SLUG from the higher-order modes of the input circuit are discussed. The microwave SLUG amplifier is expected to achieve noise performance approaching the standard quantum limit in the frequency range from 5–10 GHz, with gain around 15 dB for a single-stage device and instantaneous bandwidths of order 1 GHz.
We describe a microwave amplifier based on the superconducting low-inductance undulatory galvanometer (SLUG). The SLUG is embedded in a microstrip resonator, and the signal current is injected directly into the device loop. Measurements at 30 mK show gains of 25 dB at 3 GHz and 15 dB at 9 GHz. Amplifier performance is well described by a simple numerical model based on the Josephson junction phase dynamics. We expect optimized devices based on high critical current junctions to achieve gain greater than 15 dB, bandwidth of several hundred MHz, and added noise of order one quantum in the frequency range of 5-10 GHz.
We describe a novel scheme for low-noise phase-insensitive linear amplification at microwave frequencies based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). Direct integration of the junction equations of motion provides access to the full scattering matrix of the SLUG. We discuss the optimization of SLUG amplifiers and calculate amplifier gain and noise temperature in both the thermal and quantum regimes. Loading of the SLUG element by the finite input admittance is taken into account, and strategies for decoupling the SLUG from the higher-order modes of the input circuit are discussed. The microwave SLUG amplifier is expected to achieve noise performance approaching the standard quantum limit in the frequency range from 5-10 GHz, with gain around 15 dB for a single-stage device and instantaneous bandwidths of order 1 GHz.
We describe the high fidelity dispersive measurement of a superconducting qubit using a microwave amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). The SLUG preamplifier achieves gain of 19 dB and yields a signal-to-noise ratio improvement of 9 dB over a state-of-the-art HEMT amplifier. We demonstrate a separation fidelity of 99% at 700 ns compared to 59% with the HEMT alone. The SLUG displays a large dynamic range, with an input saturation power corresponding to 700 photons in the readout cavity.