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SLUG microwave amplifier. (a) SLUG layer stackup. GND is the groundplane, JJ are the Josephson junctions, BE is the bottom electrode and TE is the top electrode. Ib is the external bias current, Idc establishes the quasistatic flux bias point, and Iμ w is the microwave signal to be amplified. (b) Layout of SLUG element, as seen from above (not to scale). (c) Schematic diagram of the SLUG microwave amplifier. A 50 Ω source is capacitively coupled to the SLUG via a microstrip transmission line with characteristic impedance Z 0 = 5.6 Ω and a bare quarter-wave resonance at 3.36 GHz. The transmission line is terminated in an inductive short to ground through the TE trace of the SLUG.
DC characteristics of the SLUG measured at 30 mK. (a) I-V curve of the device. The two traces correspond to a flux bias that maximizes (dash) and minimizes (solid) the critical current of the SLUG. (b) V-Φ curve of the device for three current biases: 16 μA (dash), 18 μA (solid), and 20 μA (dot).
Forward gain of the SLUG amplifier. (a) Gain at the first resonant mode for Ib = 20 μA and various flux bias points. (b) Amplifier gain at the second resonant mode for two different bias points.
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