A Josephson transmission line buffer for reducing leak current in a high- Tc superconductor sampler

Abstract

We are developing a sampler circuit, which is the main part of a sampler system for measuring repeated electrical signal waveform with high-resolutions, for a digital application of a high- T c superconductor. The sampler circuit consists of three parts: a pulse generator, a comparator and a readout SQUID. The three parts are connected by short superconducting lines. Signal current to the comparator and trigger current to the pulse generator are distributed to other parts by parasitic inductances. The distributed current reaches other parts (leak current) and causes jitter, waveform distortion and noise in sampler operation. In order to reduce the leak current, we inserted Josephson transmission line (JTL) buffers between each part. The leak current is gradually decreased by flowing through the JTL. It was experimentally confirmed that a two-stage JTL buffer between the comparator and the readout SQUID reduced signal current leakage to the readout SQUID from 34% to 7.7%. A sampler circuit including two six-stage JTL buffers operated properly in the measurement of 7-GHz sinusoidal wave.