Dc SQUID based on unshunted Josephson junctions: experimental results

It has been proposed that a new SQUID type be constructed by using unshunted Josephson junctions instead of resistively shunted junctions. Using this approach we are able to present a SQUID having a high output dynamic resistance and a device characterized by a negative output resistance and an extremely low noise. The first mentioned is called a high-gain SQUID, the latter an unshunted SQUID. This article describes preliminary experiments obtained from the SQUIDs based on this concept. The iv characteristics are shown to resemble the theoretically predicted characteristics but the noise performance was higher than expected. Most likely the reason for the high noise was the imperfect damping of the SQUID in the frequencies near 1 GHz.<>