From SLUGs to Macroscopic Quantum Phenomena

For my research at Cambridge University my thesis advisor, Brian Pippard, suggested a project requiring a voltage sensitivity of 10^-12 to 10^-13 VHz^-1/2. The state of the art was 10^-9 VHz^-1/2. Shortly after my arrival, Brian Josephson presented a seminar on his research. From the knowledge I learned from his talk, I invented the Superconducting Low-Inductance Undulatory Galvanometer, which consisted of a blob of PbSn solder melted around a piece of Nb wire. This enabled me to achieve a sensitivity of 10^-14 VHz^-1/2 and thus write my thesis. Early in 1968 I moved to the University of California, Berkeley. In 1980 John Martinis joined my group as a graduate student and in 1982 Michel Devoret joined as a postdoctoral scholar. We worked on a challenging question proposed by Tony Leggett: Do macroscopic variables obey quantum mechanics? This research resulted in the publication of two Physical Review Letters in 1985: Energy-Level Quantization in the Zero-Voltage State of a Current-Biased Josephson Junction and Measurements of Macroscopic Quantum Tunneling out of the Zero-Voltage State of a Current-Biased Josephson Junction. The combination of these two publications provided overwhelming evidence for the existence of energy quantization and macroscopic quantum tunneling in an electrical circuit.