When John Clarke became one of the first Darwin students in 1965, he had previously intended to take a break from Cambridge. Born in the city in 1942, he attended the Perse School on an academic scholarship, before completing his undergraduate studies at Christ’s College, where he was fortunate in being able to live in College for all three years. Feeling the time was right for a change of scene, he applied to both Oxford and Cambridge for his PhD. But fate had other ideas.
“It’s incredible how things work out sometimes,” he says now. “It turned out that the acceptance date for Oxford was two weeks after that for Cambridge, so I would have to turn down Cambridge and hope Oxford would accept me. I decided that was too big a risk, so I stayed in Cambridge. That was the best possible decision I could ever have made. It was a great turning point in my life.”
John’s research was at the Royal Society Mond Laboratory on Free School Lane, near the centre of Cambridge. Although he was still associated with Christ’s, he was no longer able to live in the College. Instead, John found himself sharing a house near Girton.
“I got a little bit fed up with having to cycle to and from the lab,” he recalls, adding, as only a true Cambridge native could claim, “especially because it was very uphill going towards Girton.”
After the first year, John learned that Cambridge’s first postgraduate College—and first College to admit both men and women—was opening on Silver Street, not far from Free School Lane.
“This really piqued my interest, so I went there to meet the Master, Frank Young. He was a very nice guy, and we chatted for a while about this and that, and I said that I was a little tired of having to cycle all the way out to Girton every night. He asked me about my research and then he looked at me and said ‘Well, would you like to join Darwin College?’ I instantly accepted his offer.”
John made the transition, becoming not only one of Darwin’s first students but also the first President of the newly formed Darwin College Student Association (DCSA).
“I lived in College for the entire two and a half years in this wonderful room in the Old Granary. Outside there was a balcony overlooking the Mill Pond. Darwin had a plentiful supply of punts, and I remember punting along the River Cam many times. It was just a great life.”
The lack of hierarchy between the students and Fellows made for a significant change after the 500-year-old traditions of Christ’s.
“One of the great benefits was that most nights I would have dinner in College. And at the time, I have a feeling there were more Fellows than research students. So we would enjoy our excellent dinner and wine and chat about all kinds of different things. It was a wonderful experience talking to these somewhat older, more knowledgeable people about your life in general and research in particular. I especially remember many discussions with Abe Yoffe, one of the Founding Fellows of Darwin. He and I stayed in touch for many years after I left Cambridge.”
During his time at the Mond Laboratory, John’s research was focused on experiments based on the Josephson Effect. The theory had been invented a couple of years earlier by Brian Josephson, who won the Nobel Prize in 1973. Both Brian and John were supervised by Professor Brian, later Sir Brian, Pippard (Cavendish Professor of Physics 1971 -1984).
“This was at a time when, although Josephson tunnelling had been observed experimentally, there were no well-developed methods to fabricate Josephson junctions. The basic structure consists of two superconductors that are “weakly coupled” – for example by being separated by a thin oxide layer – so that the maximum supercurrent (“critical current”) is very small. I investigated various techniques of making Josephson junctions, with limited success. These experiments often involved the use of niobium wire and tin-lead solder, both of which become superconducting when immersed in liquid helium at atmospheric pressure. Tin-lead solder has a very low melting point and is widely used to connect wires in electrical circuits with the aid of a soldering iron.”
The breakthrough came, in true Cambridge tradition, during afternoon tea.
“One day I went to have daily tea with other research students and several faculty members, and expressed my frustration at the difficulty of making a Josephson junction. I explained some of the methods I had attempted. Suddenly, one of my fellow students, Paul Wraight, looked at me and said ‘Why don’t you try melting a blob of solder on a piece of niobium wire? Niobium has a surface oxide layer.’ I rushed off to my lab to try his idea. Fortunately, I had some liquid helium left in the cryostat from my experiments earlier that day. I connected some measurement wires to the device and cooled it in liquid helium. And there was my Josephson junction! I was incredibly excited. I removed the device from the cryostat and put it on my desk. Brian Pippard came into my lab the next morning to ask how it was going, and I started to explain about the experiment I had carried out the night before. He looked at the device on my desk and said ‘John, it looks as though a slug crawled under the window last night and expired on your desk overnight’. And thus the SLUG was born.”
John subsequently discovered that, with a current between the solder and the wire to develop a voltage, this voltage oscillated as a function of the current through the niobium wire, enabling him to develop a voltmeter that was significantly more sensitive than any of its predecessors.
“After I moved to Darwin, I published a paper – my first – on this voltmeter that attracted a great deal of interest. SLUG was interpreted as ‘Superconducting Low- inductance Undulatory Galvanometer’. In the summer of 1966, I attended my first conference – in Moscow – to give a talk about my voltmeter. During my visit, I vividly remember watching a performance by the Bolshoi Ballet–the first time in my life I had seen a ballet. I spent the remainder of my time at the Mond and Darwin developing the SLUG, which eventually lead to the thin film Superconducting Quantum Interference Device (SQUID).”
John took up a postdoctoral position at the University of California, Berkeley in 1968, becoming an Assistant Professor in 1969. He remains there today as a Professor of the Graduate School.
“During my career I’ve developed many different applications involving SQUIDs. I’ve worked on geophysics, developing a new technique to look for minerals under the surface of the ground by pulsing the ground with a large magnetic field. I became very involved in astrophysics projects because it turns out we could make very sensitive devices for looking at stars. I was also involved in medical physics and ultralow field magnetic resonance imaging. Two members of my group, John Martinis and Michel Devoret, and I showed that under appropriate conditions macroscopic circuits obey quantum mechanics. One of my current projects, with many colleagues, is searching for the axion, a candidate particle for cold dark matter. Something I’ve always been really happy about is the fact that I could take the basic idea of the SQUID and apply it to many very many fields.”
While his enormously distinguished career has largely taken place on the far side of the Atlantic, John still credits his time at Darwin with setting it in motion.
“Darwin had a huge impact on my life.”
