Daniel Frank Walls (1942–1999)
Text by Crispin Gardiner
Dan Walls, like Jack Dodd, was born in Hawkes Bay, but in Napier rather than Hastings. Even though I was myself was born in Hastings only a month later, he and I first met in Auckland, in form 3A at Auckland Grammar School in 1956. For the next five years we were rivals (in the atmosphere of competition still in favour in that institution) for the top places in physics, science and other subjects. We were then both students in the Physics Department of Auckland University from 1961 to 1965, and both of us then left, after completing MSc degrees, for foreign parts, he to Harvard, and myself to Oxford.
In the early seventies, after five years of doctoral and post doctoral work, we both returned to New Zealand, and by 1972, were both senior lecturers in the University of Waikato. The science departments were established there only in 1970 - hence our job was to set up theoretical physics there. As eager young Turks, we set about doing this withan enthusiasm which did not always meet with approval from the local establishment.
Dan had been a student of Roy Glauber, one of the winners of the 2005 Nobel Prize in physics, who set up the concepts and formalism of Quantum Optics, the study of those aspects of optics in which quantum mechanics played a dominant role. Jack Dodd’s work on quantum beats was a pioneering piece of work in this field, done before the introduction of the laser as a practical experimental tool. Glauber’s work formulated the concepts necessary to describe the physical phenomena now available for study because of the availability of the intense coherent monochromatic light produced by lasers. Dan had also spent a postdoctoral year in Stuttgart with Hermann Haken, one of the pioneers of the quantum theory of the laser, and arrived in New Zealand well equipped to spread the word here. My work overseas had been in particle physics, but like Jack Dodd, I felt something else was appropriate for New Zealand, and I joined Dan in his research.
In the first few years in Waikato we branched out into nonequilibrium thermodynamics of chemical reactions, a field of study that had many formal similarities to the methods of quantum optics, and which led to the formation of significant international connections. However, we soon moved more seriously back into quantum optics, especially after Howard Carmichael arrived from Auckland University to work on a D Phil with Dan and myself as 1st and 2nd supervisor. Dan first became prominent in quantum optics in his prediction, with Howard Carmichael, of the existence of photon antibunching in the spectrum of resonance fluorescence, and thus initiated the study of non-classical light as an experimentally realizable subject.
During this initial period John Harvey, also a graduate of Auckland University who had done doctoral studies overseas took up a postdoctoral fellowship in Auckland University. John had worked on theoretical nuclear physics, but also decided a change was in order. He a Dan decided it might be a good idea to start work on biophotonics. Among other things this led to a joint research effort with the then Ruakura Agricultural research Station on laser measurement of bull sperm motility. Thus John became an experimental laser physicist, and ultimately a photonics researcher.
Dan later became very active in the theory of optical bistability, and in the formulation of proposals for the production of squeezed light. He, Matthew Collett, at that time doing research for his MSc thesis, and myself developed the crucial theoretical foundations required to describe the experiments on squeezed light, predicting correctly the full squeezing spectrum measured by the team of Jeff Kimble (then in Texas, now in Caltech.) After these achievements, the "New Zealand School" of quantum optics was firmly recognized, and we both wrote definitive books on Quantum Optics. Dan’s book was authored with Gerard Milburn, whom he had attracted from Australia to do doctoral work in Waikato, and who is now internationally famous as an expert on quantum information, and is the director of an Australian Centre of Research Excellence in Brisbane based on that subject.
In 1987 Dan took up the Chair of Theoretical Physics at Auckland University, while I stayed at Waikato, and we established a programme of ongoing joint meetings once a month, alternately in Auckland of Hamilton. The effect of Dan’s move was to double the size of the theoretical Quantum Optics effort in New Zealand.
Later Dan moved into theoretical atom optics, and in the last few years he was also active in the field of Bose-Einstein condensation. His ability to identify what the most important problem of the time, and to motivate research students (as well as other researchers from all round the world) to join in research progammes on such problems,was one of his most prominent characteristics. Typically, too, he could see that many problems could be reduced to very simple models, which were amenable to rapid solution.