Melvin Lax, a pioneer in the field of quantum optics, died of cancer on 8 December 2002 at his home in Summit, New Jersey.
Born in New York City on 8 March 1922, Lax received his BA in physics from New York University in 1942. He then attended MIT, where he earned two degrees in physics: his SM in 1943 and his PhD in 1947. From 1942 to 1945, he carried out research at the MIT Underwater Sound Laboratory under the direction of Philip M. Morse, Richard H. Bolt, and Herman Feshbach.
After receiving his PhD, he took the post of assistant professor of physics at Syracuse University, where he advanced to full professor. In 1955, he joined Bell Telephone Laboratories in Murray Hill, New Jersey, where he worked as a member of the technical staff. During 1962–64, he headed the theoretical physics research department there. He left Bell Labs in 1971 to join the faculty at City College of New York as a Distinguished Professor of Physics, but continued as consultant to Bell Labs’ solid-state electronics research laboratory.
Lax’s contributions underlie many aspects of present-day design of optical communication devices. In 1963, he developed a quantum regression theorem and a quantum theory of non-commuting noise sources. His results permitted the calculation of laser spectral line shapes, general photocount distributions, and similar observables. He had a deep and lifelong interest in random processes. Before his death, he had been completing a monograph entitled Random Processes (Oxford U. Press, in press) and had been writing a text on modern quantum optics. He applied the separation of signal and noise to different condensed matter systems. That approach led him to analyze how impurities that are randomly placed in a semiconductor medium or that have randomly varying interaction strength affect the energy levels and hence the medium’s conduction properties. The approach also formed the basis for the so-called coherent potential approximation. Late in life, Lax studied the scattering and transmission of laser light through turbid media such as human tissue. He aimed to devise noninvasive optical diagnostic tools for breast cancer without the drawbacks associated with x-ray use.
In 1962, Lax developed a technique to obtain symmetry-related selection rules, including time-reversal, that govern the “go/no-go” alternatives for physical processes—for example, scattering or optical transitions—in crystalline solids. In 1971, with Donald Nelson, he considered the structure of the photoelastic tensor response of a crystal—a problem that had been dormant for more than 80 years—and found that a new physical effect, the antisymmetric microscopic rotations of the atomic constituents, had to be included to explain the experiments. Scientists going back to Friedrich Pockels (in 1893), a founder of the study of optical properties of matter under external fields, had assumed that those effects vanished. As was typical of Lax’s approach, the papers Lax and Nelson subsequently produced were detailed and comprehensive, covering the range from microscopic atomistic force models to macroscopic measurable consequences.
The interplay between electronic and vibrational degrees of freedom was a theme that, like random processes, bridged Lax’s earliest work with work he did late in his career. Of particular interest was how that interplay determines the absorption and emission of light and the electrical resistivity of material systems. In his early (1952) papers on the Franck-Condon energy in bulk crystals, he meticulously analyzed the subtle electron-phonon effects. Forty years later, with Wei Cai, he invoked those effects to explain how out-of-equilibrium “hot” electrons and “hot” phonons can interact to give negative carrier mobilities in quantum well, or heterostructure, systems. This was a counterintuitive result.
Lax served the physics community in many roles. As a member of the American Physical Society’s publications committee from 1980 to 1982, he was instrumental in helping to create a system for electronic submission and processing of manuscripts. He was ahead of his time in recognizing that he should master computers so he could carry out theoretical physics research. He became so expert in computers that one of his early papers on evaluating Fourier integrals was published in Mathematics of Computation in 1982. He also was on the board of editors of the Physical Review, Quantum Optics , and the International Journal of Modern Physics.
In the university setting, Lax had a lasting effect: His students will remember him with admiration, respect, and gratitude. He prepared his lectures with exceptional care; for his advanced courses, he frequently prepared extensive and detailed notes that he himself photocopied and distributed. He gave his time generously to students, colleagues, and others with whom he freely and graciously shared his ideas and insights. He served the City College of New York system on the University Committee on Research, for which he was the university physics liaison.
Beyond the campus, Lax served from the mid-1980s as a member of the National Academy of Sciences’ basic research advisory committee, which evaluated research proposals. He provided scientific services also to the Naval Research Laboratory, Los Alamos National Laboratory, the US Army Research Office, and the Department of Energy.
In 1999, the Optical Society of America awarded Lax the Willis Lamb Medal for his work in quantum optics.
Lax was an irreplaceable member of the CCNY physics department. An eminent physicist with an international reputation, he was also an exceptionally kind and gentle human being and a caring friend. His colleagues share a profound sense of the loss of a unique scientist and are grateful to have had the opportunity to work with him.
