Roy Glauber

Roy Jay Glauber was born in New York City on September 1, 1925, into a family of modest means during the early months of the Coolidge presidency. His father was a traveling salesman, his mother kept the household, and Roy grew up moving between apartments in the Bronx and Manhattan as the family chased work through the Depression. He was the sort of child who took clocks apart to see what was inside, and the sort of student who tested into the Bronx High School of Science when it was a brand-new experiment in concentrating gifted teenagers in one building. He finished there at sixteen, won a scholarship to Harvard, and arrived in Cambridge in 1941 with a battered suitcase and no idea that the war about to engulf the country would also pick him up and carry him into one of the strangest chapters of twentieth-century physics.

Two years into his undergraduate degree, in the autumn of 1943, a recruiter approached him on Harvard’s campus. The recruiter was vague about the work and specific about the secrecy. Glauber was eighteen years old. He boarded a train, was met at a station in New Mexico, and discovered that he had been hired to compute the critical mass of fissile materials at what would later be revealed as the Manhattan Project’s Los Alamos laboratory. He worked in the T-Division (the theoretical group) under Hans Bethe, alongside people whose names he had only read in textbooks. He carried out diffusion calculations on a mechanical calculator, watched the Trinity test from a hillside on July 16, 1945, and afterward returned to Harvard to finish the bachelor’s degree he had abandoned for the bomb. He was twenty.

His doctoral work, under Julian Schwinger, was finished in 1949 and concerned the quantum theory of electrodynamic processes. He spent a year at the Institute for Advanced Study in Princeton in the company of Oppenheimer, two years on the faculty at Caltech, and in 1952 returned to Harvard as an assistant professor. He would remain at Harvard for the rest of his career.

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The work that would eventually bring him the Nobel Prize began in the early 1960s and grew out of a specific puzzle. The laser had just been invented (Theodore Maiman’s ruby device fired in 1960) and physicists were arguing about what, exactly, was new about laser light. Was it simply very bright thermal radiation, or was it something fundamentally different at the level of the quantum field? The dominant theoretical language of the day, going back to Dirac in 1927, described the electromagnetic field as a collection of harmonic oscillators with discrete photon numbers. That picture was correct but it was clumsy for talking about coherence, the property that lets two light beams interfere to produce sharp fringes.

In a pair of papers in 1963, Glauber recast the problem. He showed that one could describe states of the quantized field using what he called coherent states, eigenstates of the photon-annihilation operator that had been introduced for the harmonic oscillator three decades earlier by Schrödinger and reinvented in 1961 by his Harvard colleague George Sudarshan. The coherent state turned out to be the closest a quantum field can come to behaving like a classical wave. Its amplitude has a definite phase. Its photon number is Poisson-distributed around a mean. Two coherent fields interfere exactly as two classical waves would. Glauber went further and built a rigorous theory of higher-order coherence (correlations of intensity, not just amplitude) which explained the famous Hanbury Brown and Twiss experiments of the 1950s in which two photodetectors looking at a star registered coincident clicks more often than chance would predict. His framework gave a sharp mathematical meaning to “coherent” and showed how truly nonclassical states, those without a classical analogue, would announce themselves through specific photon-counting statistics.

The Nobel Prize in Physics for 2005 was awarded to Glauber for his “contribution to the quantum theory of optical coherence,” with the other half of the prize shared by John Hall and Theodor Hänsch for laser-based precision spectroscopy. He was eighty years old. The announcement was, by then, a long-overdue recognition: every undergraduate quantum-optics textbook had been built on his 1963 papers for forty years. He flew to Stockholm, gave the Nobel lecture, and returned to Cambridge to keep teaching.

For two decades before the Nobel he had presided over one of the most peculiar traditions in academic science. The Ig Nobel Prizes, awarded each autumn by the Annals of Improbable Research for work that “first makes people laugh, then makes them think,” are handed out at a chaotic ceremony at Harvard’s Sanders Theatre. The audience throws paper airplanes onto the stage by the thousand. For many years the post-airplane cleanup was performed by Glauber himself, who served as the official “Keeper of the Broom” and swept the stage in academic regalia between segments, occasionally being interrupted by his own absent-minded commentary on the proceedings. He was a real Nobel laureate sweeping up paper at a fake Nobel ceremony, and the joke was multilayered enough that he never seemed to tire of it. He kept the broom routine going into his late eighties.

He died at home in Newton, Massachusetts on December 26, 2018, at the age of ninety-three. His students filled professorships around the world. His coherent-state formalism is now the standard language for everything from quantum communication and squeezed-light interferometry to the LIGO detectors that registered gravitational waves in 2015 (and which depend critically on quantum optics for their sensitivity beyond the classical limit). The quantum theory of optical coherence was a small, technical-sounding contribution that turned out to underwrite an entire branch of modern physics. Roy Glauber gave us the mathematics by which light, the oldest object of human study, finally became a quantum object we could compute with as casually as we compute with electrons.

If Dirac taught the electromagnetic field to count photons, Glauber taught it to interfere with itself like a wave again, in a language precise enough to power the lasers and detectors of the twenty-first century.