In the early hours of 9 February 2001, the physics community lost one of its most distinguished members, Leonard Mandel, after a long illness. Mandel was the Lee DuBridge Professor Emeritus of Physics and Optics at the University of Rochester, having become emeritus only a few months before he died, at the age of 73, at his home in Pittsford, New York.
Born on 9 May 1927 in Berlin, Germany, where his father had emigrated from Eastern Europe, Mandel received a BSc in mathematics and physics in 1947 and a PhD in nuclear physics in 1951 from Birkbeck College, University of London, in the UK. He became a technical officer at Imperial Chemical Industries Ltd in Welwyn, UK, in 1951. In 1955, he became a lecturer and, later, senior lecturer at Imperial College, University of London. He remained at Imperial College until 1964, when he joined the University of Rochester as a professor of physics.
Mandel is widely credited as being one of the founding fathers of the field of quantum optics. Although he made seminal contributions across most of quantum optics, the central theme of his research was the exploration of the nature of light through insightful theoretical analyses and a set of pioneering experiments that have become landmarks in the field. Not since the beginning of quantum mechanics has an individual so intimately investigated and so dramatically advanced our understanding of the quantum aspects of light.
Mandel’s early interest in quantum optics was stimulated by the Hanbury Brown–Twiss experiments, in which correlations in the fluctuations of photocurrents from two photodetectors illuminated by light from a distant source reveal information about the spatial coherence properties of the light. Mandel’s entrance into the debate about the appropriate theoretical description of this experiment was a basic paper published in 1958 in the Proceedings of the Physical Society (London) in which the so-called Mandel formula for photoelectric counting made its first appearance.
In 1968, Mandel’s research group performed the now famous experiment involving interference at the single photon level: Two beams derived from independent lasers were attenuated to a level such that the average interval between photon emissions was very long compared to the transit time through the apparatus. Interference fringes were nonetheless recorded. As in the Hanbury Brown–Twiss experiment, the detection scheme explored interference of the fourth order in the amplitudes of the fields, as opposed to the more familiar second-order interference (as in a Michelson interferometer, for example), but now with an ingenious extension. Over the next three decades, Mandel continued to refine fourth-order interferometry into a powerful tool for investigating the quantum nature of light. It is now a standard technique used worldwide.
Perhaps spurred by the debate about the quantum statistical properties of lasers and other light sources, Mandel and his students were among the first to carry out basic experiments from 1967 to 1968 to elucidate the coherence properties of the laser in the transition from incoherent to coherent emission around threshold. In the ensuing decades, those early studies grew into diverse investigations of more complex laser behavior, including mode correlation and competition in ring lasers, and optical bistability and first-order phase transitions in dye lasers.
Mandel played a leading role in the quest to investigate states of the electromagnetic field with manifestly quantum or nonclassical characteristics. In 1977, his group made a historic advance in this endeavor with the first observation of photon anti-bunching in the case of the fluorescent light from a single atom. Subsequently, Mandel and one of his students made the first measurements of another nonclassical effect, namely, sub-Poissonian photon statistics.
In the early 1980s, Mandel initiated what would become a landmark set of experiments involving pairs of photons produced by the fission of a pump photon in the process of parametric down conversion. Following earlier work by others on the correlation between photon pairs in atomic cascades, Mandel and his students first demonstrated a key nonclassical aspect of the down conversion process and were able to achieve a localized one-photon state.
Pursuing the theme of fourth-order interference with pairs of photons in the late 1980s, Mandel and his students demonstrated quantum spatial beating, violations of local realism, and phase memory due to quantum entanglement with the vacuum. His group showed that P. A. M. Dirac’s well-known statement about single-photon interference must be modified to assert that, in fourth-order interference, a pair of photons interferes only with the pair itself. He and his students also introduced what became known as the Hong-Ou-Mandel interferometer.
Beyond what had by then become “the standard model” of manifestly quantum effects, Mandel’s genius for probing the quantum realm led to advances throughout the 1990s that challenged and ultimately redefined our understanding of the quantum character of light and, more generally, of quantum measurement. For example, in 1991, Mandel and his students reported a remarkable experiment involving interference from the superposition of fields from two different but coherent parametric processes. They showed that the mere possibility of making a measurement destroyed an interference pattern, even if this possibility went unrealized. In 1995, Mandel’s group, in an experiment with photon pairs, demonstrated a central tenet of quantum mechanics: that there is no physical reality for elemental quantum processes in the absence of a measurement.
Mandel supervised the thesis research of 39 students, many of whom have subsequently become leading figures in science and technology. He earned the lifelong respect of his students for his detailed and conscientious stewardship of their scientific development. He made frequent rounds of his laboratories to interact with each of his students and to provide help ranging from optical alignment and electronic design to theoretical calculations. Not wanting to interrupt a student at work, he would sometimes quietly approach in his soft-soled shoes to view the activity at hand, leading to more than a few startled heart skips. In response to a student’s query, “Can you see a signal yet?” as some component of an apparatus was being aligned, would come his distinctively British reply, “Not a sausage!” Once, when the need arose to regulate the water cooling for an oil diffusion pump, he solved the problem the next morning with a flow switch commandeered from a washing machine at home.
In addition to his work at the frontiers of physics, Mandel was an excellent teacher. He had a special interest in neophyte science students and developed the first course at the University of Rochester for non-science majors, which he taught regularly for 20 years. From 1966 to 1995, he was one of the main organizers of a well-known series of international conferences called the Rochester Conferences on Coherence and Quantum Optics.
Mandel was the coauthor of Optical Coherence and Quantum Optics (Cambridge U. Press, 1995) and the coeditor of several conference proceedings. In 1980, a paper on coherence properties of optical fields, which he coauthored in 1965, was designated a Citation Classic by the Institute of Scientific Publications. In 1988, that paper was listed as one of the 100 most-cited articles published in the Review of Modern Physics since 1955. Mandel received many top awards, including the Frederic Ives Medal (1993) and the Max Born Medal (1982) from the Optical Society of America. He received the Marconi Medal (1987) from the Italian Research Council and the Thomas Young Award (1989) from the British Institute of Physics. In May 2001, Mandel was elected posthumously to membership in the National Academy of Sciences.
Mandel was a scientist with a warm personality, humility, compassion, and a delightful sense of humor. He enjoyed a happy family life with his wife Jeanne, a ballet teacher with whom he shared the love for ballet, and with their two children. He adored his four grandchildren and took lively interest in all their activities. Those of us who were fortunate to have been associated with Mandel as students, colleagues, or friends will remember him with deep affection and utmost respect. He will be greatly missed.
