University of Rochester

Physicist Leonard Mandel, a Founder of Quantum Optics, Dies

February 13, 2001

Leonard Mandel, one of the world's leading physicists, died Friday, Feb. 9. He was 73.

Mandel, DuBridge Professor Emeritus of Physics and Optics at the University of Rochester, was one of the founders of a burgeoning branch of physics known as quantum optics-the study of the physics of light at its most fundamental level. Mandel was known internationally for his groundbreaking experiments on the nature of light and was the first to actually observe certain remarkable phenomena predicted by quantum theory.

Mandel's career spanned an era when the influence of quantum theory, which describes the behavior of matter at the subatomic level, expanded to touch upon every area of physics. Mandel was at the forefront, putting quantum theory through its paces and actually demonstrating in the laboratory some of the bizarre outcomes predicted by quantum mechanics.

It was Mandel who most elegantly demonstrated what Einstein once called "spooky action at a distance," the idea that any action has an effect on another, seemingly unrelated, action elsewhere-an idea physicists call "quantum entanglement." With a laboratory set-up as simple and disarming as his smile, Mandel showed that a change to one laser beam in his laboratory affected another, unrelated beam elsewhere in the laboratory. The experiment was one of the clearest demonstrations ever of the validity of quantum mechanics, whose bizarre implications even the most experienced scientists have difficulty accepting. Nevertheless, the theory has held up whenever physicists like Mandel have been clever enough to find a way to test it experimentally.

"Len systematically tested quantum theory, producing the finest experiments in the world to test the foundations of quantum mechanics," says longtime colleague and friend Emil Wolf, Wilson Professor of Physics and Optics at the University, who persuaded Mandel to join him in Rochester in 1964. "He was an excellent teacher and a first-class researcher, and he was truly one of the founders of quantum optics. He was also a kind man, and incredibly generous, praising my work when the reality is that he was a much better physicist than I."

Mandel laid claim to several achievements through a series of experiments from the 1950s through the 1990s. He and his students were the first to demonstrate the interference of single photons with themselves; first to demonstrate non-classical interference between two photons; and first to design and carry out an experiment to observe "photon anti-bunching," demonstrating that a single atom illuminated by a laser beam emits photons uniformly spaced in time like soldiers marching past in a column. His measurements of the statistical properties of light gave rise to the "Mandel formula," which addresses the topic of photon detection. In one experiment his team measured the time interval between the arrival of two photons more accurately than anyone had ever done before. The team worked constantly at the boundary between classical and quantum physics, says Ian Walmsley, professor and director of the University's Institute of Optics.

"Over all the years Len was able to put his finger precisely where the boundary lay between classical and quantum physics, in a very clear way. His experiments were models of simplicity and elegance. It was the way you would do the experiment if you were as smart as someone like Len Mandel," says Walmsley.

"He came up with incredibly ingenious ways of identifying simple situations in which the quantum result was just completely at odds with your intuition. The elegance and simplicity of those experiments really caught people's imaginations. He knew how to ask the right questions."

Mandel's work helped form the basis for a growing field of knowledge that today includes quantum encryption, quantum computing, and quantum communications-radically new ways to process, transmit, and retrieve information that offer super-secure protocols and extraordinary power. Several research centers around the world owe their existence to initial work by Mandel, including the University's Center for Quantum Information.

Mandel's experiments typically employed laser beams and simple equipment to split the beams and bounce them in various directions in the laboratory, along with sensitive equipment to measure the results. His experiments with photons even earned a nickname: "quantum pinball."

Using such a set-up 10 years ago, Mandel showed that the Heisenberg Uncertainty Principle is more subtle than had previously been thought. By checking interference patterns of photons, Mandel showed that just the possibility of making a measurement is enough to change the outcome of an experiment, whether or not anyone actually makes a measurement.

Four years later, Mandel demonstrated unequivocally in the laboratory a central tenet of quantum mechanics, that an event is not real until it has been measured. In other words, no matter how much is known, what will happen next can never be known with certainty. Mandel's experiment had its roots in a 1935 paper by Albert Einstein and other physicists which asserted that if an event can be predicted with certainty, the event is real, even if it is never measured. Backers of quantum theory demurred, with Niels Bohr, Werner Heisenberg and Max Born insisting that there is no reality until a measurement is made. Sixty years later, with careful study of photons in the laboratory, Mandel demonstrated with dramatic clarity that quantum theory is correct.

"Len took what seemed like an intellectual exercise and actually demonstrated that these were real phenomena with real consequences in the real world," says Nicholas Bigelow, who succeeded Mandel as the DuBridge Professor of Physics and Optics at the University.

Mandel earned his doctorate in nuclear physics in 1951 from the University of London, where he also earned undergraduate degrees in mathematics and physics. As a graduate student, he studied cosmic rays, and as part of his work he climbed high into the Alps to make measurements. It wasn't long before he began to wonder how his cosmic ray detectors actually worked; that curiosity led him to a career asking fundamental questions about photon detection and measurement. He was senior lecturer in physics at Imperial College of the University of London for nine years before joining the University of Rochester in 1964.

At the University, he was known as an outstanding teacher, training 39 doctoral students who permeate physics programs around the world. In 1990 one of his graduate students, Zhe-Yu Ou, was honored as one of the world's top graduate students by the New York Academy of Sciences, and in 1992 Mandel was awarded the University's Faculty Award for Graduate Teaching. Walmsley recalls Mandel's class on quantum optics as "the gold standard for what an advanced class should be. It's one of the classes that inspired my career here."

In addition to his work at the highest level of physics, Mandel also had a special interest in neophyte science students. He developed the University's first physics course geared specifically for non-science majors and taught that course regularly for 20 years, and was known for his gift explaining the most complex topics in ways anyone could understand. Mandel would quietly work his way across a blackboard, gently explaining the finer points of quantum mechanics to a room full of students who had never taken physics before. "Any student who appreciated the twinkle in his eye couldn't help but be inspired by him," says Carlos Stroud, professor of optics, who works the boundary between classical and quantum physics much as Mandel did.

Mandel was recognized with nearly all the top honors an optical physicist can accrue, including the Frederic Ives Medal and Max Born Award of the Optical Society of America, the Italian National Research Council's Marconi Medal, and the Thomas Young Medal from the British Institute of Physics. He was a fellow of the American Physical Society, the Optical Society of America, and the American Academy of Arts and Sciences.

A prolific author, Mandel wrote more than 300 scientific papers, including one with Wolf on the coherence properties of optical fields that became one of the most-cited articles published in physics. Mandel and Wolf also wrote the textbook Optical Coherence and Quantum Optics, a 1,166-page volume on the nature of light. The pair and others also have organized an ongoing series of Rochester Conferences on Coherence and Quantum Optics, considered one of the world's premier conferences in the field since the first one in 1960.

Mandel is survived by his wife, Jeanne, of Pittsford, a suburb of Rochester; daughter and son-in-law Karen and Len Hanson, son and daughter-in-law Barry and Christine Mandel, and four grandchildren: Roselle, Gregory, Julie, and Tracy, all of Penfield; and his father, Naftali Mandel of London.