Daniel Kleppner

Daniel Kleppner, admired by atomic physicists for his pathbreaking research, leadership, teaching, and extraordinary service to the community, died on 16 June 2025, shortly after collapsing at a Father’s Day celebration with his family.

Denied tenure at Harvard, Dan came to MIT in 1966, starting a small atomic physics group that grew to become ranked first or second in US News and World Report’s rankings for the last two decades. Dan (and Dave Pritchard) mentored four Nobel Prize winners: Eric Cornell, William Phillips, Carl Wieman, and Wolfgang Ketterle, who has been an MIT professor since 1993. This group, together with their alumni, led advances that define atomic, molecular, and optical (AMO) physics today: atomic frequency standards, slow and trapped atoms, Bose–Einstein condensation, quantum information, atom interferometry, precision measurement, and the use of atomic physics to address fundamental problems in quantum mechanics and condensed matter. Dan was the founding director of the MIT–Harvard Center for Ultracold Atoms, an NSF Physics Frontier Center.

Dan was cherished for his good judgment, a sense of fairness that included sharing latest results with competitors, and his popular writing and speaking—all embellished with his eloquent and playful use of the English language. His thesis title, “The Broken Atomic Beam Resonance Experiment,” described the usual state of thesis experiments and also that a beam of cesium atoms retained its coherence after bouncing off surfaces. This result enabled Dan and his thesis adviser, Norman Ramsey, to invent the hydrogen maser, the first frequency standard based on trapped atoms and so far the only atomic frequency standard in which the atoms directly generate the reference signal. The maser engendered a lifelong love of hydrogen, as Dan himself admitted: “For me, hydrogen holds an almost mystical attraction.”

Overcoming Dan’s attraction to hydrogen, Rick Freeman, Ted Ducas, and Mike Littman convinced him to shift his attention to hydrogen’s cousin—the alkali Rydberg atom—which may be thought of as a one-electron atom with a core (rather than a bare nucleus). Dan’s lab pioneered the field of Rydberg atoms, demonstrated a clever way of making them in circular states—those with the highest possible angular momentum—studied their Stark structure (made richer by their cores), and demonstrated state-dependent field ionization, setting Rydberg atoms up as a new laboratory for fundamental physics.

Schrödinger’s equation is linear, hence does not exhibit chaotic solutions even for potentials characterized by chaotic motion in the classical limit. This raises the question: What are the quantum precursors to classical chaos? Dan’s group beautifully confirmed Eugene Wigner’s prediction of relatively evenly spaced quantum levels of the underlying quantum system and discovered additional unanticipated regularities in the spectra of Rydberg atoms in a strong magnetic field that stimulated many theoretical studies.

Dan is the pioneer of what he called “wrecking the vacuum,” the idea broached in “Inhibited Spontaneous Emission ” (1981), showing that spontaneous emission is not a God-given right of excited atoms but can be suppressed (or enhanced) by a nearby resonant cavity. This was demonstrated experimentally in “Inhibited Spontaneous Emission by a Rydberg Atom ” (1985). These two works are foundation stones for the field that in their 1989 Physics Today article Serge Haroche and Dan called cavity quantum electrodynamics—now an active field worldwide, a platform for quantum information processing with atoms and light, and the progenitor of photonic bandgap materials in which light propagation is forbidden at certain frequencies.

Motivated by William Stwalley and Lewis Nosanow’s 1976 proposal that cold polarized hydrogen (↓H) might form a Bose–Einstein condensate (BEC), Dan and Tom Greytak explored confining ↓H in a container coated with superfluid helium before employing a magnetic trap (for ↑H). Their lab invented evaporative cooling, which was eventually used, along with laser cooling, to produce a BEC in alkali atoms (1995). Their experiment still not working, Dan once compared their efforts to Moses, “who showed the promised land to his people but never reached it himself.” Ultimately (1998) they succeeded, with Dan receiving a standing ovation at the annual international BEC conference acknowledging his 20-year-long quest and his seminal role in the field.

Asked his profession, Dan consistently replied “teacher,” which certainly includes his textbooks: Quick Calculus (Kleppner and Ramsey, 1965) helps students learn a working knowledge of calculus, and An Introduction to Mechanics (Kleppner and Kolenkow, 1973) gives students—and their professors—a rigorous appreciation of introductory Newtonian mechanics. This textbook is famous for its in-depth explanations and examples and is frequently used at selective institutions for the advanced course in freshman mechanics.

Dan the “teacher” extended the usual expectations of professorship to include activities that directly helped students toward impactful careers. He co-created two summer programs to help young scientists move forward in their careers: JAMS (created with Heisuke Hironaka of Harvard) to bring together young scientists from Japan and the US for a week combining presentations, thoughtful reflection, and lively discussions, designed to plant the seeds of long-lasting international scientific relationships, many of which still flourish; and TOPS (Teaching Opportunities in the Physical Sciences, with Ted Ducas of Wellesley College), developed to give undergraduates interested in teaching physics at the precollege level a chance to practice the teaching of real students, often recruited from underserved populations.

As a capstone to his leadership of the MIT AMO group, Dan led the principal investigators from MIT and Harvard in building the Center for Ultracold Atoms, of which he was a founding father and the leader for the first decade—indeed, he volunteered to remain as its director and later codirector even after his formal retirement from MIT. He also led the physics department’s division of atomic, plasma and condensed matter physics and was associate director of the Research Laboratory for Electronics at MIT.

At the American Physical Society, he served on the executive committee, as counselor, on the Panel on Public Affairs, and as head of DAMOP (the APS Division of AMO Physics), and he chaired the Physics Planning Committee, the Study Group on Boost-Phase Intercept Systems for National Missile Defense, and the Forum on the History of Physics. He also served on the National Academy of Sciences Board on Physics and Astronomy and played a major role in the National Research Council’s Physics Survey Committee, authoring the section on atomic physics and editing the other sections. He also chaired the International Union of Pure and Applied Physics’s Committee on Fundamental Constants and served on the C15 Commission on Atomic and Molecular Physics and Spectroscopy.

At Williams, a liberal arts college, Dan discovered a love of and an ability for writing, developing a voice that was humane, gentle, and wryly humorous while being informative, clear, and precise—an ability that greatly benefited his career in research and service. His enjoyment of writing led him to become a physics blogger long before the internet. He wrote about 30 Reference Frame articles for Physics Today and responded to the related letters to the editor. These are collected here and cover a gamut of topics of interest to physicists and even wider audiences, including politicians. He loved the history of science and those who did it. Examples are his Reference Frames on “The Yin and Yang of Hydrogen,” “About Benjamin Thompson (Count Rumford),” “The Master of Dispersion” (Fraunhofer), and “Master Michelson’s Measurement.”

His articles often featured original sources (Did you know that Michelson transferred time from the National Observatory to his speed-of-light experiment using a temperature-stabilized tuning fork?), which often led to speaking engagements, including his last public talk (2023) “Otto Stern’s Magical Decade ” at the Pontifical Academy’s Quantum Science and Technology conference.

Important theories or experiments were clearly presented in “The Gem of General Relativity” (pulsar decay rate), “Memoirs of Schrödinger’s Cat,” “Professor Feshbach and his Resonance,” “Rereading Einstein on Radiation,” “With Apologies to Casimir,” and “Hanbury Brown’s Steamroller,” and they often included a historical perspective on precision measurement, as in “On the Matter of the Meter” and “Time Too Good to Be True.” Some articles elegantly explained interesting and impactful developments in small science: “Cavity Quantum Electrodynamics” (with Haroche), “Where I Stand” (GPS), “The Most Tenuous of Molecules,” “The Fuss About Bose–Einstein Condensation,” “A Beginner’s Guide to the Atom Laser,” “MRI for the Third World,” and “A Passion for Precision” (the Rydberg).

He also wrote about his intellectual home—the university—in “The Choice” (founding of Leiden University) and “Kudos for a Radical College” (Collège de France, est. 1530). He noted that the figures representing mens and manus (Latin for “mind and “hand) in MIT’s logo faced away from each other, symbolizing the challenge of balancing theoretical and experimental science and technology in a university.

A common theme of many of these is the huge scientific and technological payoff from small-scale university research in small laboratories. Coupled with a plea for funding such activities, “Night Thoughts on the NSF,” and his testimony before Congress, Dan became an unofficial spokesman for small science, a small counterbalance to the professional publicity staffs at large scientific installations. In today’s scientific climate, we should remember his prophetic words from the last century: “Any scenario for a decent future of our nation and the world must include a reasonable component of science that is devoted to the search for new knowledge. We cannot afford to abandon this vision under a barrage of criticism, no matter how eloquent or powerful the critics.”

Dan won the Davisson–Germer Prize (APS, 1986), William F. Meggers Award (Optical Society of America, 1991), Wolf Prize in Physics (2005), Benjamin Franklin Medal in Physics (2014), Senior BEC Award (2015), and APS Medal for Exceptional Achievement in Research (2017) for AMO research. As appreciation of the significance and impact of his science, teaching, and leadership, he received a number of general awards, including the Julius Edgar Lilienfeld Prize (APS, 1991), James Rhyne Killian Jr Faculty Achievement Award (MIT, 1995–96), Oersted Medal (American Association of Physics Teachers, 1997), Leo Szilard Lectureship Award (APS, 2005), and the National Medal of Science (2006).

But none of these awards celebrates the fullness of Dan’s manifold achievements as perfectly as a special one-time-only award from the DAMOP executive board. Purchased in a sporting goods store, the award is a gold-plated statue of a baseball player with bat at the ready and the inscription: “To Dan Kleppner who went to bat for Atomic Physics.” It solidified Dan’s sobriquet as “Mr. Atomic Physics.”

Dan was born in New York City in December 1932. His mother, Beatrice (Taub) Kleppner, instilled her love of literature in Dan, while possibly his economy of language came from his father, Otto, who ran a small advertising agency in New York City. Dan loved science toys, building boats, and sailing his Snipe sailboat, in which he became the junior sailing champion of Long Island Sound. His interest in science was kindled by his high school physics teacher, Arthur Hussey, who allowed Dan to work in his laboratory during a football team rally, causing the principal to admonish Dan for “lack of school spirit.” This experience was mirrored at Williams by the president, who extolled to Dan the merits of fraternity life when Dan met with him to argue that fraternities were anti-intellectual. Dan became a physics major with an unofficial minor in English, graduated in three years, and continued his education at Cambridge University (on a Fulbright scholarship).

While onboard the SS United States on his way to Cambridge in 1954, Dan met the vivacious Beatrice Spencer, whom he married in 1958. They made their Belmont and Vermont homes welcoming places for Dan’s growing AMO groups—even expanding their house as needed—creating an indelibly warm home for the entire Cambridge AMO community. Beatrice survives Dan, along with their three children: Paul and Andrew, who are professionals in computer-related businesses, and Sofie, who is associate vice provost at Stanford, and six grandchildren.

In keeping with Dan’s commitment to helping others to improve the future, his last words were a toast to his grandson, recently graduated from high school: “To Darwin and all youth who have new and exciting ideas.” Moments later, he collapsed and was taken to Stanford Medical Center to no avail. His death brings to mind one of his bon mots: “You get but one life to live—if that.” We can all salve our grief by reflecting on Dan’s life, which was filled with science, teaching, writing, and service that shaped a field and a generation of atomic physicists. And his voice—that reverberates in the stability of a hydrogen maser, the quantum coherence of a Bose–Einstein condensate, and the wry humor of a well-turned Reference Frame article.

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