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Daniel Kleppner, the Lester Wolfe Professor Emeritus of Physics at MIT whose contributions in experimental atomic physics left a profound influence on the discipline, passed away on June 16 at the age of 92, in Palo Alto, California.
Kleppner’s multifaceted research explored the relationships between atoms and static electric and magnetic fields along with radiation. His work encompassed precision measurements with hydrogen masers, including the co-creation of the hydrogen maser atomic clock; his investigations into the physics of Rydberg atoms and cavity quantum electrodynamics; and his groundbreaking contributions to Bose-Einstein condensation (BEC).
Retiring in 2003 after 37 years at MIT, Kleppner was a well-read and persuasive scientist whose meticulous research and communication abilities helped shape the trajectory of contemporary atomic, molecular, and optical (AMO) physics. He served as associate director of the MIT Research Laboratory of Electronics (RLE) from 1987 to 2000 and filled the role of interim director in 2001. Additionally, he co-established the MIT-Harvard Center for Ultracold Atoms (CUA) in 2000, where he served as co-director until 2006.
Although he was never honored with a Nobel Prize, Kleppner’s influence in the realm of atomic physics and quantum optics, along with his generous mentoring, facilitated the Nobel successes of many others. His patient and rigorous pursuit of knowledge provided essential research insights that led to significant breakthroughs. His extensive studies of the minuscule atom contributed foundational knowledge crucial for monumental advancements, such as the global positioning system (GPS), magnetic resonance imaging (MRI), and quantum computing.
“He was a prominent figure in the department, and a leader in the American Physical Society,” states Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT and a 2001 Nobel laureate. “He was a statesman of science. He was an articulate individual, a master of language who could convey ideas in memorable ways, while also embodying a sense of humility.”
“Dan Kleppner was a titan in the field of AMO physics, and in scientific circles more broadly,” remarks John Doyle PhD ’91, co-director of the Harvard Quantum Initiative and a mentee of Kleppner who assisted him in creating the Bose-Einstein condensate from atomic hydrogen. “His most significant legacy may be fostering a culture of respect and a supportive community that all scientists in the area of AMO physics benefit from today. Not only did his scientific contributions pave the way for current research avenues, but his kindness, intellect, and dedication to community service continue to shape the field of AMO physics. He was both a mentor and a friend to me.”
Kleppner’s daughter Sofie Kleppner mentions: “Individuals who worked on the early lasers never anticipated we would be scanning groceries at the checkout. When they developed the hydrogen maser, they were a group of enthusiastic individuals eager to grasp the nuances of Einstein’s theory of relativity. This was foundational for GPS, enabling timely flights. Our father firmly believed that fundamental research today could lead to a plethora of valuable developments in the future.”
Early life and career
Born in Manhattan on December 16, 1932, Kleppner was the progeny of Vienna native and advertising agency founder Otto Kleppner, author of the bestselling book “Advertising Procedure.” His mother, Beatrice (Taub) Kleppner, raised in New Jersey, graduated from Barnard College and assisted with Otto’s manuscripts. Daniel was the second of three children; his brother, the late Adam Kleppner, was a mathematics professor at the University of Maryland, while his sister, Susan Folkman, served as a research psychologist at the University of California at Berkeley.
“As a teenager, I simply enjoyed constructing things,” Kleppner once reflected. “That proved to be quite beneficial when I became an experimental physicist. I had a crystal radio that allowed me to listen to stations through earphones. The notion that the signals were just emanating from the atmosphere struck me as absolutely remarkable. In fact, it still does. The concept of the electromagnetic field, while well-understood in physics, still feels miraculous to me.”
During high school, he found inspiration from his physics teacher, Arthur Hussey, who encouraged Kleppner to spend countless hours in the labs. “Once, while the whole school was engaged in a pep rally and I was indifferent to cheering for football, I stayed and worked in the lab. The principal noticed my absence and called me in for a reprimand due to my lack of school spirit.”
He remained unfazed. Hussey engaged Kleppner in discussions about quantum mechanics, which “sparked my interest,” and introduced him to a bit of calculus. “Physics was incredibly fashionable back then, in the post-war years, as physicists were regarded as heroes for bringing an end to the war through the atom bomb and the development of radar.”
By that time, he felt “destined to dedicate my life to physics,” he expressed in a video interview for InfiniteMIT. “It was a time when it was easy to be captivated by physics, and I certainly was.”
While studying physics at Williams College, he became fascinated with Albert Einstein’s theory of general relativity. He constructed a programmable device he termed a precursor to cybernetics. Williams also instilled a lasting passion for literature within him, and he nearly chose an English major. However, he found the school’s fraternity culture to be “playboy” and “anti-intellectual,” prompting him to graduate rapidly within three years, in 1953.
Kleppner deferred his admission to Harvard University with a Fulbright Fellowship to attend Cambridge University, where he encountered the young physicist Kenneth Smith, who specialized in atomic beam resonance. Smith introduced him to Norman Ramsey’s book “Nuclear Moments” and shared a proposal by Ramsey’s advisor I.I. Rabi, who developed a technique capable of making an atomic clock precise enough “to observe the effect of gravity on time as predicted by Einstein,” Kleppner recalled.
“I found that utterly astonishing,” Kleppner reflected. “The idea that gravity influences time was difficult for me to visualize.”
When Kleppner strolled through Harvard’s corridors in 1955, he was thrilled to spot a door bearing Ramsey’s name. He was intrigued by Ramsey’s studies on molecular beam magnetic resonance, atomic clocks, and precision measurements. “Luckily, I arrived at a time when he had an opening in his research group,” Kleppner recounted.
A new atomic clock
As Kleppner’s advisor, Ramsey motivated him to invent a new kind of atomic clock, believing the existing cesium and ammonia masers, which employed amplified microwaves, lacked the precision necessary to assess the gravitational effect on time.
Kleppner’s dissertation focused on adopting the principles behind an ammonia maser to move towards a hydrogen maser, which utilizes the inherent microwave frequency of hydrogen atoms and amplifies it via stimulated emissions. Kleppner discovered that coherent cesium atoms could bounce off appropriately prepared surfaces without losing their coherence.
Following his 1959 PhD, Kleppner remained at Harvard, ascending to the role of assistant professor in 1962.
Kleppner’s investigations into hydrogen led to a technique that allows hydrogen atoms to be contained in a glass vessel for extended observation…
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of duration. The outcome, showcasing hydrogen atoms oscillating within a microwave chamber, is utilized to stabilize the frequency of a timepiece to a precision exceeding one microsecond annually.
In 1960, he and Ramsey effectively devised a novel atomic clock whose remarkable stability could validate the subtle effects of gravity on time, as foreseen by Einstein’s theory of general relativity.
The latest generation of optical clocks “are capable enough to detect the gravitational redshift for slight variations in height, which is quite remarkable and has produced extraordinary results,” remarked Kleppner. “We had to reevaluate what we truly understand by time.”
While the hydrogen maser did confirm Einstein’s hypothesis regarding time and gravity, it took over ten years before it gained widespread use, initially among radio astronomers. Nowadays, atomic clocks like the hydrogen maser are employed in fields demanding high short-term stability, such as synchronizing terrestrial timing systems that monitor global positioning satellites, as well as for timekeeping and communication at naval observatories to uphold a precise and stable time reference known as UTC (USNO); very long-baseline microwave interferometry (VLBI) that allows astronomers to achieve exceptionally high resolution and investigate distant radio sources, including black holes; and indirectly, in magnetic resonance imaging.
“When we initially set out to create these atomic clocks, our objectives were about as impractical as one could imagine,” Kleppner stated in an interview with the MIT Physics Department. “From being a fairly abstract concept that you’d like to observe, it transitions into a pressing requirement for managing human affairs.”
Ramsey later received the Nobel Prize in Physics in 1989 for his contributions through the separated oscillatory fields technique and its application in the hydrogen maser and atomic clocks.
MIT, ultracold gases, and BEC progress
Kleppner anticipated he wouldn’t achieve tenure at Harvard, “because despite Norman’s kindness and goodwill, he casts a long shadow, and it was beneficial for me to be at just the right distance. Upon arriving at MIT, I had a range of experiments I wanted to undertake, alongside some educational ideas I was eager to explore, making the transition very straightforward.”
Kleppner joined the Institute in 1966, and his Harvard PhD student (and current MIT faculty member post-tenure) David Pritchard followed him to work on scattering experiments: Kleppner focused on pulsed lasers, while Pritchard concentrated on continuous-wave (CW) lasers.
“He was youthful, articulate, and seemed to possess fresh ideas,” Pritchard reminisces. “We anticipated how crucial lasers would become. For a significant period, it was just Dan and me. That was indeed the time when lasers began to dominate. Dan and I ventured into lasers; he worked on Rydberg atoms, while I explored collisions and spectroscopy of weakly bound molecules and two-photon spectroscopy.”
Kleppner steered the dynamic MIT Atomic Physics Group to eventually be recognized as the US News and World Report’s top-ranked atomic physics group in 2012. “Dan was pivotal in this achievement,” Pritchard recalled. “To initiate from non-tenure and elevate it to the number-one ranked department in your subfield is a remarkable career accomplishment.”
The group evolved into what Pritchard referred to as “the supergroup” of laser innovators, including Charles Townes, who was awarded the Nobel for his contributions; Ali Javan, who established a prominent laser research center at MIT; and Dolly Shibles. Pritchard became a faculty member in 1970, and Ketterle joined in 1990 as his postdoc. “We were pioneers, and the result was our collective impact was significantly greater.”
“He’s not just the patriarch of the field; he is like a scientific father to me,” expresses Pritchard. “When I’m drafting something that isn’t going smoothly, I often ponder, ‘What would Dan suggest? What counsel would he provide?’”
Together with MIT low-temperature physicist Tom Greytak ’63, PhD ’67, Kleppner innovated two groundbreaking techniques — magnetic trapping and evaporative cooling. When the scientific community integrated these methods with laser cooling, atomic physics embarked on a significant new trajectory.
In 1995, a team of researchers, led by Kleppner’s previous students Eric Cornell PhD ’90 and Carl Weiman ’73, produced a BEC using rubidium atoms, while Ketterle succeeded with sodium atoms. For this accomplishment, they were awarded the 2001 Nobel Prize in Physics. Kleppner described BEC as “the most thrilling advancement in atomic physics in decades.”
During a BEC conference in 1996, Ketterle reminisces about Kleppner discussing his contributions: “’I feel akin to Moses, who guided his people to the Promised Land but never arrived himself.’ This was precisely what Dan accomplished. He revealed to us the Promised Land of Bose-Einstein condensation. He unveiled what was achievable … He was the godfather of Bose-Einstein condensation.”
However, he did indeed reach the Promised Land. In 1998, when only a few groups had successfully created BECs, Kleppner and Greytak achieved a hydrogen BEC. When he presented their findings at the summer school in Varenna shortly afterward, he received an enduring standing ovation — after 20 years of dedication, he realized his aspiration.
“It is ironic that when Dan initiated this research, hydrogen was the sole element capable of achieving the low temperatures necessary for BEC,” Ketterle notes. Nevertheless, it turned out that hydrogen possesses unique properties that rendered reaching BEC much more challenging compared to other atoms.
Rydberg atoms
In 1976, Kleppner was at the forefront of the Rydberg atoms field, a highly excited type of atom that shares the fundamental characteristics of hydrogen. Kleppner demonstrated that these states could be stimulated by an adjustable laser and easily identified through field ionization. He subsequently charted their responses in high electric and magnetic fields, providing new scientific insights into the links between quantum mechanics and classical chaos.
In 1989, his investigations into atomic energy levels under conditions where the associated classical motion is chaotic detailed the positions of thousands of quantum levels in relation to laser frequency and applied field via high-resolution laser spectroscopy. His findings offered new comprehension regarding the impact of classical chaos on quantum systems.
“I regard Dan as the originator of Rydberg atoms,” states Dan’s former student William Phillips PhD ’76, a physicist at the Institute of Standards and Technology (NIST). “Certainly, Rydberg atoms are phenomena that nature provides, but Dan was the individual who truly grasped that these could be utilized to achieve innovative and remarkable results.”
Such atoms have proven beneficial for examining the transition between quantum mechanics and classical chaos. Kleppner’s 1976 article on Rydberg atoms’ robust interactions, extended lifetimes, and responsiveness to external fields has propelled ongoing scientific inquiries and multimillion-dollar startups focused on developing the promising Rydberg quantum computer; highly precise measurements.
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of electrical and magnetic fields; and in quantum optics experiments.
“Primarily due to Dan’s groundbreaking blueprint, Rydberg atoms have emerged as atomic physics’ E. coli for exploring the interaction of radiation with matter,” wrote Ketterle in his nomination for Kleppner’s 2017 APS Medal for Exceptional Achievement in Research. “They are being utilized by others in attempts to develop experimental systems to actualize Schrödinger’s cat, as well as for constructing a quantum computer.”
In 1981, Kleppner proposed in a theoretical article the potential of inhibiting spontaneous emission through a cavity: excited atoms cannot decay when the cavity lacks the oscillatory modes to receive their emissions. This was succeeded by his demonstration of this phenomenon, which initiated the field of cavity quantum electrodynamics (cQED), the examination of how light confined within a reflective cavity interacts with atoms or other particles. This domain has led to advancements in new lasers and photonic devices.
“This research fundamentally transformed how physicists perceive the process of spontaneous emission by demonstrating that it is not an intrinsic feature of a quantum state, but can be adjusted and manipulated,” remarked Ketterle. “Current applications of these principles, which Dan refers to as ‘wrecking the vacuum,’ encompass thresholdless lasers and the development of photonic bandgap materials where light propagation is prohibited at specific frequencies.”
MIT-Harvard Center for Ultracold Atoms
In 2000, Kleppner obtained National Science Foundation funding to jointly establish the Center for Ultracold Atoms (CUA), a collaboration between MIT and Harvard that connected RLE with the Harvard Department of Physics to investigate the physics of ultracold atoms and quantum gases. Kleppner was its inaugural director until 2006 and was part of a group that included MIT professors Ketterle, Pritchard, Vladan Vuletic, Martin W. Zwierlein, Paola Cappellaro PhD ’06, and Isaac Chuang ’90.
“Many centers fade away after 10 to 20 years; sometimes their mission is accomplished,” states Ketterle, the CUA director from 2006 to 2023. “But considering the enthusiasm and the rapid progress in atomic physics, the CUA is a highly active center brimming with exhilaration, and we recently received renewed funding. That’s partly due to Dan’s contributions. He established the tradition of atomic physics at MIT. We rank among the finest atomic physics groups globally. And we truly are a family.”
Boost-phase intercept report
Kleppner co-wrote a notably impactful 2003 report that assessed the technical viability of boost-phase intercept, a concept central to President George H.W. Bush’s contentious Strategic Defense Initiative (SDI), nicknamed “Star Wars,” which claimed to make nuclear weapons obsolete. The primary focus of the APS Study on Boost-Phase Intercept for National Missile Defense, published as a special supplement to Reviews of Modern Physics, was on the physical and engineering challenges of intercepting a missile during its boost phase.
“This was a matter on which I had no technical background whatsoever,” Kleppner recollected, expressing his appreciation for the expertise of co-chair Fred Lamb from the University of Illinois. “But the APS [American Physical Society] believed it was crucial to provide information for the public … and no one had any knowledge about it. It was a moment in my life where I felt I could contribute. And I believe there’s an obligation to act when the need arises and you have the ability.”
The outcome? “Technically, it would not succeed, except under very limited conditions,” Kleppner remarked. Added Pritchard, “It drastically altered the nation’s trajectory.”
“He was the ideal individual to lead the committee,” states Ketterle. “He was exceptional in maintaining neutrality and objectivity, and in producing a straightforward report. I believe the APS was very proud of this document. It demonstrates how physicists analyze issues that were at that time of tremendous political and societal significance. This report clarified what laser weapons can and cannot achieve. The gradual disappearance of (SDI) may have been influenced by this report.”
Devoted educator
Kleppner has educated generations of physicists, including serving as an advisor to 23 PhD students who have progressed to secure positions at prestigious universities and achieve significant scientific accolades.
He received the Oersted Medal from the American Association of Physics Teachers in 1997, and earned the Institute’s esteemed 1995-1996 James R. Killian, Jr. Faculty Achievement Award for his service to MIT and society in the realm of atomic physics. “He has generously devoted his time and effort to shaping national science policy, and has served the Institute with distinction as an educator, administrator, and mentor,” stated the Killian committee.
Kleppner and Ramsey produced the widely utilized text “Quick Calculus” in 1972 — the book’s third edition was refreshed in 2022 in collaboration with MIT Department of Physics’ Peter Dourmashkin. Alongside Robert J. Kolenkow, Kleppner also authored “An Introduction to Mechanics” in 1973, and its second edition in 2013. Physics department head Deepto Chakrabarty ’88 described it as “a masterpiece:” “It has laid the groundwork for our freshman 8.012 course for aspiring physics majors for over half a century and has provided a profound, elegant, and mathematically intricate introduction to classical mechanics for physics students all over the U.S. It was my initial exposure to serious physics as an MIT freshman in 1984.”
Recently, while Kleppner was being wheeled into surgery, one of the medical staff recognized that his patient was the author of that book and exclaimed, “Oh my God, I still ponder one of those problems that I found so challenging,” remembers his wife, Bea, with a laugh.
Kleppner referred to his teaching approach as “a connection with the students and with the subject matter.” He stated that his teaching role model was his wife, who instructed psychology at Beaver Country Day High School. “Fortunately, at MIT, the students are phenomenal. There’s nothing difficult about teaching here, except striving to stay ahead of the students.”
He leaves behind a legacy of appreciative physicists who were influenced by his generous teaching methodology.
“I’ve always felt incredibly fortunate to be part of Dan’s group,” states Phillips, who was at Princeton when his research into magnetic resonance caught Kleppner’s attention, leading to his invitation to MIT. “Dan extended this idea to placing this hydrogen maser in a significantly higher magnetic field. Not many individuals are trained by someone of Dan Kleppner’s caliber in the art of precision measurement.”
Kleppner
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also presented Phillips with a device he constructed for his thesis, which reduced the duration of the laser cooling experiments that culminated in Phillips’ Nobel.
Ketterle attributed his success at MIT to Kleppner’s guidance. “He was an older, seasoned individual who had faith in me. He had more confidence in my abilities than I initially had myself. I felt that whenever I faced a dilemma, I could approach Dan for counsel. When I submitted a paper for him to review … there was red ink everywhere, but he was indeed correct on nearly all points.”
In 2003, Kleppner was troubled by the fact that over 60 percent of middle and high school physics instructors lacked a foundation in the subject. He initiated the CUA’s Teaching Opportunities in Physical Science summer program alongside his then-postdoc Ted Ducas to educate physics majors on how to prepare and instruct physics content for middle and high school students. Throughout its 14-year duration, they collaborated with 112 students.
Ducas mentioned that one survey “reveals that over 60 percent of our undergraduates intend to enter pre-college education — a greater percentage than anticipated, given that physics majors have numerous other career paths often offering considerably higher pay. The potential positive effect of having that many highly skilled and motivated educators is significant.”
Kleppner also collaborated with Japanese mathematician Heisuke Hironaka on the mentorship initiative Japanese Association for Mathematical Sciences (JAMS), which linked American college science students with their Japanese peers. “His commitment to ensuring that future generations recognize the importance of international communities was embodied in JAMS,” states Sofie Kleppner.
Acknowledgments and civic involvement
Kleppner ascended to the role of professor in 1974 and directed the physics department’s Division of Atomic, Plasma, and Condensed Matter Physics from 1976 to 1979. In 1985, he was appointed the Lester Wolfe Professor of Physics.
Engaged in the intersection of physics and public policy, he participated in more than 30 committees. For the APS, he was a member of the Panel on Public Affairs (POPA), chaired both the Physics Planning Committee and the Division of Atomic, Molecular and Optical Physics, and contributed to a report on the development and mentoring of young physics academics. He chaired a report for the National Academy of Sciences on atomic physics, which he presented to various congressional committees, served on the National Research Council’s Physics Survey Committee, and was chair of the International Union of Pure and Applied Physics’ Commission on Atomic and Molecular Physics. At MIT, he also acted as an ombudsman for the Physics Department.
Kleppner was a fellow of the American Academy of Arts and Sciences, the American Association for the Advancement of Science, OSA (now Optica), the French Academy of Sciences, and the American Philosophical Society; a member of the National Academy of Sciences; and a Phi Beta Kappa lecturer.
His passion for literature at Williams evolved into a secondary career as an author, including decades spent crafting witty and insightful, yet accessible, articles for Physics Today, encompassing his “Reference Frame” columns on the history and policy of physics.
Kleppner received numerous accolades, including the esteemed Wolf Prize in 2005 “for pioneering work in atomic physics of hydrogenic systems, encompassing research on the hydrogen maser, Rydberg atoms, and Bose-Einstein condensation.” Additional honors include a 2014 Benjamin Franklin Medal and a 2006 National Medal of Science, awarded by U.S. President George W. Bush. He also earned the Frederic Ives Medal (2007), the William F. Meggers Award (1991), the Lilienfeld Prize (1991), and the Davisson-Germer Prize (1986).
His writings, congressional testimonies, and advocacy on behalf of physicists worldwide at one point inspired his Physics Planning Committee colleagues to present him with a Little League trophy featuring a golden baseball player, inscribed “Dan Kleppner — Who Went to Bat for Atomic Physics.”
Kleppner mentioned that he was influenced by his mentor, Ramsey, to engage in the scientific community. “It’s a privilege to be a scientist in this country,” Kleppner stated. “And I believe that one bears some responsibility to contribute for the privilege, whenever possible.”
He expressed, “Any prospective vision for a decent future for our nation and the world must incorporate a reasonable portion of science dedicated to the pursuit of new knowledge. We cannot afford to relinquish this ideal amidst a barrage of criticism, no matter how eloquent or formidable the critics.”
Family and life in retirement
Kleppner encountered his future spouse, Beatrice Spencer, in 1954 aboard the USS United States, while both were traveling to England and in their second year at Cambridge. They began as friends and ultimately wed in 1958, in Ipswich, Massachusetts. They nurtured their three children, Sofie, Paul, and Andrew, in their home in Belmont, Massachusetts, and at their vacation property in Vermont.
Kleppner’s family characterized him as an optimist who did not subscribe to lying, worrying, or unethical actions. He and Bea generously opened their home to anyone in need. “As we were growing up, we had the international community in our home,” reminisces Sofie. “He was an incredibly generous individual. At my father’s 80th birthday festivities at MIT, there were three hours of five-minute tributes. It was truly touching to hear how many individuals felt that simply having the door open at my parents’ house made a difference during challenging periods.”
In his retirement, Kleppner continued with his woodworking pursuits, creating beds, lamps, cabinets, a striking spiral staircase, a cradle shaped like the hull of a boat, and bookcases featuring super ellipses, a closed curve that fuses aspects of an ellipse and a rectangle.
“I take pleasure in designing,” he stated in one video. “It reflects the same instinct for making things function in experimental physics. It’s delightful to create a piece of equipment that begins to operate, and even if the experiment doesn’t yield the desired result, there’s always joy when the apparatus first turns on and functions.”
His final article for Physics Today appeared in 2020. In his later years, he maintained connections with his colleagues, exchanging book ideas with Ketterle’s wife, Michele Plott, and, since the Covid-19 pandemic, participated in regular Zoom meetings with a group of former students, organized by Mike Kash; and another, which they referred to as “The Famous Physicists,” including Phillips and their Brazilian associate Vanderlei Bagnato.
“In recent times, I would still consult Dan for guidance on challenging issues,” Phillips reflects, “sometimes concerning physics, sometimes merely about life and public policy, because I always sensed that if there was anything you needed resolved in which physics or science played a role, Dan would be the ideal person to approach.”
His family reports that Kleppner unexpectedly became ill during a Father’s Day dinner. According to his wife, his final words before being taken to the hospital were a toast to his grandson, who had recently graduated from high school: “To Darwin and all youth filled with fresh and exciting ideas.”
Bea remarks, “He consistently said that one needs to be optimistic to be a scientist because patience is essential. Things seldom proceed smoothly, and there are numerous complications. His last words were ones that instill a sense of hope for the future.”
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