Caltech Mourns the Passing of Jeff Kimble (1949-2024)

Harry Jeffrey Kimble, the William L. Valentine Professor of Physics, Emeritus, passed away on September 2, 2024, at the age of 75. A giant in the field of physics, Kimble—who was also known as H.J. Kimble and Jeff Kimble—performed groundbreaking experiments in quantum optics and quantum information science. His achievements include some of the earliest demonstrations of quantum squeezing and quantum teleportation, among many other trailblazing experiments.

Kimble is perhaps best known for his pioneering work in a field known as cavity quantum electrodynamics, or cavity QED, which is the study of single atoms trapped in cavities where they strongly interact with single photons of light. Kimble's cavity QED experiments formed the basis of many quantum technologies being developed today, such as quantum computers. His experiments also laid the foundations for research into quantum networks—essentially a quantum internet—in which ensembles of atoms are entangled.

"Jeff had a very deep influence on me as a scientist and as a person," says John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics. "I was a particle physicist originally but became interested in quantum computers around 1994 and 1995 when they were first starting to get attention. If not for Jeff, I wouldn't have made the pivot that I did from particle physics to quantum information science. He was a very inspiring and daring scientist whom I am grateful for."

Oskar Painter, Caltech's John G Braun Professor of Applied Physics and Physics, says Kimble "was an explorer at heart, who deeply loved the challenge of a solo backpacking trip to Patagonia almost as much as he loved uncovering the mysteries of the quantum world. He was the most intense and inspirational person I have ever worked with, and he had a tremendous impact on numerous generations of students like myself, who dreamed of one day taming quantum systems and applying them to new ways of communicating and processing information."

Kimble summed up his own adventuresome spirit toward science in a 2016 lecture at Caltech titled, "The Future of Physics." In this talk, he highlighted advances in quantum information science, a field in which the curious effects of quantum physics are exploited to better communicate and process information. "The amazingly exciting thing about quantum information science is this explosion in our ignorance, not our knowledge," he said.

Squeezing the Darkness

Some of Kimble's earliest work involves quantum squeezing, a method in which light is manipulated to reduce the quantum noise that limits precision measurements. Kimble himself eloquently explained squeezing in an article he wrote for Caltech's Engineering & Science magazine in 1993 called "New Light on the Nature of Darkness." Even a vacuum, he said, has some light due to fundamental fluctuations in the electromagnetic field. He and his team in essence squeezed that light out of the darkness such that "light coming into our detector is four times darker than the darkness that the detector would see if it viewed empty space."

Kimble's experiments were "the first to demonstrate squeezing and its application to noise reduction in interferometry in a really convincing way," says Carlton Caves (PhD '79), a former research fellow and now Distinguished Professor Emeritus at the University of New Mexico, who had identified squeezing as the key to controlling quantum noise. Kimble and his colleagues conceived and demonstrated the methods for generating squeezed light that are now employed at LIGO (the Laser Interferometer Gravitational-wave Observatory), which regularly detects ripples in space-time known as gravitational waves.

Later, in 2001, Kimble came up with a theoretical idea that would ultimately enable LIGO to squeeze light in an optimal manner across the range of gravitational-wave frequencies it detects—a method called frequency-dependent squeezing that goes beyond traditional squeezing to circumvent a fundamental limit imposed by Heisenberg's uncertainty principle.

Nobel Laureate Kip Thorne, a co-founder of LIGO and the Richard P. Feynman Professor of Theoretical Physics, Emeritus, had challenged a group of Caltech colleagues to come up with ways to work around this Heisenberg quantum limit. Kimble, a participant in that group, figured out how to do this by using a specialized filter cavity, sometimes referred to as a Kimble filter cavity, to process the squeezed light. That cavity is in use today at LIGO along with a squeezer of the type that Kimble and colleagues had conceived and demonstrated. 

"The LIGO team spent nearly 20 years perfecting Kimble's techniques at LIGO's audio frequencies, making them robust, and then implemented them into LIGO," Thorne says. "Today, they are a major contributor to LIGO's remarkable sensitivity and ability to see several black hole collisions a week by comparison with one every six weeks in 2015 and 2016."

Quantum Teleportation and Beyond

Squeezing methods also played a key role in Kimble's pioneering quantum teleportation experiments in the late 1990s. Not to be confused with beaming people to and from planets as seen on Star Trek, these experiments transported a quantum state of light from one side of an optical bench to another without the light traversing any physical medium in between. As with quantum squeezing, Kimble's experiments in this field were some of the most convincing. By using squeezed light in the experiment, Kimble's team achieved a high fidelity.

"In 1998, Jeff's group reported an experiment on the first unconditional quantum teleportation, the work that laid the foundation for the burgeoning field of quantum information processing with continuous variables," says Eugene Simon Polzik, a professor of physics at the Niels Bohr Institute at the University of Copenhagen, and a former associate and visiting scientist at Caltech. "My graduate student and I, then at the University of Aarhus, worked on that experiment conducted at Caltech. It was truly exciting. The paper has since been cited more than 3,000 times and is now a classical work in the field."

Adds Painter: "There is nobody I know who is as well respected as Jeff for the quality of their experiments. He would take on the hardest experiments and was the best in the world in his field. He was unmatched."

Around the same time as the teleportation work, Kimble was leading the burgeoning field of cavity QED. Much of his career was devoted to this technique, in which a small cavity is used to focus light down onto an atom. The interaction between atom and photon becomes very strong and allows for "magical things to happen," Painter says, citing Kimble's atom-cavity microscope as an example.

"He was an expert at both quantum optics and atomic physics and married the two fields," Painter says.

The cavity work was important for future quantum technologies because it provided researchers with a way to store quantum information in matter and manipulate it: in essence, to do what occurs in classical computers, but for information that is encoded at the atomic scale. Kimble's cavity research led to many of the quantum technologies that are being actively investigated today, including superconducting electronic circuits, a potential platform for future quantum computers.

He also melded his work in quantum teleportation with cavity QED to come up with schemes for quantum networks that could spread quantum information across long distances. Some of that research involved using quantum repeaters to distribute entanglement across networks. "Suppose you want to send your quantum information from LA to New York. You need quantum repeaters to do this, and Jeff's cavity work is the basis for this. Cavity QED is the birthplace of many ideas still being improved upon today," Painter says.

Kimble also helped establish the Institute for Quantum Information and Matter (IQIM) at Caltech, together with Preskill, who is currently the Allen V. C. Davis and Lenabelle Davis Leadership Chair of IQIM. The center's roots go back to 1996 when Kimble and Preskill received a grant from the Defense Advanced Research Projects Agency (DARPA) to study quantum computing. Later, in 2000, they were awarded funding from the National Science Foundation (NSF) to establish the Institute for Quantum Information (IQI), which focused on theoretical research. By 2011, Kimble, Preskill, and other colleagues had been awarded funding from the NSF to broaden the scope of their center to include experimental work. They changed the center's name to the Institute for Quantum Information and Matter, and Kimble became the inaugural director.

"IQIM would not have happened without Jeff," Preskill says. "Over the years, we did several research proposals together and those interactions were very exciting and helped to steer us scientifically."

Fiona Harrison, the Harold A. Rosen Professor of Physics and the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics and Astronomy, says Jeff's impact on Caltech went well beyond his own research. "He set unwaveringly high standards, his scientific interests were very broad, and Jeff never did anything half-heartedly," she says. "As a result, his advice was highly sought on issues ranging from who to hire onto the faculty to what new strategic directions Caltech physics should pursue. His presence as an admired colleague and friend influenced generations of Caltech scientists, including myself."

Texan at Heart

Kimble was born in Floydada, Texas, in 1949. He obtained his bachelor's degree from Abilene Christian University in Texas in 1971, and his PhD from the University of Rochester in New York in 1978. From 1979 to 1989, he served as a professor of physics at University of Texas at Austin. Kimble became a professor of physics at Caltech in 1989, the William L. Valentine Professor in 1997, and professor emeritus in 2021.

In an IQIM blog post, one of Kimble's former postdocs, Jun Ye, now a professor of physics at the University of Colorado Boulder, recalled shaking hands with Kimble when they first met. "His grip was more than just firm; it actually squeezed the bones of my hand. So naturally, I took the handshake as a sign that he really wanted me to join his group. When an offer of a Caltech fellowship arrived three months later, I accepted it without hesitation. In 1997, I had no way of knowing that Jeff's way of doing science would leave a profound mark on my career and that his deep friendship would continue to enrich my life and that of my family for many years … He is a daring pioneer, an original thinker, a groundbreaking technologist, and a relentless seeker of ultimate truth and knowledge."

Caves says that Kimble never "forgot his roots on the high plains of West Texas." He recalls, as an example, a talk Kimble gave on his squeezing work. "Interrupted by a somewhat hostile question, Jeff, after hearing cries of, 'repeat the question,' stepped forward and said, 'He tried to draw on a Texan.'

"He was a giant of physics, the soul of scientific integrity. A Kimble experimental result was as solid and unshakeable as a rock," Caves says.

Kimble's own advice about the challenges and rewards of doing science can be heard in an animated video produced by PHD Comics. "If you want to be on the cusp of the future, you have to be willing to fail," he said at the end of the movie. "We've come up some climb, and it didn't work; you better go down, try another way. You better believe it's scary, but it's an exhilarating intellectual adventure."

During his research career, Kimble mentored more than 50 PhD students and postdocs, and at least 40 have become professors and group leaders at universities worldwide, according to Polzik. "Counting Jeff's research 'grandchildren'—PhD students and postdocs of his former group members—results in close to a thousand young researchers. This is the contribution to science that is as valuable as Jeff's own research achievements," Polzik says. "Jeff was one of the most honorable men I have ever met. His dedication to science and to his collaborators, his ingenuity, and his perseverance were unparalleled."

Kimble received many awards for his pioneering research, including the Leonard Mandel Quantum Optics Award (2024), the Herbert Walther Award (2013), the Julius Edgar Lilienfeld Prize (2004), the Albert A. Michelson Medal of the Franklin Institute (1990), the Einstein Prize for Laser Science (1989), and Optica's Max Born Award (1996). He was a member of the U.S. National Academy of Sciences and a fellow of Optica, the American Association for the Advancement of Science (AAAS), and the American Physical Society, as well as a National Security Science and Engineering Faculty Fellow.

Family was always central to Kimble. "What also stood out to me about Jeff was his devotion to his family," Painter says. "He really drew strength from them and prioritized them in a way I have rarely seen in someone so driven by their work."

Kimble is survived by his wife, Midge Kimble; their children, Katie Grooms and Megan Kimble; and their grandchildren, Ellie, Bryn, and Madeline. He is also survived by two brothers, Jim Kimble and John Kimble.