Wine glasses, combs, and bicycle wheels: these are some of the surprising objects that come up when talking about photonics, quantum optomechanics and optical microresonators with Tobias Kippenberg, whose breakthroughs earned him the 2025 Marcel Benoist Prize. These scientific fields have become essential to today’s technological landscape and remain – rest assured – far removed from oenology, trichology (the study of hair), or cycling. Or maybe not so distant after all…
It was while riding his bicycle that the now 49-year-old German physicist discovered his vocation for science. One winter morning in Bremen, where he grew up, he slipped on asphalt covered with black ice – a thin, transparent layer formed by the condensation of water vapour. Practically invisible because it contains virtually no air bubbles, the black ice had formed even though the air temperature was not particularly low. The teenager was taken aback – both by his accident and by his inability to discern the road conditions. He then wondered how cars could detect such an icy surface. But such sensors did not exist at the time. Coming across a poster for ‘Jugend forscht’, a contest for young scientists, he decided to enter with this idea in mind. He researched the subject and came across scientific literature describing the study of glaciers by satellite. He then approached local companies, which agreed to supply him with equipment. Aided by his passion for computer programming, he developed an experimental device to detect black ice using microwaves and infrared. He won that contest and the European one after that. His path was clear.
Like motorcyclists in a circus
After earning a Bachelor’s degree in both physics and electrical engineering from RWTH Aachen University, he recalls, “I wanted to work in a field that would combine physics and engineering.” He applied to several universities and was accepted by the prestigious California Institute of Technology (Caltech), even securing a scholarship. After completing his Master’s degree, he embarked on a PhD: “When it came time to choose a research topic for my PhD, I followed the advice of my future supervisor, Kerry Vahala, and explored optical microresonators – an area considered exotic at the time – and became fascinated by microscopic glass spheres.” These objects trap light particles (photons), causing them to circulate in loops, much like motorcyclists riding inside the stationary steel spheres seen in circuses.
“I wanted to work in a field that would combine physics and engineering.”
Kippenberg studied these structures and developed another form. Instead of a sphere, he created a glass ring about 30 micrometres in diameter – roughly three times thinner than a human hair – resembling a miniature bicycle tyre. This innovation led to his first major discovery: When laser light is coupled tangentially into this transparent toroid, an exceptionally high photon recirculation rate is achieved: the photons can circulate up to a million times before dissipating. In the process, they exert a force on the cavity walls known as radiation pressure, which can become strong enough to make the ring vibrate. In other words, light induces mechanical oscillations.
To explain this phenomenon, Kippenberg often uses the analogy of a wine glass: “When you run a wet finger around the rim, the contact amplifies the glass’s vibration until you can hear it. In our experiments, light plays the role of the finger, amplifying the mechanical vibration of the ring – a vibration that can also be measured.” He began this line of research during his postdoctoral work at Caltech and later continued it in Germany as an independent researcher at the Max Planck Institute of Quantum Optics. These efforts opened the entirely new field of quantum optomechanics. Kippenberg has since published numerous scientific papers on the subject, which he attributes, above all, to a certain degree of serendipity.
“In our experiments, light plays the role of the finger, amplifying the mechanical vibration of the ring – a vibration that can also be measured.”
By chance, Kippenberg discovered a book by Russian physicist Vladimir Braginsky, that described the effect of laser beams reflecting off mirrors in instruments known as interferometers. In 1969, Braginsky was developing concepts for future gravitational-wave detectors, which required extremely precise optical systems. He predicted that the radiation pressure exerted by laser light could destabilise these systems by amplifying the mechanical oscillations of the mirrors. “In fact, the entire theoretical basis was laid out in his book,” recalls Kippenberg. “And I realised that this very phenomenon was occurring in our ring resonators! All that remained was to link the experiment to the theory – to connect the dots, so to speak.” The young German physicist had thus observed a prediction made nearly 50 years earlier, known as dynamical backaction.
A Eureka year
Even more striking, in 2006, Kippenberg demonstrated the opposite effect. By detuning the laser so that the optical field extracts energy from the resonator, the mechanical oscillations are significantly damped – a process known as radiation-pressure cooling via dynamical backaction. In this regime, the glass microresonator is almost frozen, or ‘cooled’, as physicists say. This behaviour, too, was already predicted in Braginsky’s work.
“I always kept in mind the words of the famous physicist Richard Feynman: ‘The easiest person to fool is yourself.’”
The findings were remarkable in every respect. But would the physicist call this a ‘eureka moment’? “It was more like a eureka year, because it all unfolded over a certain period of time. I always kept in mind the words of the famous physicist Richard Feynman: ‘The easiest person to fool is yourself.’ We still had to prove that what we observed was not caused by laser-induced heating, but rather by radiation pressure, which is a purely mechanical effect.” Subsequent experiments would soon settle the matter and make the field even more exciting.
An exceptional clean room
Kippenberg joined EPFL in 2008 and transferred his laboratory there from the Max Planck Institute in 2010, thus deepening his engagement in this field in Switzerland. “Beyond the recognised excellence of the two federal institutes of technology (Lausanne and Zurich) and the stability and overall security that Switzerland offers on so many different levels, there were two main factors that drew me here: First, the quality of the clean room – a dust-free laboratory where I knew I could develop the technologies that I had envisioned.
Second, EPFL was one of the few European universities to have introduced a tenure-track system under the leadership of former president Patrick Aebischer.” This system allows researchers to be hired on multi-year contracts with the prospect of obtaining a permanent position as a full professor. Kippenberg received his tenure in 2013.
Even more unusual is the principle of ‘superposition’ where the same subatomic particle can exist in several states or locations at once.
In this context, he and his team published a groundbreaking article in the prestigious journal Nature, showing that the quantum dynamics of laser light confined in optical microresonators can exert radiation pressure strong enough to couple to the resonator’s mechanical vibrations. This interaction – known as optomechanical coupling – opened the door to controlling mechanical motion with light. To appreciate why this is remarkable, we need to recall two of the counterintuitive principles of quantum physics. One is ‘quantum entanglement’, where two photons located hundreds of metres apart remain connected so that acting on one affects the other. Even more unusual is the principle of ‘superposition’ where the same subatomic particle can exist in several states or locations at once. Such phenomena usually belong to the quantum realm and vanish in the classical world of macroscopic objects. Yet here, in these tiny ring-shaped resonators, vibrations behave in ways that bring us closer to observing quantum effects in mechanical systems! Astonishing.
Optical frequency combs
In the end, Tobias Kippenberg demonstrated that light, as described by quantum physics, can induce mechanical effects that are not only remarkable but also useful. One of the techniques he helped pioneer, called ‘laser backaction cooling’, has given rise to motion sensors with extraordinary sensitivity, capable of detecting displacements between ten and a thousand times smaller than the diameter of a proton.
“Of course, I am deeply honoured. However, with retirement still a long way off, this recognition does nothing to diminish my curiosity…”
All these discoveries have given a tremendous boost to fundamental research in quantum optomechanics, establishing Tobias Kippenberg as one of the most highly cited scientists in his field and a sought-after member of leading scientific academies, such as the US National Academy of Engineering or Germany’s National Academy of Sciences Leopoldina. He has received prestigious grants, including the Synergy Grant from the European Research Council (ERC), and major awards such as the ZEISS Research Award and Switzerland’s 2014 National Latsis Prize. And now, the Marcel Benoist Prize: “Of course, I am deeply honoured. However, with retirement still a long way off, this recognition does nothing to diminish my curiosity…” This is especially true since, from his PhD days, he has also ventured into another promising area of research: optical frequency combs.
This is a highly specialised application of a pulsed laser, in which a device splits the spectrum into a series of equally spaced frequency components. Visually, one can picture a laser beam that, after passing through this device, emerges as a sequence of discrete spectral lines, each at a different frequency but evenly spaced – like the teeth of a comb. This spectral structure, known as an optical frequency comb, serves as a precise ruler for measuring frequencies in a wide range of optical applications.
The discovery of optical frequency combs earned its creators, American physicist John L. Hall and German physicist Theodor W. Hänsch, the 2005 Nobel Prize in Physics. “When I returned to Germany, I worked with Hänsch,” recalls Tobias Kippenberg. “As at Caltech, when I came across the writings of the Russian physicist, I was once again fortunate enough to be in the right place at the right time – this time to consolidate another observation we had made with our toroidal microresonators.” Namely: the laser light emerging from the resonator appeared as a sequence of discrete spectral lines. Could this be a perfect optical frequency comb? “We were able to show that this was indeed the case,” explains the physicist. But with a major advantage: whereas the device invented by Theodor Hänsch to generate optical frequency combs required a full optical table and significant power, Tobias Kippenberg’s device fits into a tyre-shaped microstructure smaller than a human hair and consumes orders of magnitude less power.
The fact that his team was able to explain this phenomenon through fundamental research experiments was the icing on the cake. The vibrations of the microresonator, induced by the incident laser, are converted back into light as they exit. “However, in the meantime, this light has self-organised in a nonlinear way,” explains the physicist. “The outgoing light is no longer continuous, as it was when entering the ring, but is transformed into ultrashort pulses – like tiny packets of light – known as dissipative solitons. These solitons are what generate optical frequency combs on a micrometric scale.”
Highly practical applications
The potential for highly practical applications quickly became clear: in optical communications, where multiple light channels carrying information at closely spaced but distinct frequencies can travel through the same fibre-optic cable; in astronomy, where spectrometric instruments can be calibrated to analyse light from exoplanets – planets located outside our solar system; and even in the development of new ‘neuromorphic’ computer architectures, which mimic the massively parallel and highly adaptive functioning of billions of neurons to accelerate computing power.
“In fact, they are simply physical principles present in nature that we were able to observe. It’s beautiful and fascinating!”
Looking back on all these discoveries, Tobias Kippenberg recalls a piece of advice given to him by one of his PhD supervisors at Caltech, Israeli professor Amnon Yariv, when he was considering the next step in his scientific career: “If you are going to do something, do something beautiful or something useful – but not something in between!” The physicist believes he has come close to that aesthetic ideal. “In fact, this convergence between the worlds of classical mechanics and quantum physics – these solitons, these optical frequency combs – are simply physical principles present in nature that we were able to observe. It’s beautiful and fascinating!”
As for the practical impact of his research, this goal has also been achieved: Tobias Kippenberg used the Latsis Prize funds to co-found a start-up based on his discoveries, LIGENTEC SA, which now employs around 70 people at the EPFL campus and in France. “Initially, we only intended to commercialise these frequency combs,” he explains. “But we soon realised that the field of next-generation integrated photonic circuits, which consume extremely little energy, offered much greater potential.” So much so that the professor now wishes to expand the range of expertise within his research group, but often struggles to secure the necessary funding: “Switzerland does a great deal to support science,” he notes, “but this support is mainly limited to providing project funding and expanding core budgets. A truly transformative approach would be to provide substantial direct funding to individuals for a specified period, with an evaluation at the end – similar to what is done in the United States by the Howard Hughes Medical Institute (HHMI), a programme that has proven remarkably successful.” In his view, current European funding systems require scientists to string together multiple grants for different projects – “like beads on a necklace” – in order to pursue large-scale undertakings. In Switzerland, a programme similar to HHMI could catalyse research excellence,” he says, “particularly in areas that are very costly and require larger teams.” Such an approach would also help convert Switzerland’s strong innovation potential – where it often ranks among the global leaders – into tangible industrial advantages. Kippenberg points out that roughly 75% of his group’s funding comes from the United States or the EU. His research – especially on integrated photonic circuits – would not be possible without this non-Swiss support.
Importance of the EU for research
The physicist also recalls with dismay the recent episode that led to the temporary exclusion of scientists based in Switzerland from European research programmes such as Horizon and the Quantum Flagship. “It left its mark on my laboratory,” he says. “In particular, I lost a project we were coordinating, along with many other funding opportunities in the field of quantum science and technology. The interim financial measures quickly introduced by the Swiss National Science Foundation (SNSF) were certainly welcome, but they did not offset the long-term impact.” In 2020, he told the European online magazine Science|Business that he would not rule out leaving Switzerland if no solution were found – even though he would have deeply regretted giving up his favourite pastimes: hiking in the mountains and cycling, especially on the winding roads of the countless Alpine passes.
“Follow your professional aspirations, but don’t expect things to be easy!”
Today, in 2025, the situation has improved: scientists working in Switzerland have regained access to most European programmes – though not all. For now, this is reassuring for Tobias Kippenberg, who lives just a few minutes’ walk from the EPFL campus with his wife, also a researcher at the institution, and their two young children, aged four and one. “I’ve been lucky,” he admits. However, he keeps in mind the motto he hopes to instil in his two sons: “Follow your professional aspirations, but don’t expect things to be easy!”
