For his experiments in mechanics, Tobias Kippenberg, a professor at the Laboratory of Photonics and Quantum Measurements at École Polytechnique Fédérale de Lausanne (EPFL), uses not wine glasses, but ultra-tiny glass objects that look like wheels. Each one of these “microresonators” measures about one-tenth the diameter of a single strand of hair.
“While a wine glass typically vibrates about 1,000 times per second, a micromechanical system like this glass torus does so almost 100 million times,” the physicist explains. The tiny glass wheel is coupled to a laser on a chip, and the assembly is then refrigerated to -272.55°C – just 0.6 degrees Celsius above absolute zero. Despite the cold temperatures, the wheel vibrates very slightly.
But using the laser, Kippenberg manages to further dampen this vibrating movement. While the tiny object vibrates, laser light is guided into the torus by means of an ultrathin optical glass fiber. The beam of light is then actually “caught” in the tiny wheel and runs in a circle. At the same time, photon bombardment exerts pressure on the object – radiation pressure, as physicists say – thereby countering the minimal vibrations. The result is that the tiny wheel cools still further. This principle, which Kippenberg’s team developed, is called optomechanical laser cooling.
When Atoms Stand Still
Theoretically, all movement ceases at absolute zero – minus 273.15° C. The atoms stand absolutely still. The only thing that still moves is the subatomic particles, the quanta that make up the atoms, which fluctuate, creating the merest whisper of a tremble. These quantum fluctuations have already been proven to exist and measured for atoms and individual molecules. But not for comparatively gigantic objects consisting of several million atoms, like this micromechanical glass torus.
Now, thanks to Kippenberg’s experiments, this has become possible. The change in the resonance frequency of the laser light that is generated by the mechanical movement is minimal, but still enough to make it possible to take exact measurements of ultra-tiny changes in the object’s movement.
Kippenberg will be presented with this year’s Klung Wilhelmy Science Award on November 19 for his research in the field of resonator optomechanics. The award, which carries prize money of 75,000 euros, is given out in alternating years to physicists and chemists. It is among the highest-endowed privately financed awards for top young German researchers. By granting Kippenberg the award, the jury is recognizing his “groundbreaking work on the interaction of light and micro- and nanomechanical systems.”
Kippenberg grew up in the Netherlands and in the German city of Bremen. He came to the sciences without any bias – his father is a comparative religion scholar, and his mother is a teacher of English and geography. No, he literally fell into physics in 1994. One winter morning, when he was heading off to school by bike, as usual, he came upon a patch of black ice – and took a tumble onto the asphalt. Muttering and cursing to himself, he picked himself back up and wondered: Aren’t there any sensors that could show the road conditions? How is a driver supposed to see black ice from their car if I can’t even do it on my bike? No, nothing like that existed – but that would really be something if it did!
Kippenberg explored the library, looking for literature on the interaction of light and matter. He came across a series of studies from the California Institute of Technology (Caltech) on using radar technology to investigate polar ice. American research institutions had already caught Kippenberg’s eye anyway, since the family had gone with the father to Princeton for six months the year before. “I wanted to have a chance to study at such an impressive campus myself later on,” he says.
As he was taking physics as an honors course, he got right down to work and developed what was missing. With his ice sensor for vehicles – consisting of a source of microwave radiation and an infrared laser – he not only took first place in the prestigious “Jugend forscht” youth research competition in 1996, but also won the European Union Contest for Young Scientists, in Helsinki. “I got really positive responses at the time, and I was amazed by how easy it is to get in touch with real researchers,” he recalls. He even got a call from Daimler-Benz, which applied for a joint patent with him.
Kippenberg decided to study physics and electrical engineering at the university in Aachen. After earning his preliminary degree (Vordiplom) in 1998, his dream came true: He received a coveted doctoral study spot at Caltech in Pasadena, where he stayed for six years and wrote his dissertation about optical microresonators. He returned to Germany in 2005 to take up an offer to start a Max Planck research group at the Max Planck Institute of Quantum Optics, in Munich’s Garching district. “I started out in the department of Theodor Hänsch, who received the Nobel Prize three months later. It was an exciting and highly educational time,” he says. Incidentally, Hänsch also won a Klung Wilhelmy Science Award.
In Munich, Kippenberg developed the fundamental experiments for optomechanical laser cooling and precision measurements of mechanical systems. In 2009, he was appointed to an assistant professorship at EPFL, where he has been a professor at the Institute of Condensed Matter Physics since 2013. What else could he wish for in his career?
“Nothing, actually,” says Kippenberg, who is now 39. “California, Munich, and now Lausanne, on Lake Geneva. I’ve always had the good fortune to live and do research in wonderful places. And there’s nothing more exciting than being right at the forefront of things during the pioneering phase of a new research field.” In the next ten years, Kippenberg hopes to be able to prove quantum theory on a macroscopic scale. At the same time, though, he also has real-world applications in mind. “We want to show that high sensitivity is very useful for new technologies,” he says. “Maybe not in everyday life because much less sensitive motion sensors are sufficient in smartphones, but for special applications,” he continues.