When vibrations become quantum
The winner of the 2014 Latsis Prize, Tobias Kippenberg, studies quantum oscillation phenomena in microresonators capable of storing light in very small volumes. This work allows the development of very high precision measurement apparatus. By Anton Vos
Back in 1994, Tobias Kippenberg was riding his bicycle one winter morning in Bremen in the north of Germany. The air was cold, but as there was no frost, he had taken confidently to the road. All of a sudden, he hit a sheet of black ice that had gone unnoticed and ended up sprawled across the tarmac. He pulled himself off the ground and cursed his bad luck.
At the time this simple accident meant nothing to him. Little did he know that the final outcome would be so much happier for him, as it guided him to the world of science, to the California Institute of Technology (Caltech), the Max Planck Institute (Germany), and then the École Polytech-nique Fédéral de Lausanne (EPFL).
“Just after the accident, I began asking if it was possible to dream up a machine capable of measuring the condition of the road, and, above all, to distinguish between wet and frozen ground, something which is often impossible for the naked eye”, says Kippenberg. Today he is a professor at the Laboratory of Photonics and Quantum Measurements at the EPFL, and the recent winner of the national Latsis Prize for 2014 for his work in optomechanics.
Back then, he was already fascinated by science, and as a young researcher it wasn’t enough for him just to think about how such a device might improve the world (and his own situation). So he went headlong into actually designing it too. In the quantum library, he found a book on the interaction between light and matter, and another, written by an American researcher from Caltech, that described a radar technique applied to studying polar ice. Taking inspiration from these books and applying a lot of ingenuity he created an experimental model using a source of microwaves and an infrared laser. He gave his device the following name: Infrared-microwave radiation ice condition sensor for cars, and it has become an excellent black ice detector.
Thanks to this invention, Kippenberg took part in the Jugend forscht competition (awarded by the German foundation supporting young researchers), and promptly won it. Around the same time, in 1996, he also won the eighth edition of the European Union Contest for Young Researchers.
Back then he was still a young man, the son of a professor specialised in the comparative study of religions, and his future seemed laid out for him. He went to Aachen University to study physics and electrical engineering, obtaining his bachelor degree in 1998. His journey later took him to the United States where he was offered a place at Caltech, Pasadena.
A 12-micron bicycle wheel
It was there that he developed his first microscopic structures (microresonators) capable of storing photons for several microseconds – a considerable period for these elements of light, which in the same timespan would have otherwise travelled almost a kilometre. He still feels passionate about this field today.
In 2005 he returned to Germany and took over as head of an independent research group at the Max Planck Institute in Garching. There he rubbed shoulders with Theodor Hänsch, winner of the Nobel Prize for Physics, and took his habilitation at the Ludwig Maximilian University of Munich. In 2008 he was awarded a post at the EPFL, first as an assistant professor, then in 2013 as a full professor.
Today he is studying a minute oscillator made of glass, shaped like a bicycle wheel but with a diameter of just 12 micrometres. It was this that won him the Latsis Prize for 2014. This resonator is both mechanical and optical. It allows light to circulate in the toric part of its structure (the tyre of the bicycle wheel, as it were). The walls reflect the light, thereby producing “radiation pressure”.
In an experiment reported in Nature in 2012, the regulator was cooled to half a degree above absolute zero. Kippenberg and his team showed for the first time that it is possible to reduce its temperature even further, by injecting photons into the resonator and creating well-controlled radiation pressure. During this process, a particularly strong coupling is produced between the light and the mechanical movement, so strong in fact that the optical and mechanical properties of the structure become inseparable.
Let's put it to work!
At this point, the oscillator becomes so cold that it starts to become almost entirely submerged in what is known as its fundamental state. This state of minimal vibration is something that can only be described by quantum mechanics. The theory predicts, amongst other things, that an object can never be perfectly still, even at ab-solute zero, and that there is always a slight movement.
“We managed to cool an object composed of billions of atoms to temperatures so low that we could observe quantum phenomena”, says Kippenberg. “This really is fundamental science, and we aim to continue our work in this direction. But that doesn’t mean that we are not interested in possible applications for our research. Quite to the contrary, my passion for science has always included both”.
Indeed, this was the goal that he was pursuing whilst at the Max Planck Institute when he managed to discover another remarkable property of microresonators. Light in a laser beam connected to a microresonator using a small fibre-optic cable can produce so-called “frequency combs”. These are essential, for example, in calibrating high-precision spectrometers used in astronomy and atomic clocks. Generators of frequency combs have always been cumbersome; they are the size of tables, very expensive and very complex. Kippenberg’s, however, is tiny and is built using the same methods as for electronic chips. A first patent was filed in 2007, followed by a second in 2013 at the EPFL. This invention won Kippenberg the Helmholtz Prize for Metrology in 2009 and is very close to being put on the market, a step that Kippenberg hopes to take by launching a start-up.
Anton Vos is a science journalist, working chiefly at the University of Geneva.(From "Horizons" no. 103, December 2014)