When a particle is completely isolated from its environment, the laws of quantum physics start to play a crucial role. One important requirement to see quantum effects is to remove all thermal energy from the particle motion, in other words, to cool it as close as possible to absolute zero temperature. Researchers at the University of Vienna, the Austrian Academy of Sciences and MIT are now one step closer to reaching this goal by demonstrating a new method for cooling levitated nanoparticles.
Tightly focused laser beams can act as optical "tweezers" to trap and manipulate tiny objects, from glass particles to living cells. A key element in these research efforts is to obtain full control over the particle motion, ideally in a system where the laws of quantum physics dominate its behavior. However, laser noise and largely required laser intensities have posed a substantial limitation to these methods. "Our new cooling scheme is directly borrowed from the atomic physics community, where similar challenges for quantum control exist", says Uros Delic, lead author of the recently published study by the aforementioned researchers.
The idea goes back to early works from Innsbruck physicist Helmut Ritsch and from US physicists Vladan Vuletic and Steve Chu, who realized that it is sufficient to use the light that is scattered directly from the optical tweezer itself if the particle is kept inside an initially empty optical cavity. A nanoparticle in an optical tweezer scatters a tiny part of the tweezer light in nearly all directions. If the particle is positioned inside an optical cavity a part of the scattered light can be stored between its mirrors. As a result, photons are preferentially scattered into the optical cavity. However, this is only possible for a light of specific colors, or said differently, specific photon energies. If we use tweezer light of a color that corresponds to slightly smaller photon energy than required, the nanoparticles will "sacrifice" some of their kinetic energy to allow photon scattering into the optical cavity. This loss of kinetic energy effectively cools its motion.
This quantum cooling method has shown to be much more powerful than all the previously demonstrated schemes. In fact, without the constraints imposed by laser noise and laser power, quantum behavior of levitated nanoparticles should be right around the corner.
