Quantum optomechanics with nanospheres and ions

Control of quantum-mechanical degrees of freedom, once the realm of thought experiments, has now been achieved with diverse systems, including individual atoms and photons. An intensive effort is underway to extend this control to the macroscopic world, and in particular, to the motion of macroscopic objects. In the emerging field of cavity optomechanics, mechanical oscillators are integrated with a resonator, in which light is stored. The light exerts pressure on the oscillator, and the motion of the oscillator changes the characteristic frequency of the resonator, so that the two systems are coupled. Current experiments have made remarkable progress, but this coupling is typically linear, that is, a change in oscillator motion is proportional to a change in the light field. However, in order to prepare states of the oscillator that are nonclassical, a nonlinear coupling between light and matter must be introduced. This project will achieve the necessary nonlinearity by coupling both a macroscopic system here, a levitated glass bead and a single trapped ion to an optical resonator. One important aspect of this project is the levitation of the glass bead, which isolates it from its environment. The second essential feature is the trapped ion, which is a well-understood quantum mechanical system that provides the optomechanical nonlinearity. The key objective is to prepare the glass bead in a superposition state, that is, a quantum mechanical state in which it is in two places at once. Our long-term vision is to use these nonclassical states of the glass bead as highly sensitive detectors, as platforms for observing quantum mechanics in unprecedented regimes, and as interfaces to diverse quantum systems.

Control of quantum-mechanical degrees of freedom, once the realm of thought experiments, has now been achieved with diverse systems, including individual atoms and photons. An intensive effort is underway to extend this control to the macroscopic world, and in particular, to the motion of macroscopic objects. For example, in recent years, clamped mechanical oscillators (such as miniature drumheads) have been prepared in quantum mechanical superpositions of being in two places at once. The enabling step in these experiments was a nonlinear coupling between the oscillator's motion and light (or, more broadly speaking, electromagnetic or acoustic waves). Furthermore, it was pointed out that levitated mechanical oscillators, that is, oscillators that are not clamped to their environment but are instead suspended against gravity using optical or electromagnetic forces, would have specific advantages as macroscopic quantum objects: they could serve as the basis for highly sensitive detectors and would enable quantum mechanics to be observed in unprecedented regimes. The aim of this project was to achieve a nonlinear coupling between the motion of a levitated mechanical oscillator and light, enabling the preparation of quantum states. For a macroscopic oscillator, we chose to use a glass nanosphere that was levitated in an electromagnetic trap known as a Paul trap. First, we showed that a nanosphere can be loaded into a Paul trap at the ultra-low pressures necessary for quantum-mechanical experiments. Next, we published four research papers on novel methods to slow down the motion of a nanosphere in a Paul trap. Our extensive work on this topic reflects the fact that preparing truly quantum mechanical states requires the nanosphere to be moving very slowly, close to the limits imposed by quantum mechanics. Moreover, in three further publications, we examined "hybrid traps," combinations of Paul traps and laser traps that take advantage of the strengths of each setup. Finally, our two most recent results offer a promising outlook for preparing quantum states. On one hand, we can "ring" a nanosphere, just like a bell, by giving it an (electrical) kick. It turns out that the nanosphere will ring for a long time, in fact, for months, before it stops. This finding shows that the nanosphere is extremely well isolated from its environment, another requirement for reaching the quantum domain. On the other hand, we have levitated a nanosphere together with an atomic ion in the same trap, the first time that two such different objects have been trapped together. The atomic ion is a well-understood quantum mechanical system that, in future experiments, will provide the nonlinearity we have been seeking.