Single iON And phoTon INterActions (SONATINA)

The objects that we see around us are visible because they deflect light, the deflected light reaches our eyes, and we see them. This description of how things become visible can be traced down to the smallest building blocks of matter: atoms and molecules. How does an atom deflect light? This happens in two steps. First, an atom absorbs energy from light and stores this energy internally, by changing its status or excitation level to an excited state. The atom then spontaneously leaves the excited state by releasing the internally stored extra energy in the form of a small package of light: a photon. This spontaneous energy release is called spontaneous emission and, although it is fundamental for being able to see objects, there are situations in physics experiments in which its effects are undesired. For example, spontaneous emission limits how long an atom can remain in the excited state, thereby reducing the time we can store quantum information in those states and use the atom as memory storage. Finally, once the atom releases the extra energy in form of the photon, this emission perturbs the atoms motion, which is a limiting factor if the aim of the experiment is to identify the atoms position precisely. In this project, we will prevent the spontaneous emission from happening and counteract these undesired effects, that is, we want to increase the time the atom remains in the excited state. We achieve this by coupling an atom to a mirror, which reflects the light back onto the atom and thereby regulates the emission process. This scheme is effective because an emission event takes some time and light reflected at a short distance can be guided back quickly enough to interfere with the emission process while it is still underway. Our mirror is unlike any other: it is half-spherical to control the emission in any direction of space and it is manufactured with extraordinary precision to meet the stringent experimental requirements that are necessary to control the emission. Our experimental challenge is to combine the mirror and a single isolated atom to control the atoms spontaneous emission to the finest degree possible. We want to make the atom invisible by completely suppressing its emission. All findings coming from this project can find applications in future quantum devices, for example, to extend the storage time of quantum information, increase the encoding efficiency, control the internal states of atoms, and improve the sensitivity of light-based sensing protocols such as for position measurements. Furthermore, our methods for controlling visibility with a spherical mirror can be applied to any object that can scatter light and are not limited to atoms. The wide applicability provides excellent perspectives for the findings of these studies to be adopted also in other fields of science such as in microscopy or the study of quantum mechanics with microscopic particles.

We built a highly innovative setup in which a single atomic ion is held at the center of curvature of a hemispherical mirror. The setup is fully operational, and this achievement paves the way for fundamental research that will make it possible to shape and control how a single atom emits light-an important step toward future quantum technologies. The apparatus now traps single ions reliably, moves them with great precision, and detects their light at rates comparable to the best systems worldwide, enabling cutting-edge experiments. We have published research and filed patents on controlling light using this setup and complex reflective boundary conditions, with impact on quantum optics, quantum information, and quantum optomechanics. The project has attracted strong international attention, led to invited talks across Europe, and sparked new collaborations with leading groups in Germany and the UK. It has trained students and earlycareer researchers, including a postdoctoral fellow who has since become a professor.