Gravitationally induced phase shift on a single photon

Quantum mechanics and general relativity are two fundamentally different theories and have both been tested independently with very high precision. Quantum mechanics allows for describing nature at very small scales whereas general relativity gives access to descriptions at very large scales. But even after a century of research, the interplay of those two very different theories has never been tested experimentally. The ongoing search for a unified framework, capable of combining all known forces of nature, suffers from this lack of experimental guidance. This project aims to explore the interface between quantum mechanics and general relativity by performing high-precision experiments at the level of single quanta of light, the photons. Such quantum systems allow one to examine the influence of gravity on interference effects. For this purpose, a high-precision interferometer whose paths are subject to different gravitational potentials will be used. If the mass-energy equivalence principle holds, the photons energy must provide its gravitational mass. Then a relative phase shift between the different paths of the interferometer located at different potentials can be detected. This kind of experiments would therefore directly probe the gravitational mass-energy equivalence principle of relativity. The import insights of this experiment will influence the design of even more sensitive interferometers that allow for a genuine experimental verification of the interplay between quantum mechanics and general relativity in a table-top experiment. According to general relativity, clocks at different heights within the gravitational field of the Earth are not ticking at the same rate. If this time difference is comparable to the length of the photon inside the interferometer, then each path leads to a different arrival time. This results in a drop in the interferometric visibility, which shows up in addition to the relative phase shift.

The project "Gravitational phase shifts in single photons" was extremely successful with the creation of experimental research at the interdisciplinary interface between quantum physics and gravitation. The aim of this project was the preparation and implementation of precision measurements in the laboratory in which the influence of gravity on quantum light, and in particular on individual light particles, is measured. In order to make this possible, methods for the stabilization of an optical interferometer were developed, the arm lengths of which are given by 100 km long optical glass fibers. The influence of different disturbance factors was also examined and suitable isolation concepts have been developed for the respective cause of noise. The knowledge gained for the stabilization of interferometers and the processing of individual photons in the course of this FWF project was very helpful for the implementation of other quantum experiments. Among other things, the robustness against noise within different interferometric quantum network structures for quantum communication could be examined and compared. Another experiment benefited from the implementation of fiber-integrated quantum light switches which are useful for various quantum technology applications. The results of this project have contributed significantly to new quantum technology for optical precision measurements, as well as laid the foundation for fundamental experiments with the aim of gaining new knowledge about the interplay between gravitation and quantum physics.