Ultra stable optical references for applications on Earth and in space

Modern optical frequency references are among the most stable known "timing-signals" of our universe and are about to revolutionize the concepts of current time measurements and metrology. They are also used in many precision experiments to test the foundations of modern physics with highest accuracy. However, for the step from the laboratory to actual usability in technical and scientific applications the two main obstacles are the lack of (a) truly mobile optical frequency standards and (b) suitable stable local oscillators. This project has the goal to minimize these problems. Through continuous knowledge, experience and technology exchange shall the locally already in the research groups from Germany and Australia existing optical reference systems continuously being improved and novel, more robust and long-term stable ones be created: (a) Ultra-precision spectroscopy of molecular iodine The German group is currently working on a very robust iodine standard based on a monolithic-type Zerodur-setup with a planned absolute frequency stability of better than 10^-15. Parallel to that has the Australian group successfully loaded molecular iodine in hollow-core optical fibers. This technology would allow realizing a complete fiber-optics-based iodine standard, which could thus achieve unprecedented robustness and compactness. Both approaches will be fully developed in this project. (b) Ultra-stable cryogenic optical resonators Both groups are currently working on ultra-stable cryogenic optical resonators made of sapphire or silicon, which should have a much better short-and long-term stability than the conventional glass ceramic resonators at room temperature. The goal is a relative frequency stability of a few 10^-17 and a high level of maturity, so that the resonators can be used as local oscillators in various precision measurement in continuous operation, for example for tests of Lorentz invariance and the gravitational redshift.