Limits and potential of distributed quantum metrology

High precision measurements of physical quantities are of major importance in natural sciences and beyond, with high relevance also for technical applications. Precise measurement of time using atomic clocks is e.g. the basis for determining position via GPS. Quantum mechanics thereby offers a significantly improved precision as compared to classical approaches, and one tries to realize this advantage for practical applications. So far these attempts where limited to local sensing, e.g. of magnetic or gravitational fields. Recent development in quantum technologies enable however not only local control of individual atoms or other quantum objects, but also to distribute, manipulate and entangle them over long distances. This project deals with such non-local quantum networks and their usage as sensor networks. Coupled, spatially separated sensors allow for improved measurements of spatial dependent quantities such as field changes or waves, or to improve resolution as in telescope arrays. In this research project we develop methods and protocols to use entangled quantum systems as sensor networks. We investigate which quantum systems can be used, and how to generate and measure them. Small fluctuations and noise are however a serious problem, as they jeopardize the quantum advantage. A central part of this project is concerned with methods to deal with such (correlated) noise processes in order to maintain the quantum advantage. We also investigate different applications, e.g. for telescope arrays. However, sensor networks are not limited to situations where sensors are distributed over long distances basically any arrangement of sensors at different positions is a sensor network. This includes also multiple atoms at different positions in an electromagnetic trap, or multiple NV-vacancy centers in diamond. We aim for using our developed methods and protocols also in this context, e.g. to improve or stabilize atomic clocks, or for NV-center microscopes. Despite the fact that the corresponding systems are very distinct, the methods as such are universally applicable. We will use the methods to deal with correlated noise processes not only for sensor networks, but also in the context of quantum entanglement switches and quantum simulators.