Coherent Optical Metrology Beyond Dipole-Allowed Transitions

The development of modern physics, in particular quantum mechanics, went hand in hand with the continual improvement of precision in metrology, aiming to create the most precise clocks and to detect the most elusive phenomena. Current efforts to test physics beyond the Standard Model require conceptually new measurement methods that are sensitive enough to investigate the time/space invariance of the fundamental physical constants. The goal of this project, COMB.AT, is to develop experimental and theoretical methodology that allows to measure fundamental constants (fine-structure and strong-force-interaction constants, proton- to-electron mass ratio) at a conceptually more sensitive level. In high-precision experiments, narrow transitions of molecules, atoms, or nuclei are typically measured, serving as precise frequency references. Minute changes in fundamental constants result in similarly small alterations in transition frequency. To achieve an ideal experiment, the aim is to identify transitions highly responsive to such changes, ideally as narrow as possible to establish optimal frequency references, yet as strong as possible to ensure high signal-to-noise ratios. However, a fundamental challenge arises: transitions are either narrow and weak or wide and strong. Within COMB.AT, our primary objective is to efficiently couple light to narrow transitions, thereby enhancing light-matter interaction to elevate precision measurement sensitivity. Specifically, we aim to boost sensitivity by employing "twisted" light, which is light carrying orbital angular momentum (OAM), to transitions that are electric-dipole forbidden yet magnetic-dipole and electric-quadrupole allowed. Our precision measurements focus on two systems: a nuclear transition in 229Th and carbon monosulfide, a model for heteronuclear diatomic molecules. The 229Th nucleus is highly sensitive to variations in the fine-structure constant, while rovibrational transitions in molecules are sensitive to changes in the electron- to-proton mass ratio. We will pioneer a revolutionary molecular spectroscopy approach utilizing mid-infrared frequency combs with controlled OAM, in tandem with electric fields. These experiments will facilitate fundamental investigations into the interaction of OAM-light with the simplest diatomic heteronuclear molecular structure, as exemplified by hydrogen deuteride. The project will be implemented by 3 experimental and 2 theory groups. Andrius Baltuška and Thorsten Schumm (TU Wien) will focus on spectroscopy of 229Th nucleus, Adriana Plffy (University of Würzburg) will continue to develop theory for nuclear excitations with twisted light. Oliver Heckl (University of Vienna) will focus on molecular spectroscopy with twisted light, and Mikhail Lemeshko (IST Austria) will develop the theory for molecules interacting with twisted light.