Researchers Discover Magic Wavelength for Molecular Hydrogen Spectroscopy

In the realm of atomic and molecular physics, the study of fundamental physics and the testing of quantum theory are essential for advancing scientific understanding. The accurate spectroscopy of simple atomic and molecular systems plays a crucial role in this pursuit. Notably, atomic hydrogen has been a cornerstone in the development and testing of quantum electrodynamics, providing a reference for ab initio quantum calculations and contributing to the adjustment of fundamental constants.

While atomic hydrogen has been instrumental in these endeavors, other systems like the helium atom, HD+ ion, exotic atoms, and the hydrogen molecule also play significant roles in testing quantum theory and exploring new physics beyond the standard model. Among these, the hydrogen molecule stands out for its unique set of ultralong living rovibrational states, making it a promising candidate for studying fundamental physics with exceptional accuracy.

The precision achieved in molecular hydrogen spectroscopy has been greatly enhanced by advancements in technology, such as the implementation of optical frequency combs. These developments have enabled high-accuracy measurements using techniques like infrared-ultraviolet double resonance spectroscopy and cavity-enhanced spectroscopy. Notably, the highest accuracy of 13 kHz has been attained for the HD isotopologue, showcasing the progress in molecular hydrogen spectroscopy.

To further advance the field of molecular hydrogen spectroscopy, trapping cold H2 samples in an optical lattice is essential. This approach requires addressing the AC Stark effect caused by the trapping laser field, which poses a challenge due to the large energy of the first electronic excitation in H2. Traditional methods for finding a magic wavelength, commonly used in atomic spectroscopy, are not directly applicable to the H2 molecule due to the unique characteristics of its polarizability.

In a groundbreaking study, a new approach to finding a magic wavelength for rovibrational transitions in molecular hydrogen has been proposed. By leveraging the anisotropy of the dipole polarizability in H2, researchers have identified a rovibrational transition where the AC Stark shift can be effectively compensated. This compensation is achieved by tuning the trapping laser to a specific magic wavelength, at which the quadrupole polarizability cancels the residual dipole polarizability, eliminating the AC Stark effect.

The choice of a magic wavelength is critical for experimental realization, taking into account factors such as detuning, Rabi frequencies, and light shifts of trap laser resonances. Through meticulous analysis and calculations, a magic wavelength of 2413 nm has been identified for the 1–0 S(0) transition in the H2 isotopologue. This wavelength, located 0.23 MHz red-detuned from the center of the Q(2) 1–0 line, offers optimal conditions for reducing the AC Stark shift in molecular hydrogen spectroscopy.

Looking ahead, future research directions include exploring the application of the magic wavelength concept to ortho-H2 and other molecular hydrogen isotopologues with nonzero nuclear spin. The hyperfine structure of these isotopologues presents additional complexities but also opportunities for further advancements in experimental implementation of magic wavelengths in molecular spectroscopy.