We are building a new experiment to laser cool radium-containing molecules, in collaboration with MIT (Garcia Ruiz group), Caltech (Hutzler group), and the Facility for Rare Isotope Beams (FRIB).
Why Radium?
Radium-containing molecules offer unparalleled opportunity to study some of the open questions in our understanding of the universe, such as the root cause of the observed matter/antimatter asymmetry, the long-sought search for CP violation in the strong interaction, and the potential existence of yet-undetected particles and forces [1, 2, 3]. The octuple deformation of radium nuclei [4] is predicted to amplify both parity (P) and time-reversal (T) violating nuclear properties by more than three orders of magnitude compared to stable molecules [5, 6, 7].
Recent results in the spectroscopy of radioactive molecules have demonstrated that radium-containing molecules, such as 225RaF and 225RaOH, additionally possess a relatively simple structure that is very favorable for laser cooling (see Fig. 1) [7, 8, 9]. This combination makes these molecules excellent quantum sensors for fundamental physics, opening the door to searches for P,T violating nuclear effects, such as the nuclear Schiff moment, manifesting as electric dipole moments (EDMs) of molecules [1, 2, 3].
Experimental Work
The roadmap to ultra-sensitive measurements with radium-containing molecules begins with the less radioactive isotope 226Ra (τ1/2 = 1600 yr), simplifying the experimental apparatus and molecule structure, and enabling rapid prototyping at university laboratories. To demonstrate the first laser cooling and trapping of a radioactive molecule, we will work with the well-characterized diatomic molecule 226RaF, which has an ideal molecular structure for laser cooling [9]. Concurrently, we will also test optical cycling (the key property that enables laser cooling) in 226RaOH, where the polyatomic structure generically provides parity doublets, a powerful structural advantage for state-of-the-art precision measurements [10, 11, 12, 13, 14, 15]. Fortunately, RaF and RaOH molecules have isoelectronic structure; hence sharing common production methods and similar optical transitions, accessible with common laser technologies. Many of the initial experimental breakthroughs in RaF can pave the way for advances with RaOH, and their similarity enables changing from RaF to RaOH in an experiment with the metaphorical “flip of a switch”.
Initially, experimental prototyping will be performed in cold (∼ K) molecular beams, an essential tool for measuring, cooling, and trapping molecules. This experiment will overcome challenges in producing and cooling 226Ra-containing molecules, establishing the path for future work to capture the molecular beam in a long-lived trap at ultracold temperatures (< mK). This work will establish the experimental basis for eventual enhanced hadronic symmetry violation measurements with 225Ra (τ1/2 = 14.9 d) containing molecules at the Facility for Rare Isotope Beams (FRIB).