Research

Polyatomic molecules offer the unique ability to probe CP-violation at the PeV scale through laser cooling and trapping combined with robust systematic error rejection.

Reaching the PeV Scale

Searches for the electron EDM depend on the ability to perform spin precession measurements with long interaction time and large numbers. Both of these can be achieved with laser-cooled and trapped neutral polyatomic molecules, which also offer robust systematic error rejection techniques via internal co-magnetometer states. The rapid advances in laser cooling of diatomic and polyatomic molecules have made such an experiment within reach, and will enable significant increases in sensitivity. As an example, consider 10⁶ heavy, eEDM-sensitive molecules in an optical trap with 10 s coherence time. After 1 week of integration, such an experiment would have sensitivity to CP-violating physics at the PeV scale. This is beyond the reach of conceivable accelerators. The ability to combine long coherence times, large numbers, internal co-magnetometers, and the ability to further enhance sensitivity via quantum control techniques is unique to polyatomic molecules.

To realize this experiment, we have worked with three molecules that combine these advantageous features: CaOH, SrOH, and YbOH.

CaOH - "EDM Pathfinder"

CaOH is not very sensitive to the eEDM due to its light mass; however, as it is the pioneering platform in ultracold, laser-cooled polyatomics, we have implemented a number of the critical steps toward an EDM experiment as a proof-of-principle demonstration of the power of this new approach. Using optically-trapped, ultracold CaOH at Harvard, we transferred into the EDM-sensitive bending mode, performed high-resolution spectroscopy of the science state, created a coherent electron spin superposition, observed free electron spin precession, and performed readout of the electron spin state - all of the steps required for an EDM experiment. We were able to verify the proposed advantages of polyatomics, including large polarization at small fields, tunable electromagnetic interactions, and co-magnetometry.

SrOH - Electron EDM in trapped, ultracold polyatomics

The heavier nucleus of SrOH gives it increased sensitivity to the electron EDM while maintaining the advantages of laser-coolability afforded by CaOH. SrOH was in fact the first polyatomic molecule to be laser cooled; since then, the branching ratios in SrOH have been measured to very high precision, and confirm that a sufficient number of photons can be scattered to slow, cool, trap, and control these molecules for an EDM experiment. We are currently developing an SrOH-based electron EDM experiment which will therefore benefit from all of the advantages of ultracold, trapped, polyatomic molecules.

YbOH - Increased sensitivity and nuclear CP-violation

The Yb atom is a powerful atom for precision measurement. Its large mass induces highly relativistic motion of valence electrons near the heavy nucleus, making it a strong probe of CP violating physics. YbF has been used to search for the eEDM, and the highly deformed nucleus of ¹⁷³Yb makes it highly sensitive to hadronic CP-violation via a nuclear magnetic quadrupole moment (MQM). The electronic structure of Yb is nearly identical to an alkaline earth atom, so Yb-bearing molecules are amenable to laser cooling, though with additional challenges due to the complexity of the Yb electronic and nuclear structures.

Future Directions - New metals and new ligands

Symmetric top molecules such as YbOCH₃ may offer even more advantages due to their very low-lying metastable states with parity doublets that are even more polarizable, and heavy, radioactive species such as Ac, Ra, or Pa can offer extremely large sensitivity to the electron EDM as well as nuclear symmetry violations.

Figure: Symmetric tops such as YbOCH₃ should maintain the ability to cycle photons for laser cooling while offering low-lying, fully-polarizable states corresponding to rotations of the CH₃ group around the symmetry axis.

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