We aim to laser cool and optically trap heavy, polyatomic molecules in order to search for the electric dipole moment (EDM) of the electron.
Our work has recently been featured in Physics World. Read about the PolyEDM experiment here!
Despite its many successes, the Standard Model of particle physics has several problems which must be addressed. For example, the Standard Model, as it stands, does not predict a universe dominated by matter, with scarcely any anti-matter. Yet such is the universe in which we find ourselves.
It is known that any theory explaining this matter/anti-matter asymmetry requires the presence of new time-reversal-violating interactions , and EDMs of fundamental particles are one direct signature of such symmetry violation . Progress over many decades has demonstrated that heavy atoms and molecules possess very high sensitivity to the electron EDM [3-6].
At its core, an electron EDM experiment simply searches for a shift in the electron’s energy due to the presence of an electric field. Performing such a measurement precisely requires large electric fields, long interaction times, and many particles. All of these can be achieved using laser-cooled and trapped, neutral, heavy polyatomic molecules. Furthermore, polyatomic molecules (as opposed to diatomics) promise robust systematic error rejection via internal co-magnetometer states. While some diatomic species offer a subset of these advantages, polyatomics are the only identified platform which simultaneously offer all of them .
As one example, an experiment probing 106 such molecules in an optical trap with a 10 s coherence time would probe T-violating physics at the PeV scale after about 1 week of averaging. This is well beyond the energy reach of existing or conceivable accelerators.
Polyatomic Molecules as EDM Probes
Laser-coolable polyatomic molecules are ideally suited for such an experiment. Polyatomic molecules containing a heavy nucleus, such as Sr or Yb, possess large relativistic enhancements for the electron EDM . At the same time, molecules like SrOH are isoelectronic to CaF, a species for which laser cooling and optical trapping have already been demonstrated [9-12]. Our group has even demonstrated laser cooling of the heavy polyatomic molecules SrOH and YbOH [13, 14] and trapping of the lighter molecule CaOH.
A particularly important aspect of the structure of these molecules is the presence of ‘parity doublets’ arising from nuclear orbital angular momentum . The bending mode of SrOH consists of a pair of states separated by about 20 MHz. Small laboratory electric fields, ~100 V/cm, will mix these states are orient the molecule either along or against the laboratory field. The electron near the Sr nucleus, on the other hand, interacts with an internal electric field of order 1 GV/cm . By selectively populating one or the other orientation and performing a spin precession measurement, the electron EDM interaction can be reversed, thereby suppressing potential systematic errors. More complex polyatomic molecules (like SrSH or YbOCH3) have a similar pair of states due to rigid body rotation. The parity doublets in both species live for >10 s, promising long coherence times.
We have demonstrated the first laser cooling of YbOH, reducing the transverse temperature of a molecular beam from 20 mK to < 600 uK. In the images below, a clear compression of the molecular beam (shown unperturbed in panel (a)) is indicative of cooling when blue-detuned lasers are applied (panel (c)). On the other hand, a clear heating signature is apparent when the laser is red-detuned (panel (b)). This work has recently been accepted to NJP as a Fast Track Communication. You can read about the laser cooling here.
In parallel, we have made the first observations of YbOCH3 molecules, in collaboration with the Steimle group at ASU. Our initial measurements demonstrate that YbOCH3 has relatively diagonal Franck-Condon factors, and is another candidate molecule for laser cooling and trapping.
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