John Doyle at Harvard University

Development of New Methods and Molecules for Precision Measurement

Doyle Figure 1: A beam of YbOH molecules, imaged by laser excitation.
Figure 1: A beam of YbOH molecules, imaged by laser excitation. The molecular beam can be heated (b) or cooled (c) by appropriately tuning the frequency of transverse laser beams. In the cooling configuration, the on-axis phase-space density is increased by more than a factor of six. From [New J. Phys. 22 022003 (2020)].
Doyle Figure 2: High-resolution, rotationally resolved spectrum of YbOCH3
Figure 2: High-resolution, rotationally resolved spectrum of YbOCH3, a complex symmetric top molecule promising for electron electric dipole moment searches. From [Phys. Rev. A 103 022814 (2021)].


We have conducted detailed studies of numerous polyatomic molecules for laser-cooling and trapping, with applications to precision measurement and quantum science. We conducted spectroscopy of several molecules sensitive to the electron electric dipole moment, namely YbOH, YbOCH3, and SrOH, as well as analyzed in detail how the rovibrational spectrum of SrOH could probe certain classes of dark matter. Laser cooling of YbOH has been demonstrated. The first observation of YbOCH3 has been made, with an initial pathway toward laser cooling established. Vibrational branching ratios of YbOH, CaOH, and SrOH were measured with unprecedented sensitivity, enabled by optical cycling for improved detection efficiency, validating the detailed experimental pathway to deep 3D cooling and magneto-optical trapping of all three species. To explore the limitations of laser-cooling polyatomic molecules, we produced the complex aromatic compounds CaOPh (and its derivatives), CaONap, and SrONap (Ph is phenol and Nap is napthol) and observed vibrational branching ratios favorable for optical cycling—opening promising new avenues for future directions. To improve loading of complex molecules into a trap, we have demonstrated efficient Zeeman-Sisyphus slowing of polyatomic molecules with fewer than 10 photon scatters. In a complementary direction, we have also improved the reaction chemistry of metal atoms and reagent gases via promotion of the atoms to metastable states using a new method relying on only low-power lasers. This work will increase the number of ultracold molecules usable in a wide class of ultracold polyatomic molecule experiments.

Big scientific questions addressed

Our research is primarily oriented toward precision measurements that can address the origin of the matter-antimatter asymmetry of the universe (via the measurement of the electron electric dipole moment) and the microscopic nature of dark matter (via observation of time-varying rovibrational energy levels of polyatomic molecules). Key questions toward these aims include: (1) what is the limit to the types of molecules for which we can exercise full quantum control; and (2) how can we more efficiently produce and trap molecules to use in these measurements?

Researchers Whose Research Profited from these Funds
  1. Zack Lasner, a postdoc (later research scientist) in the Doyle group, was partially supported by the CFP Templeton grant between July 2019 and June 2022.
  2. John Doyle, Professor of Physics at Harvard

Notable scientific progress was enabled by the CFP Templeton grant?

The work supported by the CFP and the Templeton Foundation proceeded along two complementary avenues: (1) exploring new polyatomic molecular species with promising features for precision measurements and quantum science; and (2) developing improved methods to produce and trap laser-coolable polyatomic molecules. We describe each of these avenues in turn.

Polyatomic molecules possess unique features such as a multiplicity of rotational and vibrational modes, and generic “parity doublets” that enable full molecular polarization in small laboratory electric fields, that make them compelling candidates for a variety of precision measurements and quantum science applications. Numerous polyatomic molecules have been proposed for laser cooling but to date only SrOH, CaOH, YbOH, and CaOCH3 have been laser-cooled in one dimension, while only CaOH has been magneto-optically trapped. Exploring and validating new laser-coolable species is a critical bottleneck to exploiting the full utility of laser-cooled polyatomic molecules.

Toward this end, we have undertaken a number of studies with support from the CFP Templeton grant. For all work we describe below, the postdoc was supported by the CFP Templeton. First, we Sisyphus cooled a beam of YbOH molecules by optically cycling approximately 500 photons, increasing the phase-space density of on-axis molecules by more than a factor of 6 [New J. Phys. 22 022003 (2020)]. Due to the heavy Yb metal atom, on which the optical cycling process is localized, YbOH is subject to large perturbations, which were not well studied or understood prior to this work. The possibility of laser-cooling in this heavy polyatomic molecule regime therefore needed to be experimentally verified. In later work we studied the vibrational branching ratios, radiative lifetimes, and transition strengths of dozens of vibronic transitions in YbOH, providing a deeper understanding about the molecule and laying out an initial pathway toward full 3D laser cooling and trapping [J. Phys. Chem. A 124 3135 (2020)]. We built upon this work, and further validated the laser-coolability of YbOH, by directly measuring vibronic branching ratios to unprecedented sensitivity. This was possible by leveraging its optical cycling capability to achieve high molecular detection efficiency [J. Chem. Phys. 155 091101 (2021)]. Through this work we were able to experimentally determine which vibrational states in the ground electronic manifold are populated after more than 10,000 electronic excitations, and therefore identify vibrational repumping lasers necessary to achieve full 3D magneto-optical trapping. The same work determined the vibrational repumping lasers needed for CaOH, which was subsequently magneto-optically trapped in our lab (in an unrelated experiment).

A long-term extension of the work with YbOH is to laser cool its symmetric top analogue, YbOCH3, which could offer longer coherence times in its parity-doubled state (occurring in a rotationally excited state, as opposed to a shorter-lived vibrationally excited state for YbOH). We made the first observation of YbOCH3 and characterized it spectroscopically [Phys. Rev. A 103 022814 (2021)]. This work demonstrated that rapid optical cycling for YbOCH3 is feasible in a manner closely analogous to YbOH.

Another molecule that we have studied with support from the CFP Templeton grant is SrOH. Although SrOH had already been laser-cooled in one dimension and was fairly well understood, a detailed vibrational repumping pathway to 3D magneto-optical trapping had not been experimentally validated. We therefore measured vibrational branching ratios in SrOH to the level of approximately 10 ppm, more than sufficient to identify all vibrational repumping lasers necessary to achieve magneto-optical trapping [Phys Rev. A 106 L020801 (2022)]. Around this time, we also undertook a much more extensive study of possible systematic errors for a previously unpublished proposal for a potential dark matter search using ultracold SrOH molecules, laying the foundation for a precision measurement that we are now actively pursuing [Phys. Rev. A 103 043313 (2021)].

a precision measurement that we are now actively pursuing [Phys. Rev. A 103 043313 (2021)]. The last category of new polyatomic molecules that we explored for laser cooling applications was aromatic compounds decorated with alkaline-earth optical cycling centers [Nature Chemistry 14 995 (2022), J. Phys. Chem. Lett. 13 7029 (2022)]. We measured vibrational branching ratios of CaOPh (i.e., CaOC6H5) and its derivatives (CaOPh-3-CH3, CaOPh-3-F, CaOPh-3-CF3, CaOPh-3,4-F2, and CaOPh-3,4,5-F3), as well as CaONap (i.e., CaOC10H7) and SrONap. This was highly exploratory work, pushing the approach of alkaline-earth optical cycling centers attached to electronegative ligands to unprecedented complexity. Perhaps surprisingly, the paradigm appears to hold: even for some of the largest of these molecules, we observe a 95% vibrational branching probability to the ground vibrational state within the ground electronic manifold. These results have stimulated a robust experimental effort in our group to further understand and laser-cool aromatic compounds decorated with optical cycling centers.

The second avenue of research we pursued was enhancing the number of usable molecules in a precision measurement or quantum science application with polyatomic molecules, via increased production or trapping efficiency. Our most notable project toward this end was Zeeman-Sisyphus deceleration of CaOH (and later YbOH), which lowers the forward velocity of a molecular beam to within the capture velocity of a magneto-optically trap by scattering fewer than 10 photons [Phys. Rev. Lett. 127 263002 (2021)]. Since polyatomic molecules possess an increased number of vibrational modes to which they can decay, photon-efficient slowing and cooling methods are of greater importance compared to diatomic molecules. Zeeman-Sisyphus deceleration can be applied with extreme generality to radical species with even very modest photon cycling capabilities, and is thus a significant advance in the toolbox to slow and trap increasingly complex molecules.

In an entirely complementary direction, we have also studied improved production of SrOH molecules via enhanced reaction rates in excited atomic states. Previous work showed that the production of CaOH and YbOH, via reaction of metal atoms (Ca or Yb) with water or methanol, is increased in the metastable triplet atomic states. This had been demonstrated by directly driving to the metastable state using a weak laser transition. For SrOH, we have investigated an alternative pathway toward increased molecular production, namely two-step excitation on moderately strong transitions followed by moderately strong spontaneous decay to the metastable states. We have observed up to a factor of 5 improvement in molecular production, using low-power and inexpensive ECDL lasers. This method is expected to generalize to other atoms such as Ca and Yb, and thus offer lower-power and lower-cost alternatives to achieving improved chemical production in a wide variety of molecules. We have not yet published these results as we expect further improvements are possible.

Notable scientific progress that was Triggered, Launched or Enabled that Can be Reasonably Expected in the Next Couple of Years?
  1. Our increased understanding of YbOH, YbOCH3, and SrOH, all undertaken with CFP Templeton support, has motivated an increased focus on SrOH for both the original proposed dark matter measurement as well as an electron electric dipole moment search (which was initially planned with YbOH). We expect a magneto-optical trap of SrOH, which will be the first of its kind, within approximately one year. Subsequently, we will perform both the electron EDM and dark matter precision measurements with trapped, ultracold SrOH molecules. The work already performed with YbOH will also be a valuable foundation for future improved electron EDM measurements with MOH (M=Sr,Yb) molecules due to the increased intrinsic sensitivity of Yb compared to Sr.

  2. We have extended the paradigm of optical cycling with alkaline earth molecules functionalizing electronegative ligands to far larger and more complex species (e.g., SrONap). We are pursuing high-resolution spectroscopic studies of CaOPh with the aim of laser-cooling it in the near future.

  3. With the initial demonstration of Zeeman-Sisyphus slowing of CaOH, and later also of the more complex molecule YbOH, we have an established tool for efficient slowing of any radical species capable of scattering 10 photons. This will be a powerful tool for future work, especially with polyatomic molecules where scattering tens of thousands of photons is experimentally challenging.

  4. We have explored a low-power, inexpensive method of increasing molecular production in SrOH using a two-step excitation followed by spontaneous decay of the atomic precursor to enhance chemical reaction rates. This method is expected to generalize to both other metal atoms and other ligands, to increase molecular production in a wide variety of experimental applications.

Full citations and whether support was properly acknowledged
  1. “Pathway toward Optical Cycling and Laser Cooling of Functionalized Arenes,” D. Mitra, Z.D. Lasner, . . . , J.M. Doyle. J. Phys. Chem. Lett. 13 7029 (2022)

  2. “Functionalizing aromatic compounds with optical cycling centers,” G.-Z. Zhu, . . . , Z.D. Lasner, . . . , J.M. Doyle, and Eric R. Hudson. Nature Chemistry 14 995 (2022)

  3. “Vibronic branching ratios for nearly-closed rapid photon cycling of SrOH,” Z. Lasner, . . . , J.M. Doyle. Phys Rev. A 106 L020801 (2022)

  4. “Ultracold polyatomic molecules for quantum science and precision measurements,” J.M. Doyle, B.L. Augenbraun, and Z.D. Lasner. JPS Conference Proceedings: 24th International Spin Symposium (2022)

  5. “Zeeman-Sisyphus Deceleration of Molecular Beams,” B.L. Augenbraun, . . . , Z.D. Lasner, J.M. Doyle. Phys. Rev. Lett. 127 263002 (2021). Support acknowledged.

  6. “Accurate prediction and measurement of vibronic branching ratios for laser cooling linear polyatomic molecules,” C. Zhang, . . . , Z.D. Lasner, . . . , J.M. Doyle, L. Cheng. J. Chem. Phys. 155 091101 (2021)

  7. “Fast and high-yield loading of a D2 magneto-optical trap of potassium from a cryogenic buffer-gas beam,” Z. Lasner, . . . , J.M. Doyle. Phys. Rev. A. 104 063305 (2021)

  8. “Enhanced sensitivity to ultralight bosonic dark matter in the spectra of the linear radical SrOH,” I. Kozyryev, Z.D. Lasner, J.M. Doyle. Phys. Rev. A 103 043313 (2021)

  9. “Observation and laser spectroscopy of ytterbium monomethoxide, YbOCH3,” B.L. Augenbraun, Z.D. Lasner, . . . , J.M. Doyle, T.C. Steimle. Phys. Rev. A 103 022814 (2021). Support acknowledged.

  10. “Branching Ratios, Radiative Lifetimes, and Transition Dipole Moments for YbOH,” E.T. Mengesha, . . . , Z.D. Lasner, J.M. Doyle. J. Phys. Chem. A 124 3135 (2020).

  11. “Laser-cooled polyatomic molecules for improved electron electric dipole moment searches,” B.L. Augenbraun, Z.D. Lasner, . . . , J.M. Doyle. New J. Phys. 22 022003 (2020).

Resulting publications reasonably expected in the next couple of years
  1. Magneto-optical trapping of SrOH molecules

  2. Increased molecular production via strong optical pathways to excited-state chemistry

  3. Laser-cooling of complex asymmetric top molecules, e.g. CaOPh

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