EDM with ThF+ (Gen III)

Cornell group fall 2023

Members of ThF+ Gen III.

Top row, from left: Jun Ye, Patricia Hector Hernandez, Trevor Wright, Eric Cornell

Front row, from left: Anzhou Wang, Addison Hartman, Sun Yool Park, Kia Boon Ng


To have the most precise measurement of the eEDM (de), or to place an even more stringent bound on de than our experiment using HfF+, i.e., we want to have a very small uncertainty in the measurement, δde.

Figure of merit

The quality of our precision metrology of the eEDM goes as

δde ~ 1 / (Eeff τ (Nsingle·frep·Tint)1/2 )


  1. Eeff is the effective electric field that the electron sees within the molecule;
  2. τ is the coherence time of the eEDM-sensitive quantum state that we are probing;
  3. Nsingle is the number of ions that we detect per measurement (our measurement is a destructive process);
  4. frep is the repetition rate of our experiment; and
  5. Tint is the total integration time of our measurement (or the total amount of time that the graduate students are willing to take data).


In other words, we want (in order corresponding to the above):

  1. The best molecular ion: ThF+! ThF+ has a 50% larger effective electric field than HfF+ used in our ongoing experiment.
  2. The best molecular ion: ThF+! The eEDM-sensitive state in ThF+ is the ground state, which means that it is not limited to natural decay lifetimes.
  3. A big ion trap: to trap more ions!
  4. A multiplexed ion trap: the Bucket Brigade! The Bucket Brigade is a conveyor belt of ion traps, i.e., a brigade of buckets containing ions. This allows us to “multithread” our experiment and have a repetition rate that is not limited to how long it takes to perform one shot of the experiment.
  5. Hardworking and motivated graduate students! The eEDM experiment requires us to be a jack of all trades (and master of some). Technologies that we use include: (i) high power lasers, (ii) ultra high vacuum systems, (iii) high voltage electronics, (iv) simulations, (v) ion optics, (vi) cryogenics, and many more!


Current Status

We have just completed a decade worth of spectroscopy on ThF+ (and ThF), and are in the midst of assembling a prototype Bucket Brigade (the Baby Bucket). With the Baby Bucket, we hope to demonstrate some key features of the Bucket Brigade (e.g. ion translation, suppression of blackbody radiation excitation under a cryogenic environment), and also to measure a 20 second coherence time on the eEDM-sensitive state in ThF+.

Check out our selected paper selections below, come visit us or drop us an email to find out more!

Selected Paper Abstracts

Systematic and statistical uncertainty evaluation of the HfF+ electron electric dipole moment experiment

We have completed a new precision measurement of the electron's electric dipole moment using trapped HfF+ in rotating bias fields. We report on the accuracy evaluation of this measurement, describing the mechanisms behind our systematic shifts. Our systematic uncertainty is reduced by a factor of 30 compared to the first generation of this measurement. Our combined statistical and systematic accuracy is improved by a factor of 2 relative to any previous measurement.

Phys. Rev. A 108, 012804 (2023) arXiv

Spectroscopy on the eEDM-sensitive states of ThF+

An excellent candidate molecule for the measurement of the electron’s electric dipole moment
(eEDM) is thorium monofluoride (ThF+) because the eEDM-sensitive state, 31, is the electronic
ground state, and thus is immune to decoherence from spontaneous decay. We perform spectroscopy
on X 31 to extract three spectroscopic constants crucial to the eEDM experiment: the hyperfine
coupling constant, the molecular frame electric dipole moment, and the magnetic g-factor. To
understand the impact of thermal blackbody radiation on the vibrational ground state, we study
the lifetime of the first excited vibrational manifold of X 31. We perform ab initio calculations,
compare them to our results, and discuss prospects for using ThF+ in a new eEDM experiment at

Phys. Rev. A. 105, 022823 (2022) arXiv

Second-scale coherence measured at the projection noise limit with hundreds of molecular ions

Cold molecules provide a platform for a variety of scientific applications such as quantum computation, simulation, cold chemistry, and searches for physics beyond the Standard Model. Mastering quantum state control and measurement of diverse molecular species is critical for enabling these applications. However, state control and readout are difficult for the majority of molecular species due to the lack of optical cycling transitions. Here we demonstrate internal state cooling and orientation-selective photofragment imaging in a spin precession measurement with multi-second coherence, allowing us to achieve the quantum projection noise (QPN) limit in a large ensemble of trapped molecular ions. We realize this scheme for both HfF+ and ThF+ --- molecules chosen for their sensitivity to beyond Standard Model physics rather than their amenability to state control and readout.

Phys. Rev. Lett. 124, 053201 (2020) arXiv

Visible and Ultraviolet Laser Spectroscopy of ThF

The molecular ion ThF+ is the species to be used in the next generation of search for the electron's Electric Dipole Moment (eEDM) at JILA. The measurement requires creating molecular ions in the eEDM sensitive state, the rovibronic ground state 3Δ1, v+=0, J+=1. Survey spectroscopy of neutral ThF is required to identify an appropriate intermediate state for a Resonance Enhanced Multi-Photon Ionization (REMPI) scheme that will create ions in the required state. We perform broadband survey spectroscopy (from 13000 to 44000 cm-1) of ThF using both Laser Induced Fluorescence (LIF) and 1+1' REMPI spectroscopy. We observe and assign 345 previously unreported vibronic bands of ThF. We demonstrate 30% efficiency in the production of ThF+ ions in the eEDM sensitive state using the Ω = 3/2 [32.85] intermediate state. In addition, we propose a method to increase the aforementioned efficiency to ~100% by using vibrational autoionization via core-nonpenetrating Rydberg states, and discuss theoretical and experimental challenges. Finally, we also report 83 vibronic bands of an impurity species, ThO.

J. Mol. Spec. 358, (2019) 1-16 arXiv

Broadband velocity modulation spectroscopy of ThF+ for use in a measurement of the electron electric dipole moment

A number of extensions to the Standard Model of particle physics predict a permanent electric dipole moment of the electron (eEDM) in the range of the current experimental limits. Trapped ThF+ will be used in a forthcoming generation of the JILA eEDM experiment. Here, we present extensive survey spectroscopy of ThF+ in the 700-1000 nm spectral region, with the 700-900 nm range fully covered using frequency comb velocity modulation spectroscopy. We have determined that the ThF+ electronic ground state is X 3Δ1, which is the eEDM-sensitive state. In addition, we report high-precision rotational and vibrational constants for 14 ThF+ electronic states, including excited states that can be used to transfer and readout population in the eEDM experiment.

J. Mol. Spec. 319, (2016) 1-9 arXiv