Electron dipole moment searches with trapped molecular ions

Short answer: The electron's electric dipole moment (eEDM) is a quantity that new particles beyond the Standard Model could induce, and so far it has not been found. Using trapped molecular ions, a 2017 paper reported an upper bound of 1.3 x 10^-28 e cm, a 2022 preprint described the accuracy evaluation of a new measurement, and a 2023 Science paper reported a result consistent with zero that improved the previous bound by a factor of about 2.4.

Why look for an electron dipole moment?

The 2023 Science abstract explains that the imbalance of matter and antimatter in the universe motivates a search for undiscovered particles that violate charge-parity symmetry. Interactions with vacuum fluctuations of the fields of such particles would induce an eEDM. The authors say their constraints cover broad classes of new physics above 10^13 electron volts, beyond the direct reach of current colliders.

What did the 2017 measurement report?

The 2017 Physical Review Letters paper describes the first precision eEDM measurement using trapped molecular ions. The authors use spin interrogation times above 700 ms to achieve high sensitivity and strong rejection of systematic errors. Through electron spin resonance spectroscopy on hafnium fluoride ions in a metastable electronic state, they obtain an eEDM of (0.9 plus or minus 7.7 statistical plus or minus 1.7 systematic) x 10^-29 e cm. That gives an upper bound of 1.3 x 10^-28 e cm at 90 percent confidence. They say this independently confirms the then-current upper bound of 9.4 x 10^-29 e cm from a different experiment, and offers potential to improve on it in the near future.

What did the later work do?

A preprint from 2022, since listed with a Physical Review A reference, reports the accuracy evaluation of a new precision measurement of the eEDM using trapped hafnium fluoride ions in rotating bias fields. The authors describe the mechanisms behind their systematic shifts. They say the systematic uncertainty is reduced by a factor of 30 compared with the first-generation measurement, and the combined statistical and systematic accuracy is improved by a factor of 2 relative to any previous measurement.

The 2023 Science paper presents what its authors call the most precise eEDM measurement yet, using electrons confined inside molecular ions subjected to a huge intramolecular electric field and evolving coherently for up to 3 seconds. The result is consistent with zero, and improves on the previous best upper bound by a factor of about 2.4.

What do the three have in common?

The three results share a method: hold molecular ions in a trap, let the electron spins precess in a very strong internal electric field, and look for a tiny extra precession that would signal an eEDM. Longer coherent evolution times make the measurement more sensitive, which is why the 2017 paper stresses spin interrogation times above 700 ms and the 2023 paper quotes up to 3 seconds. The 2022 preprint is mainly about controlling the systematic shifts that could mimic a signal, so that the improved accuracy can be trusted.

How to read this

All three results are limits and not detections. The measurements use trapped molecules as precision sensors, with long coherence times as the key resource, which is why they fit a quantum-sensing theme. This post is based on abstracts, and full texts were not checked. It makes no claim about products, and does not claim any new particle exists.

FAQ

Was an electron dipole moment found?

No. The 2023 result is consistent with zero.

What is the most recent bound reported here?

An improvement by a factor of about 2.4 over the previous best bound, per the 2023 abstract.

Why use molecular ions?

The authors cite a huge intramolecular electric field and long spin interrogation times.

How long did the electrons evolve coherently?

Up to 3 seconds in the 2023 paper.