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arXiv:2602.00713 · Physical Review Letters

Spin interferometry in a beam of ultracold molecules

R. A. Jenkins, M. T. Ziemba, F. J. Collings, X. S. Zheng, F. Castellini, E. Wursten, J. Lim, B. E. Sauer, M. R. Tarbutt

physics.atom-phquant-phSubmitted Jan 31, 2026• Announced Feb 3, 2026• 0 citations• Revised May 22, 2026 (v2)
v1 · Jan 31, 2026v2 · May 22, 2026
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Abstract

We describe a spin interferometer using ultracold YbF molecules and develop the complete set of techniques needed to measure the electron's electric dipole moment, $d_e$, with this apparatus. The molecules are cooled in an optical molasses and prepared in a single internal quantum state. A Raman transition prepares a spin superposition which evolves in parallel magnetic and electric fields before a second Raman transition maps the phase onto the populations of two hyperfine states. These populations are read out using detectors that have spatial and temporal resolution and approach unit efficiency. We characterize the efficiencies and fidelities of all these steps and evaluate the sensitivity of this approach to measuring $d_e$.

AI Summary

The paper tests whether laser-cooled neutral molecules with a large internal effective electric field can support an electron electric-dipole-moment (eEDM) measurement. It demonstrates a spin interferometer in a transversely cooled YbF beam, integrating optical pumping, Raman preparation and recombination, spin precession in parallel electric and magnetic fields, and state-selective fluorescence readout. Molecules are cooled to approximately $100\,\mu\mathrm{K}$ and optically pumped into one hyperfine state with efficiency $0.738(11)$. Two Raman pulses separated by $0.77\,\mathrm{m}$ provide approximately $5\,\mathrm{ms}$ of precession. Velocity-resolved measurements give weighted mean splitter and recombiner transfer efficiencies of $0.88$ and $0.76$. Magnetic-field scans demonstrate interference with mean contrast $0.65$, reaching approximately $0.8$ at low speeds. Independent measurements of transfer efficiencies, background populations and detector cross-talk largely reproduce the observed fringes. Camera-based molecular detection efficiency is estimated at $54\%$. Using the demonstrated contrast, precession time and $2.0\times10^6$ molecules per shot at $5\,\mathrm{Hz}$, the projected shot-noise-limited eEDM uncertainty is $8.6\times10^{-30}\,e\,\mathrm{cm}$ in 24 hours, assuming a $50\%$ duty cycle. Approximately 100 days would therefore permit statistical uncertainty below $10^{-30}\,e\,\mathrm{cm}$; no eEDM measurement or bound is reported. Limitations include velocity-dependent Raman transfer, residual population backgrounds, detector cross-talk, and the challenge of attaining the projected shot-noise limit while controlling electric-field-correlated magnetic shifts. Longer precession times and larger molecular flux are proposed routes to uncertainties below $10^{-31}\,e\,\mathrm{cm}$, rather than demonstrated performance.

No sign of AI-generated text

Error found: The abstract describes detection as approaching unit efficiency, although the demonstrated molecular detection efficiencies are 54% for EMCCDs and 4.5% for PMTs. Appendix C's contrast formula has an incorrect first-order cross-talk correction: its factor 1 − 2χ1 should be 1 − 2χ1 − 2χ2 + 3χ1χ2, producing a small error in the predicted contrast.

Written by gpt-6.1-sol from the paper's full text. Automated: it can be wrong.

Journal reference
Phys. Rev. Lett. 136, 253401 (2026)
DOI
10.1103/6g7x-y15q
Comments
8 pages, 6 figures

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We describe a spin interferometer using ultracold YbF molecules and develop the complete set of techniques needed to measure the electron's electric dipole moment, ded_ede​, with this apparatus. The molecules are cooled in an optical molasses and prepared in a single internal quantum state. A Raman transition prepares a spin superposition which evolves in parallel magnetic and electric fields before a second Raman transition maps the phase onto the populations of two hyperfine states. These populations are read out using detectors that have spatial and temporal resolution and approach unit efficiency. We characterize the efficiencies and fidelities of all these steps and evaluate the sensitivity of this approach to measuring ded_ede​.
The paper tests whether laser-cooled neutral molecules with a large internal effective electric field can support an electron electric-dipole-moment (eEDM) measurement. It demonstrates a spin interferometer in a transversely cooled YbF beam, integrating optical pumping, Raman preparation and recombination, spin precession in parallel electric and magnetic fields, and state-selective fluorescence readout.
Molecules are cooled to approximately 100 μK100\,\mu\mathrm{K}100μK and optically pumped into one hyperfine state with efficiency 0.738(11)0.738(11)0.738(11). Two Raman pulses separated by 0.77 m0.77\,\mathrm{m}0.77m provide approximately 5 ms5\,\mathrm{ms}5ms of precession. Velocity-resolved measurements give weighted mean splitter and recombiner transfer efficiencies of 0.880.880.88 and 0.760.760.76. Magnetic-field scans demonstrate interference with mean contrast 0.650.650.65, reaching approximately 0.80.80.8 at low speeds. Independent measurements of transfer efficiencies, background populations and detector cross-talk largely reproduce the observed fringes. Camera-based molecular detection efficiency is estimated at 54%54\%54%.
Using the demonstrated contrast, precession time and 2.0×1062.0\times10^62.0×106 molecules per shot at 5 Hz5\,\mathrm{Hz}5Hz, the projected shot-noise-limited eEDM uncertainty is 8.6×10−30 e cm8.6\times10^{-30}\,e\,\mathrm{cm}8.6×10−30ecm in 24 hours, assuming a 50%50\%50% duty cycle. Approximately 100 days would therefore permit statistical uncertainty below 10−30 e cm10^{-30}\,e\,\mathrm{cm}10−30ecm; no eEDM measurement or bound is reported.
Limitations include velocity-dependent Raman transfer, residual population backgrounds, detector cross-talk, and the challenge of attaining the projected shot-noise limit while controlling electric-field-correlated magnetic shifts. Longer precession times and larger molecular flux are proposed routes to uncertainties below 10−31 e cm10^{-31}\,e\,\mathrm{cm}10−31ecm, rather than demonstrated performance.
Abstract Measurements of the electron’s electric dipole moment (eEDM) are demanding tests of physics beyond the standard model. We describe how ultracold YbF molecules could be used to improve the precision of eEDM measurements by two to three orders of magnitude. Using numerical simulations, we show how the combination of magnetic focussing, two-dimensional transverse laser cooling, and frequency-chirped laser slowing, can produce an intense, slow, highly-collimated molecular beam. We show how to make a magneto-optical trap of YbF molecules and how the molecules could be loaded into an optical lattice. eEDM measurements could be made using the slow molecular beam or using molecules trapped in the lattice. We estimate the statistical sensitivity that could be reached in each case and consider how sources of noise can be reduced so that the shot-noise limit of sensitivity can be reached. We also consider systematic effects due to magnetic fields and vector light shifts and how they could be controlled.
We propose an experiment to measure the electric dipole moment (EDM) of the electron using ultracold YbF molecules. The molecules are produced as a thermal beam by a cryogenic buffer gas source, and brought to rest in an optical molasses that cools them to the Doppler limit or below. The molecular cloud is then thrown upward to form a fountain in which the EDM of the electron is measured. A non-zero result would be unambiguous proof of new elementary particle interactions, beyond the standard model.
Cold molecules are attractive for a wide range of scientific applications, including quantum computation, the study of chemical reactions, and tests of fundamental physics. Laser cooling has proved to be an invaluable technique in the cooling of atoms. This technique was once thought to be infeasible for molecules, because it is difficult to find a closed cycling transition due to their vibrational structure. Recently, laser cooling of several diatomic species has been demonstrated. These molecules possess electronic transitions with highly diagonal Franck-Condon matrices, which make it possible to drive a quasi-closed cycling transition. Ytterbium fluoride (YbF) molecules are amenable to laser cooling and are especially interesting because they are used to measure the electron's electric dipole moment (eEDM). Measurements of the eEDM test the prediction of theories that extend the Standard Model of particle physics. The sensitivity of an eEDM experiment could be greatly increased by using ultracold molecules produced by direct laser cooling. This thesis presents work done towards producing a laser-cooled beam of YbF for an eEDM experiment. This work includes the construction of the cooling experiment, a novel method for efficiently combining laser beams of very similar frequencies, results of spectroscopic measurements to find the required transitions for laser cooling, the results of initial optical cycling experiments, and the first laser cooling results of YbF. Using a one-dimensional optical molasses, a beam of molecules is Doppler cooled in one transverse direction to a temperature of approximately 70 mK. Preliminary evidence of cooling to lower temperatures through a Sisyphus mechanism is also presented. Finally, paths towards improving the laser cooling are suggested. The work opens the door to improved measurements of the eEDM using ultracold YbF molecules.
Abstract We use two-dimensional transverse laser cooling to produce an ultracold beam of YbF molecules. Through experiments and numerical simulations, we study how the cooling is influenced by the polarization configuration, laser intensity, laser detuning and applied magnetic field. The ultracold part of the beam contains more than 2 × 105 molecules per shot and has a temperature below 200 μK, and the cooling yields a 300-fold increase in the brightness of the beam. The method can improve the precision of experiments that use molecules to test fundamental physics. In particular, the beam is suitable for measuring the electron electric dipole moment with a statistical precision better than 10−30 e cm.
Heavy polar molecules can be used to measure the electric dipole moment of the electron, which is a sensitive probe of physics beyond the Standard Model. The value is determined by measuring the precession of the molecule’s spin in a plane perpendicular to an applied electric field. The longer this precession evolves coherently, the higher the precision of the measurement. For molecules in a trap, this coherence time could be very long indeed. We evaluate the sensitivity of an experiment where neutral molecules are trapped electrically, and compare this to an equivalent measurement in a molecular beam. We consider the use of a Stark decelerator to load the trap from a supersonic source, and calculate the deceleration efficiency for YbF molecules in both strong-field seeking and weak-field seeking states. With a 1 s holding time in the trap, the statistical sensitivity could be ten times higher than it is in the beam experiment, and this could improve by a further factor of five if the trap can be loaded from a source of larger emittance. We study some effects due to field inhomogeneity in the trap and find that rotation of the electric field direction, leading to an inhomogeneous geometric phase shift, is the primary obstacle to a sensitive trap-based measurement. 1