New techniques for a measurement of the electron’s electric dipole moment

Abstract

The electric dipole moment of the electron (eEDM) can be measured with high precision using heavy polar molecules. In this paper, we report on a series of new techniques that have improved the statistical sensitivity of the YbF eEDM experiment. We increase the number of molecules participating in the experiment by an order of magnitude using a carefully designed optical pumping scheme. We also increase the detection efficiency of these molecules by another order of magnitude using an optical cycling scheme. In addition, we show how to destabilise dark states and reduce backgrounds that otherwise limit the efficiency of these techniques. Together, these improvements allow us to demonstrate a statistical sensitivity of 1.8 × 10 −28 e cm after one day of measurement, which is 1.2 times the shot-noise limit. The techniques presented here are applicable to other high-precision measurements using molecules. © 2020 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische GesellschaftNew J. Phys. 22 (2020) 053031 C J Ho et alfactors of E eff /E ext = 120 and −585 respectively 4 . The linear dependence of E eff on E ext indicates that the atoms are only weakly polarised in the external electric field. In polar molecules, the interaction energy is larger because it is easier to polarise these molecules in an electric field. It is more appropriate to write E eff = ηE eff,max for molecules, where η is the degree of polarisation of the molecule, and E eff,max is the maximum effective field seen by the electron when the molecule is fully polarised, η = 1. The latter is typically in the range 10 GV cm −1 to 100 GV cm −1 , which is much larger than electric fields that can be applied in the laboratory.In 2011, the precision of atomic measurements was surpassed in an experiment using YbF, setting a new upper limit 5 of |d e | < 1.06 × 10 −27 e cm [10]. The enhancement of YbF was E eff ≈ −14.5 GV cm −1 . Crucially, a systematic effect which is large for atoms-the Zeeman interaction with the motional magnetic field mimicking the EDM interaction-is highly suppressed in molecules due to their strong tensor polarisability [11]. In 2014, the ACME collaboration pushed the limit down to |d e | < 8.7 × 10 −29 e cm using a beam of ThO molecules in an Ω-doublet state [12]. Molecules in this state are fully polarised in a small applied electric field, giving E eff = E eff,max ≈ 84 GV cm −1 . The Ω-doublet can also be used conveniently for internal co-magnetometry. In 2017, a measurement using trapped HfF + molecular ions (E eff ≈ 23 GV cm −1 ) reported the limit |d e | < 1.3 × 10 −28 e cm [13]. This experiment benefited from the long coherence times available in a molecular ion trap, but was limited by the relatively low number of ions trapped. In 2018, the ACME collaboration improved on their limit, reaching |d e | < 1.1 × 10 −29 e cm [14]. This last result constrains any new physics arising from T-violating effects to energy scales above 3 TeV [14].Many new ideas are now emerging on ho...

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