Attaining the shot noise limit in the ACME measurement of the electron electric dipole moment
Experimental searches for the electron electric dipole moment, $d_e$ probe new physics beyond the Standard Model. Recently, the ACME Collaboration set a new limit of |d_e| <1.1\times 10^ 29} $e\cdot \textrm{cm} [Nature \textbf{562} 355 (2018) constraining time reversal symmetry (T) violating physics in the 3 100 TeV energy scale. ACME extracts $d_e$ from the measurement of electron spin precession due to the thorium monoxide (ThO) molecule's internal electric field. This recent ACME II measurement achieved an order of magnitude increased sensitivity over ACME I by reducing both statistical and systematic uncertainties in the measurement of the electric dipole precession frequency. The ACME II statistical uncertainty was a factor of 1.7 above the ideal shot noise limit. We have since traced this excess noise to timing imperfections. When the experimental imperfections are eliminated, we show that shot noise limit is attained by acquiring noise free data in the same configuration as ACME II.
Journal of Physics B: Atomic, Molecular and Optical Physics