# Attaining the shot-noise-limit in the ACME measurement of the electron electric dipole moment

## TL;DR

This paper demonstrates how the ACME experiment can reach the fundamental shot-noise limit in measuring the electron electric dipole moment by identifying and eliminating timing imperfections, thus improving measurement sensitivity.

## Contribution

It shows that eliminating timing imperfections allows ACME to attain the shot-noise limit, enhancing the precision of electron EDM measurements beyond previous results.

## Key findings

- ACME II's excess noise was due to timing imperfections.
- Removing imperfections enables shot-noise-limited measurement.
- Achieved order of magnitude sensitivity improvement.

## Abstract

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.