A precision measurement of the electron's electric dipole moment using trapped molecular ions

Abstract: We describe the first precision measurement of the electron's electric dipole moment (eEDM, $d_e$) using trapped molecular ions, demonstrating the application of spin interrogation times over 700 ms to achieve high sensitivity and stringent rejection of systematic errors. Through electron spin resonance spectroscopy on $^{180}{\rm Hf}^{19}{\rm F}^{+}$ in its metastable $^{3}\Delta_{1}$ electronic state, we obtain $d_e = (0.9 \pm 7.7_{\rm stat} \pm 1.7_{\rm syst}) \times 10^{-29}\,e\,{\rm cm}$, resulting in an upper bound of $|d_e| < 1.3 \times 10^{-28}\,e\,{\rm cm}$ (90% confidence). Our result provides independent confirmation of the current upper bound of $|d_e| < 9.3 \times 10^{-29}\,e\,{\rm cm}$ [J. Baron $\textit{et al.}$, Science $\textbf{343}$, 269 (2014)], and offers the potential to improve on this limit in the near future.

• The paper presents an advanced eEDM measurement using trapped 180Hf19F+ ions with spin precession times over 700 ms.
• It employs high-sensitivity electron spin resonance spectroscopy with RF traps to reduce systematic errors compared to traditional beam methods.
• The experiment reports an upper bound of |dₑ| < 1.3×10⁻²⁸ e·cm, confirming existing limits and opening avenues for future sensitivity enhancements.

Precision Measurement of the Electron's Electric Dipole Moment Using Trapped Molecular Ions

The paper by Cairncross et al. details a precision measurement of the electron's electric dipole moment (eEDM, $d_e$) using trapped molecular ions. This methodology presents a significant advancement in the field of precision measurement, leveraging the properties of trapped ions to enhance sensitivity and reduce systematic errors. The study focuses on an eEDM measurement using $^{180}$Hf$^{19}$F$^+$ molecular ions confined within a radio frequency (RF) trap, applying spin precession techniques over extended interrogation times exceeding 700 ms.

Experimental Approach and Methodology

The use of the $^{180}$Hf$^{19}$F$^+$ ion in its metastable $^3 \Delta_1$ electronic state is central to the experiment. The authors perform high-sensitivity electron spin resonance spectroscopy, capitalizing on the relativistically enhanced eEDM-induced energy shift observed in the heavy polar molecule. This method builds on and enhances traditional thermal beam approaches, offering improvements in systematic error rejection and increased coherence times due to ion confinement.

The experimental setup involves a sophisticated system where HfF molecules are produced through ablation, trapped, and cooled, followed by laser-based ionization to form HfF$^+$. Ion confinement is maintained using a combination of DC and RF electric fields, with a rotating electric field applied for molecular polarization. The setup's design allows for extended interrogation times due to significantly reduced decoherence rates from ion-ion collisions relative to those in traditional beam experiments. Notably, the ability to achieve long spin precession times, enabling sensitive measurements despite lower initial signal-to-noise ratios, is a crucial aspect of their methodology.

Measurement Results and Implications

The reported measurement for the eEDM is $$d_e = (0.9 \pm 7.7_{\rm stat} \pm 1.7_{\rm syst}) \times 10^{-29}\,e\,$$cm, resulting in an upper bound of $|d_e| < 1.3 \times 10^{-28}\,e\,$cm at 90% confidence. This result aligns with existing upper limits set by previous experiments while providing confirmation using a distinct physical system and technique. The approach shown enhances independent verification of eEDM measurements—a critical aspect given the implications for physics beyond the Standard Model (SM).

Systematic Errors and Statistical Analysis

The authors employ an intricate data analysis process, utilizing channels to account for variations and potential systematic errors within their measurements. This includes corrections for staging background effects and addressing stray magnetic field interactions, which are potential contributors to experimental inaccuracies. Scaling factors were applied to address over-scattering in the dataset, ensuring statistical reliability.

Future Work and Speculations

The findings present important implications for refining eEDM measurements and suggest promise for future sensitivity enhancements. The authors express confidence in further reducing systematic errors to the $10^{-30}\,e\,$cm level and propose improvements for a second-generation trap with enhanced ion confinement properties. They anticipate using a third-generation experiment with $^{232}$Th$^{19}$F$^+$ ions, targeting an even higher sensitivity through longer coherence times.

Overall, this work by Cairncross et al. constitutes a substantive contribution to precision measurement techniques and the ongoing search for physics beyond the SM, offering both experimental rigor and promising avenues for increased sensitivity in future measurements.