# Second-Scale Coherence Measured at the Quantum Projection Noise Limit with Hundreds of Molecular Ions

## TL;DR

This paper demonstrates second-scale quantum coherence at the quantum projection noise limit using hundreds of trapped molecular ions, enhancing precision measurement capabilities for fundamental physics tests.

## Contribution

It introduces a method to achieve long coherence times at the QPN limit with molecular ions sensitive to the $e$EDM, enabling improved precision in fundamental physics experiments.

## Key findings

- Achieved second-scale coherence at the QPN limit with molecular ions.
- Simultaneous measurement of two states with opposite $e$EDM sensitivity.
- Enhanced sensitivity for fundamental physics tests using molecular ions.

## Abstract

Cold molecules provide an excellent platform for quantum information, cold chemistry, and precision measurement. Certain molecules have enhanced sensitivity to beyond Standard Model physics, such as the electron's electric dipole moment ($e$EDM). Molecular ions are easily trappable and are therefore particularly attractive for precision measurements where sensitivity scales with interrogation time. Here, we demonstrate a spin precession measurement with second-scale coherence at the quantum projection noise (QPN) limit with hundreds of trapped molecular ions, chosen for their sensitivity to the $e$EDM rather than their amenability to state control and readout. Orientation-resolved resonant photodissociation allows us to simultaneously measure two quantum states with opposite $e$EDM sensitivity, reaching the QPN limit and fully exploiting the high count rate and long coherence.