Static Laboratory-Frame Polarization of Molecular Ions Enables CP-Violation Searches Despite Trapping Constraints

The search for new physics beyond our current understanding drives experiments seeking to detect subtle violations of fundamental symmetries, and recent advances focus on measuring permanent electric dipole moments. Fabian Wolf from the Physikalisch-Technische Bundesanstalt, along with colleagues, now demonstrates a crucial step towards realising highly sensitive searches for these violations within trapped molecular ions. The team reveals that, contrary to previous assumptions, these ions can be statically polarised using electric fields while remaining securely confined within the trap, a feat achieved through a delicate balance between electrostatic and trapping forces. This breakthrough overcomes a significant obstacle in the field, paving the way for the development of exceptionally precise molecular clocks capable of probing new physics and ultimately enhancing the search for charge-parity (CP) violation.

It was previously assumed that experiments utilising static polarisation from direct current electric fields were infeasible, as the ion’s charge would either shift it to a field-free region or eject it from the trap. This constraint appeared to render single ion quantum-logic clocks, among the most precise measurement devices available, incompatible with electric dipole moment measurements. However, this work demonstrates that, under typical trapping conditions, heavy molecular ions with small energy differences between opposite parity states can be polarised by a static electric field while remaining confined in the Paul trap. This effect arises from a cancellation between the electrostatic force and the trap’s ponderomotive force, resulting in an equilibrium position where the ion experiences a stable force.

Molecular Ions Probe CP Violation and EDMs

This research centers on highly precise experiments designed to search for physics beyond the Standard Model, specifically focusing on violations of Charge-Parity (CP) symmetry. CP violation is a crucial ingredient in explaining the matter-antimatter asymmetry in the universe. The experiments utilise molecular ions as sensitive probes for these subtle effects, measuring electric dipole moments (EDMs), and the existence of an EDM would indicate CP violation and new sources of physics. Molecular ions, such as thorium monoxide (ThO), radium monofluoride (RaF), acetic acid (AcOH), and protactinium-229 (Pa-229), are employed because they can amplify the effects of subtle interactions, making them easier to detect.

These experiments rely on trapping ions using electromagnetic fields, allowing for long observation times and precise control over their quantum states. Laser spectroscopy is then used to probe the energy levels of the trapped ions with high precision, and changes in these energy levels due to CP-violating interactions can be detected. Radium monoxide (RaO) and radium monofluoride (RaF) are particularly promising molecules due to their strong dipole moments and favorable properties for EDM searches.

Static Polarization of Trapped Molecular Ions

Scientists have demonstrated that heavy molecular ions can be statically polarised while confined in a Paul trap, a breakthrough previously considered incompatible with high-precision measurements. The research overcomes a long-standing limitation in experiments designed to detect permanent electric dipole moments (EDMs), which signify violations of fundamental symmetries in nature. The team discovered a cancellation between the electrostatic force acting on the ion and the ponderomotive force of the trap, creating an equilibrium position where a strong static electric field can be applied without ejecting the ion. Experiments reveal that for radially applied static electric fields, the combined forces allow full polarisation of molecular ions with small energy differences between opposite parity states.

This polarisation occurs because the oscillating trap field is too fast for the molecule to follow, effectively locking the molecular dipole in place. The minimum electric field required to achieve this polarisation is surprisingly low; for radium monoxide ions (RaO+), with an energy difference of 81kHz, the critical field is only approximately 2V/m. This low threshold opens the door to using highly sensitive molecular species, including rare and radioactive ones, in EDM searches. Measurements confirm that this technique does not hinder sympathetic laser cooling or quantum logic spectroscopy, essential tools for precise control and measurement of ions. Calculations show that even for light ions, the fractional second-order Doppler shift introduced by the oscillating electric field is minimal, demonstrating minimal impact on clock accuracy. The team proposes a measurement scheme utilizing a Ramsey interferometer and dynamical decoupling, allowing for extended interrogation times and enhanced sensitivity to EDMs.

Static Polarization Enables Molecular Quantum Clocks

Researchers have demonstrated that heavy molecular ions can be statically polarised while confined within a Paul trap, overturning previous assumptions about the feasibility of such experiments. This achievement arises from a cancellation between electrostatic and ponderomotive forces, allowing a stable electric field component to be applied to the ion. The team successfully implemented dynamic decoupling techniques, enabling co-magnetometry schemes and extended interrogation times, crucial for sensitive measurements. This breakthrough paves the way for utilizing quantum-logic molecular radio-frequency clocks to search for electric dipole moments, offering a new approach to probing fundamental symmetries in nature.

The ability to statically polarize ions opens opportunities to leverage advanced quantum metrological tools, including entanglement-enhanced spectroscopy and noise mitigation techniques. By employing non-destructive detection methods and working with only a few ions, scientists can explore highly sensitive, rare, and radioactive molecular species, potentially achieving unprecedented levels of accuracy and precision in their measurements. While acknowledging that current experiments are limited by the availability of suitable molecular species, the researchers suggest that molecules containing radium or actinium offer promising platforms for near-term advancements. Future progress in scalable trapped-ion technologies, driven by developments in quantum computing, could further enhance these measurements and deepen our understanding of CP violation.