Ultracold Triatomic Molecules as Quantum Sensors for New Physics

Our current best model of particle physics, known as the Standard Model, says that the big bang should have created equal quantities of matter and antimatter, subsequently annihilating with one another to leave an empty, featureless universe containing only light. That there is enough left-over matter to form galaxies, stars, planets and indeed us, while we observe almost no antimatter, is known as the matter-antimatter imbalance. This existential anomaly is one of the great outstanding mysteries of modern physics.

Explanation of the matter-antimatter imbalance requires new physics which violates something physicists refer to as CP (combined charge and parity) symmetry. This means that the universe would behave differently if you were to replace all positive charges with negative ones (and vice versa) and simultaneously reflect the positions of all particles about some fixed plane.

Many extensions to the Standard Model-proposed to help explain the matter-antimatter asymmetry-add new particles and fields at higher energies than so far explored by particle colliders with CP-violating interactions. Whilst direct searches for these particles using higher-energy particle colliders are (at least) many decades away, they can also be detected by the energy shifts they produce in low-energy systems.

The higher the energy scale of the exotic new physics, the smaller the resulting energy shifts. Energy levels of atoms and molecules can be measured with precision higher than any other system making them an ideal place to search for the expected tiny shifts. To date, no such shifts have been detected but the best limits on many possible types of CP violation are set by precision measurements of mercury atoms.

Other species can have greatly enhanced sensitivity to new CP-violating physics; the same underlying mechanism produces dramatically larger energy shifts than in mercury. In particular, polar molecules have sensitivity ~3 orders higher than atoms and the sensitivity can be enhanced by a further ~1-3 orders of magnitude in systems containing heavy deformed nuclei.

Over the course of this UKRI Future Leaders Fellowship, I will develop a new tabletop experimental platform to search for CP-violating physics using deformed nuclei embedded in ultracold triatomic molecules. These molecules are a near-ideal platform for such a search: (i) the molecules have intrinsic sensitivity to CP-violating physics which is enhanced by 4-6 orders of magnitude relative to mercury (ii) they will be produced at ultracold temperatures, ideally suited to a high-precision measurement, and (iii) the triatomic structure gives them very powerful features to reject many common sources of systematic error in similar experiments.

High-precision measurements require long interrogation times. In the highest precision measurements of atoms, long interrogation times are enabled by trapping the atoms and laser cooling them to very low temperatures; one-millionth of a degree above absolute zero or less. However, the complex structure of polyatomic molecules containing heavy atoms makes the application of laser cooling extremely challenging.

Instead, we will produce ultracold triatomics by separately laser cooling a light diatomic molecule and a heavy atom with a quadrupole-deformed nucleus. One particle from each species will be trapped in an array of optical tweezers-highly focussed laser beams which allow trapping and control of single particles. The pairs will then be bound together into triatomic molecules using magnetic fields.

A measurement using this platform, the ultimate goal of the fellowship, will probe for new CP-violating physics at energy scales well beyond those previously explored. Measurement of a non-zero shift will constitute a discovery of new physics, whilst a null result would set stringent limits on the properties that such new physics could have. Either outcome will provide new understanding of the universe we live in.