Quantum Sensing for Antimatter Gravity

Why is the Universe apparently devoid of antimatter? According to our best current understanding of physics (as expressed by the Standard Model (SM) of particle physics), we should not exist; our solar system, the Milky Way Galaxy, and indeed the entire observable Universe do not make sense in this paradigm because they seem to be composed only of matter, with hardly any antimatter to be found. This is mysterious because the fundamental symmetries underlying the SM mean that, when energy was converted to matter after the Big Bang, an equal amount of antimatter should also have been created. This arises because matter and antimatter are deeply symmetrical, as are the fundamental quantum field theories that we use to describe them; breaking this symmetry does extreme damage to some of the most cherished properties of modern physics. Nevertheless, we have a crisis and something in our current understanding has to change. The great success of the SM has the ironic side effect that it doesn't tell us where to go next, even though we have many indications that it is itself incomplete. Examples are the existence of Dark Matter, the strong CP problem, the hierarchy problem, and so (there are many more). Similarly, the formation of a matter-dominated Universe should be impossible, and yet this is what seems to have happened. The observation seems to be on solid ground: a carefully balanced Universe containing both matter and antimatter in equal quantities does not seem very likely, as we don't see galaxy-scale annihilation events. Therefore it appears that the exact symmetry between matter and antimatter must somehow be broken. However, there is no clear indication of how this could happen in a way that is compatible with the SM. While it may be difficult to reconcile with the SM in a general way, some sort of antigravity effect may offer an explanation for our seemingly unreasonable existence.

The Quantum Sensing for Antimatter Gravity (QSAG) project is represents an attempt to measure the gravitational interaction of positronium (Ps) in the field of the Earth. Ps is a hydrogen-like atomic system composed of both matter and antimatter (an electron and a positron). Our aim is to measure if it falls in the same way as other matter; this is essentially a test of the weak equivalence principle, which has not previously been tested directly for systems containing antimatter. The basis of the experiment is a highly sensitive quantum sensing protocol using interference effects to probe the force of gravity on a quantum superposition of highly excited Rydberg states of Ps. This form of (anti)matter wave interferometry has not been demonstrated for Ps, but has recently been developed for Rydberg He atoms [J E. Palmer and S. D. Hogan Electric Rydberg atom interferometry, Phys Rev Lett 122, 250404 (2019)]. Experiments with Ps atoms are made more difficult by the short Ps lifetime and so it is highly advantageous to excite them into Rydberg states that do not annihilate. Thus the QSAG experiment will be able to employ optical and microwave radiation to perform interferometric measurements that will allow us to test for antigravity effects in Ps, and in so doing possibly explain how it is that we are able to exist.