Of the four fundamental forces in nature, three are well-described by quantum theory while the fourth — gravity — is not. Obtaining a unified theory that describes all four forces ranks among the most important goals in fundamental physics. Perhaps the most pressing question on the road to unification is whether gravity is a quantum force. Experiments that directly probe quantum gravity have been considered infeasible because the fundamental unit of ‘quantized’ space is exceedingly small. Directly probing this scale would require a particle accelerator the size of our galaxy. Instead, what physicists have been trying to do for many decades is to think of viable experiments that would teach us something about quantum gravity. Two recent papers envision just such an experiment. That experiment, unfortunately, is not feasible given the limitations of current experimental physics. Yet there’s considerable optimism in some physics communities that these limitations may be overcome in the reasonably near future, perhaps in five to ten years.
This grant provides funding to Gavin Morley, Professor of Physics at Warwick University, who is leading a collaboration that aims to tackle the primary challenges to performing the proposed experiment. Summarized broadly, what’s needed is the experimental capacity to create and manipulate large, heavy quantum systems and to arrange for conditions that allow gravity to be the dominant interaction between two laboratory-scale objects.
In quantum physics, entanglement refers to a non-intuitive connection between objects that’s evidenced by measuring some property of the objects and showing that the measurements are more highly correlated than is allowed by classical physics. The proposed future experiment aims to determine whether two objects (diamonds) can become entangled via their mutual gravitational attraction. If they can, then gravity must be a quantum force, as asserted by two recently published papers.
Morley and his team will address three lines of research critical to enabling that future quantum gravity experiment. First, it’s important that gravity be the dominant interaction between two objects, here micron-scale diamond samples (microdiamonds). Electromagnetism is much stronger than gravity so electromagnetic (EM) interactions must be heavily attenuated. The plan is to first measure the strength of EM interactions between two microdiamonds, and between a microdiamond and nearby experimental components. After measuring and understanding the sources of microdiamond EM interaction, the PIs will implement attenuation measures.
Next, the researchers will demonstrate matter-wave interferometry involving objects much heavier than what’s been achieved to date. Heavy objects are required because the strength of gravity scales with mass so heavy objects are more easily entangled. Interferometry is required because the entanglement will be evidenced by a gravity-induced shift in the interference pattern. Morley will release a magnetically-levitated diamond and allow it to fall onto a laser-induced diffraction grating that creates a matter-wave interference pattern. By studying the ‘sharpness’ of the matter-wave interference pattern, Morley will learn about sources of ‘decoherence’ that limit how long the superposition lasts.
Finally, the team will follow a protocol they’ve proposed to put a microdiamond into a spatial superposition of being in two places at once by using a laser to create a spin superposition (diamond simultaneously in two spin states) and then using a magnetic field to drive the two different spin components in opposite directions; thereby also achieving a spatial superposition. This will be done under conditions where EM forces are attenuated by rotating the diamond and by using a conductive screen.