Levitated Quantum Diamonds (LQD)

Atoms and molecules are very well described by quantum mechanics, but what about much larger things? Erwin Schrödinger pointed out that cats have never been shown to exist in a quantum superposition, but recent experiments are pushing back the boundaries of which objects have been shown to do this. The strongest tests require a large mass in a superposition for a long time with a large superposition distance. The superposition distance is the distance between the two components of the superposition. Pioneering experiments have been done with superconductors, superfluids and vibrating cantilevers but the most macroscopic superposition state created so far is a variant of the famous two-slit experiment for molecules made of 2000 atoms. Our project has the ambitious goal of testing whether levitated nanodiamonds made up of more than a million times more atoms can display this quantum behaviour.

The most exciting thing about this experimental frontier is that it could, in 10-15 years, lead to a test of quantum gravity. Einstein's general relativity explains gravity and is needed to make GPS work, but we don't know how to combine it with quantum mechanics to explain the gravitational effects produced by a quantum object. Successfully combining these two most fundamental theories of physics would produce a theory of quantum gravity, which has been sought for 100 years. Theories of quantum gravity such as string theory and loop quantum gravity have been proposed, but suffer from a lack of empirical evidence. Physicists such as Stephen Hawking and Roger Penrose worked on black holes and showed they are fertile playgrounds to constrain theories of quantum gravity, but black holes are not practical to experiment on.

A new proposal from us and others shows a way to test one key aspect of quantum gravity with a lab experiment on a table-top. The idea is to create two of the nanodiamond Schrödinger cats and see how they interact gravitationally. This project is only possible thanks to the advances already demonstrated by the quantum technology community, and indeed this research will, in time, lead to a new class of more sensitive sensors that would be used to detect acceleration, rotation, tilt, gravity and magnetic fields.

Having already published our descriptions for how to test macroscopic quantum mechanics and quantum gravity, we will now transform our preliminary experiments to begin the delivery of these proposals.

To reach large superposition distances and long durations we will use diamond nanoparticles (around 800 nm across) containing a single nitrogen vacancy centre (NVC). This follows our proposals which provide a clear route to achieve a superposition distance of over 1000 nm, although our initial experiments will only reach 0.1 pm. Nanodiamonds have been levitated in vacuum using optical traps by us and others, as well as in Paul traps and magnetic traps. We showed that the heating of the diamond by the trapping beam in an optical trap in vacuum is a serious obstacle. To get around this we developed (with collaborator Oliver Williams) large quantities of high-purity nanodiamonds, and have now switched to using a magnetic trap as this further minimises the heating of the levitated diamond. A magnetic trap also provides the inhomogeneous magnetic field which is required to couple the spin to the motion. The core idea is to put the NVC electron spin into a spin superposition because the inhomogeneous magnetic field then provides a superposition of forces on the diamond leading to a spatial superposition. To evidence this, we will then flip the spin to recombine the superposition components for matter-wave interferometry and repeat the interferometry as a function of experimental tilt with respect to gravity to search for interference fringes.