We propose to magnetically and optically manipulate micron-sized diamond crystals containing nitrogen-vacancy (NV) centers levitated in our recently developed magneto-gravitational trap, prepare them in macroscopic quantum superpositions by coherent interactions of the crystal center-of-mass motion with the spin of the diamond NV centers, and measure the effects of decoherence and gravity. This system combines the atomic-physics-like control over the motion of the particle due to trapping with the precision magnetic field measurement capabilities of diamond NV centers. The system we propose for quantum measurements has several unique features: (i) extreme isolation from the environment (ii) a built-in quantum sensor and quantum handle (dia- mond NV centers), (iii) explicit dependence of the oscillation frequency on gravity, and (iv) extremely high sensitivity to small external forces.
We will target two specific aims during the course of our project. First, we will create macroscopic quantum superpositions and measure them. The non-classical state is created by putting the NV center spins in the nanodiamond into a superposition state, and transferring that superposition to the center-of-mass position through a magnetic field gradient. The system is then allowed to evolve over time in this state. Finally, the superposition state is measured with the NV centers to determine the decoherence rate. If successful, this would be the first scalable approach for estimating positional decoherence for non-classical motional states. The foundational tools we develop for preparation and measurement of quantum states of motion will enable the second aim: we will design and create a double-trap geometry that will allow measurements of the effects of gravity from a second microcrystal in a quantum superposition state. Detection of such effects would be a significant milestone in understanding the interface between quantum physics and gravity.
