Levitated nitrogen-vacancy nanodiamonds in optomechanics

Short answer: A nanodiamond containing a single nitrogen-vacancy (NV) center can be levitated in an optical trap, which links its motion, its light and its electron spin. A 2015 Nature Photonics paper reported the first hybrid system of this kind, a 2013 Physical Review A paper proposed using the spin to make large quantum superposition states of the nanodiamond's motion, and a 2018 New Journal of Physics paper reported nanodiamonds that survive trapping at lower pressure. These are early laboratory and theory results, and no superposition has been reported in these abstracts.

2015: a levitated hybrid system

The Nature Photonics abstract describes progress in nanomechanical and nano-optomechanical devices, and in solid-state emitters with optically accessible spins. The authors demonstrate a hybrid nano-optomechanical system consisting of a nanodiamond containing a single NV center, levitated in an optical dipole trap. The mechanical state of the diamond is controlled by modulating the trapping potential. They report imprinting the multidimensional mechanical motion of this cavity-free oscillator into the NV fluorescence, and manipulating the mechanical system's intrinsic spin. They call it the first step toward a hybrid quantum system based on levitating nanoparticles that engages optical, phononic and spin degrees of freedom.

2013: a proposal for non-classical motion

The Physical Review A paper (Yin, Li, Zhang and Duan) is a proposal and not an experiment. The authors suggest a method to generate and detect large quantum superposition states, and arbitrary Fock states, for the oscillation mode of an optically levitated nanodiamond. The nonlinear interaction needed for non-Gaussian states comes from spin-mechanical coupling with a built-in NV center. They say the method would allow superpositions of nanoparticles with millions of atoms and observation of the associated spatial quantum interference under reasonable experimental conditions. That is their theoretical claim.

2018: the heating problem

The New Journal of Physics abstract says optical trapping at high vacuum of an NV nanodiamond would provide a test bed for new phenomena in fundamental physics. The difficulty is that nanodiamonds used so far absorbed too much trapping light and were heated to destruction, above 800 K, except at pressures above about 10 millibar where air molecules carry the heat away. The authors show that milling diamond of 1000 times greater purity creates nanodiamonds that do not heat up even when the optical intensity exceeds 700 gigawatts per square meter below 5 millibar. The introduction lists proposed uses: probes of quantum gravity, mesoscopic wavefunction collapse, phonon-mediated spin coupling and dark matter detection. These are proposals the authors cite, not results.

Limits

The 2013 paper is theoretical, and the 2018 paper reports a materials fix and not an NV experiment in high vacuum. The three papers span five years and do not form a single program. Large superposition states of a levitated nanodiamond have not been shown in these abstracts.

FAQ

What is levitated optomechanics?

Trapping a small particle with light so its motion can be controlled, as in the 2015 experiment.

Why use an NV center?

Its spin provides the nonlinearity needed for non-classical motion states, per the 2013 proposal.

What was the 2018 advance?

Purer milled diamond that avoids heating at lower pressure.

Has a quantum superposition been seen?

Not in these abstracts.

Is this a quantum computer?

No. It is a platform for fundamental physics tests.