A nitrogen-vacancy (NV−) centre in a nanodiamond, levitated in high vacuum, has recently been
proposed as a probe for demonstrating mesoscopic centre-of-mass superpositions and for testing
quantum gravity. Here, we study the behaviour of optically levitated nanodiamonds containing NV−
centres at sub-atmospheric pressures and show that while they burn in air, this can be prevented by
replacing the air with nitrogen. However, in nitrogen the nanodiamonds graphitize below ≈10 mB.
Exploiting the Brownian motion of a levitated nanodiamond, we extract its internal temperature
(Ti
) and find thatit would be detrimentalto the NV− centre’s spin coherence time. These values of Ti
make it clear that the diamond is not melting, contradicting a recent suggestion. Additionally, using
the measured damping rate of a levitated nanoparticle at a given pressure, we propose a new way of
determining its size.