https://www.theregister.co.uk/2018/07/06/luminescent_nanocrystals_could_lead_to_pbplus_optical_disks/

Australian researchers have managed to store information on light-emitting nanocrystals, and they reckon a cubic-centimetre chunk of the stuff could hold a petabyte of data.

The base tech is described in "Towards rewritable multilevel optical data storage in single nanocrystals" in Optical Express.