US Patent 7917298 - Nanocrystal taggants

http://www.freepatentsonline.com/7917298.html

This patent from Nanosys has relatively early priority (2003) and includes some basic claims to  using quantum dots to create invisible bar codes for inventory control. Claim 1 reads:

1. A composition for tagging objects, comprising:

a population of nanocrystals comprising two or more subsets of nanocrystals, the population characterized by a unique spectral code;

wherein each said subset comprises a plurality of quantum dots of the same size and/or the same composition;

wherein the unique spectral code of the population comprises at least one optical property defined by the interaction of the emission frequency of at least the first nanocrystal subset and the emission frequency of at least the second nanocrystal subset.