The search for a more perfect kilogram
Jonathon Keats, writing in Wired:
Two different approaches to linking the kilogram to a fundamental constant are in the works, but both have proven far more complicated than in the case of the meter. Borrowing tricks from quantum mechanics and techniques used to manufacture atomic bombs, the competing initiatives are finally on the verge of delivering the kind of precision necessary to displace Le Grand K.
Really great profile of the teams working on the two approaches; highly recommend taking 15 minutes to read all 4,000 words.