High precision frequency to voltage converter
The Josephson junction is used as a voltage standard because it is based on fundamental physical constants and can convert a known frequency source into an accurate DC voltage.  Since frequency can be measured to a much higher accuracy than most other physical parameters, Josephson junction voltage sources can and are used as primary voltage standards. What you are asking is if there is any other technology that can compete with the Josephson junction.  As far as I know, the answer is no.