My profane question would be:
Why is not everybody already using voltage references based on Josephson junction?
Is this because it is too expensive for a personal lab to keep the very low temperatures required, or it is just because a Josephson chip is very expensive because of its rarity? Or is it that the results between LTZ1000 and Josephson does not justify the price difference? Or maybe it's just the availability?
What is the main reason that most volt-nuts are using LTZ1000 instead of a Josepshon junction based voltage reference?
AFAIK a Josephson junction voltage depends only by universal physics constants (which physics constants are, of course, always constant), so it seems a no brainer to go for the Josephson junction, yet it is not so common. Why?