I tried to do some reading about JJ but it only confused me.
1. How does it convert AC to DC and DC to AC?
2. In terms of power,how efficient is this conversion?
PHOOL BRIDGE RECTIFIER!11 Okay, I dunno
. But wikipedia says something about "The DC Josephson effect". May be that's what people use?
Are you making a DIY JJ?
Josephson Junction is a method that is used as Voltage Standard in standard labs. This is something you should ask from metrology section, there might be some people that actually have seen a such device is practice.