The first nation to do this was “neutral” Sweden.
Now the Iranians, and we suspect the Chinese have it. So much for “neutrality”. Which, ought to make one wonder, to start with - why would a “neutral” want to employ sonar evasion in the first place, and why would, Sweden, a Western European country and a Nordic country - like our NATO allies Denmark, Norway and Iceland - NOT have delivered that technology to the US but would ALLOW it to find its way to Iran and China????
You gotta relax, man. There ain’t no such thing.
Actually, the Swedes did deliver the technology to the U.S. As I understand it, they had one of their subs stationed in San Diego (I think) so our guys could learn to find it. How successful our guys were is probably classified but they’re awfully good, so I should think they have a handle on this new technology already.