I think that it probably uses fuel cell technology to generate electricity. As for quiet, well that has a lot to do with the shape of the hull and propeller disturbing the water as little as possible. But military secrets are the hardest to keep and one weapon is soon outmoded with a countermeasure.
This is what the Swedish boats use
http://www.kockums.se/en/products-services/submarines/stirling-aip-system/stirling-aip-conversion/