State media indicated Thursday that North Korea has added the hydrogen bomb to its arsenal, a development that, if true, would represent a major leap in its nuclear weapons capabilities. But outside observers were skeptical, saying that such an advance in the country's nuclear technology seemed unlikely. Analysts in recent years have believed that North Korea may have been working toward -- but didn't yet have the capability to produce -- a hydrogen, or thermonuclear, bomb. It can be hundreds of times more powerful than an atomic bomb....