Did Soviets steal H-bomb as well as A-bomb?

Without naming names, a new book says yes, and sounds convincing.

Given the even shorter time between U.S. and Soviet hydrogen bombs vs. the four-year gap between fission bombs, and also given that the USSR first built a “cake” bomb — a souped-up A-bomb and not a real thermonuclear — before the real deal, the idea of thievery sounds convincing to me.

Now, since authors Thomas C. Reed and Danny B. Stillman don’t name names, who fits their clues of someone born in the United States, who grew up in a foreign country, fell in with communist sympathizers during the Depression, and worked at Los Alamos during World War II, then became deeply involved with H-bomb efforts there?