"We knew the world would not be the same. A few people laughed... A few people cried... Most people were silent. I remembered the line from the Hindu scripture the Bhagavad Gita; Vishnu is trying to persuade the prince that he should do his duty, and to impress him takes on his multi-armed form, and says, "Now I am become death, the destroyer of worlds." I suppose we all thought that, one way or another. "
Robert Oppenheimer was the director of the laboratory in Los Alamos, New Mexico, and the scientific director of the Manhattan project. Since so many talents were involved it's somewhat misleading to call him "the father of the nuclear bomb", but he undeniably made one of the major individual contributions.
In an interview from 1965, Oppenheimer describes the initial reactions as the fruit of their labors, the very first nuclear bomb (the Hiroshima bomb was the second one), detonated early in the morning of July 16, 1945:
The quote was something he thought, but he didn't say it.
The quote is indeed from the Bhagavad Gita ("Song of the lord"). Some suggest it's a misquote, which would explain the peculiar grammar; but "am become" is not an error but a (poetic) archaism, as in "I am become a name, for always roaming with a hungry heart" (Tennyson, Ulysses). Which in turn might be a trace of French; "Je suis devenu la mort".
Since Oppenheimer was proficient in sanskrit he read the original text, and the translation is his own; I haven't found any other translation with "am become". It certainly gives a certain something to the line, however, and it might had been at least somewhat less well known had it been "I am death" or "I have become death".