The 6 innings, 3 earned runs is of course the bare minimum for a QS. That point can't be stressed enough (LOL). I guarantee that a pitcher who produces a high number of QS's will have an ERA much lower than 4.50 in those starts.
There is also the High Quality Start which I believe has a minimum of 7 innings, 2 earned runs.
I agree with Palodios that 9 innings, 4 earned runs should be a QS, not that it happens very often.