When a person is either being sarcastic, albeit very dryly, or being serious and the person they are talking to can not tell whether or not they are being sarcastic or serious. They could be simultaneously serious or sarcastic at the exact same moment in time, thus quantum sarcasm.
Guy takes another man's sandwich and the man whose sandwich has been taken says "Go ahead and take it." in an inflected voice and direct stare. An uncomfortable moment passes because the man who took sandwich is now not sure of whether the other man is serious or is sarcastically remarking on the situation. Thus quantum sarcasm because sarcasm or seriousness are both equally valid options at the moment.
by joshfromriverside January 27, 2012