I use this lense with a Canon rebel and get an error message that it cannot communicate and to clean contacts. How do I do that?

Wow, I'm surprised to actually tells you to clean the contacts? What model Canon Rebel do you have? Okay for cleaning the contacts simplest and most effective way is with a good clean new red rubber pencil eraser. Make sure the camera is off and remove the lens, place the camera body face down on a clean lint free surface or install a body cap if you have one. Hold the lens as vertical as possible but tilted slightly to be able to see and work with the gold contacts. Lightly rub the contact surface with the eraser, it won't look as if you have done much but it does shine them up well. When you have finished cleaning them use a lens cloth (not a tissue) to lightly clean any rubber dust from around the contacts and the rest of the mount. There shouldn't be much if any you are polishing the surface not erasing. Set the lens aside and while holding the camera as much as possible lens side down take the eraser and gently clean the contacts again it doesn't take much don't forget you are polishing not erasing, a few passes should be all it takes. Once done again use the lens cloth not a tissue and clean the area of any dust that may have occurred. Assemble the lens and camera and turn it on.

On another note but a related subject. How new is your Rebel compared to the Quantaray lens? If the lens is newer then the camera that's fine but if the camera is newer then the lens this also may be part of the problem.