Sorry, forgot to follow up on this thread...

In your opinion, is a quantum system a sine qua non of quantum states?

Define what you mean by a "quantum system" and "quantum states". As far as I'm aware, all known particles act in quantum fashion; ergo, the entirety of our universe is thus a quantum system.

I suspect you're getting at the phenomena where larger objects loose the quantum nature of their constituent atoms/particles. AFAIK exactly how quantum-level phenomena translate into larger "macro processes remains unclear. None-the-less, we expect macro behaviour to be some sort of a superstition of the underlying states.

That said, we do see quantum nature reflected in high order systems. Any high-resolution optical system (which I deal extensively with) is highly impacted by the quantum nature of light. Same is true of semiconductors (i.e. every electronic gizmo on the planet). Even in nature we see aspects of quantum states - virtual particles (Cashmeir effect, Hawking radiation, etc).