Two-state quantum system

A two-state quantum system is like a house that can only have two states: on or off. It's like flipping a switch. In the "on" state, the house has energy and in the "off" state it doesn't. In a two-state quantum system, the "on" state is called an "excited state" and the "off" state is called a "ground state". These two states can be switched back and forth using energy, just like flipping a switch.