1. A method for determining whether a first state of a quantum system is occupied, the method comprising: (A) applying a signal to the quantum system at a frequency that corresponds to an energy level separation between said first state and a second state of said quantum system, wherein (i) said quantum system produces a readout frequency when said first state is occupied at a time when said signal is applied, and (ii) said quantum system does not produce said readout frequency when said first state is not occupied at the time when said signal is applied; and (B) measuring a property of a measurement resonator that is conditionally coupled to the quantum system when said quantum system produces said readout frequency, thereby determining whether said first state of said quantum system is occupied.