  We demonstrate high-contrast state detection of a superconducting flux qubit.
Detection is realized by probing the microwave transmission of a nonlinear
resonator, based on a SQUID. Depending on the driving strength of the
resonator, the detector can be operated in the monostable or the bistable mode.
The bistable operation combines high-sensitivity with intrinsic latching. The
measured contrast of Rabi oscillations is as high as 87 %; of the missing 13 %,
only 3 % is unaccounted for. Experiments involving two consecutive detection
pulses are consistent with preparation of the qubit state by the first
measurement.
