  We develop a circuit theory that enables us to analyze quantum measurements
on a two-level system and on a continuous-variable system on an equal footing.
As a measurement scheme applicable to both systems, we discuss a swapping state
measurement which exchanges quantum states between the system and the measuring
apparatus before the apparatus meter is read out. This swapping state
measurement has an advantage in gravitational-wave detection over contractive
state measurement in that the postmeasurement state of the system can be set to
a prescribed one, regardless of the outcome of the measurement.
