  We investigate qubit measurements using a single electron transistor (SET).
Applying the Schr\"odinger equation to the entire system we find that an
asymmetric SET is considerably more efficient than a symmetric SET. The
asymmetric SET becomes close to an ideal detector in the large asymmetry limit.
We also compared the SET detector with a point-contact detector. This
comparison allows us to illuminate the relation between information gain in the
measurement process and the decoherence generated by these measurement devices.
