  We evaluate the detector nonideality (and energy sensitivity) of a
normal-state single-electron transistor (SET) in the cotunneling regime in a
two-charge-state approximation. For small conductances and at zero temperature,
the SET's performance as a charge-qubit readout device is characterized by a
universal one-parameter function. The result shows that near-ideal,
quantum-limited measurement is possible for a wide range of small bias
voltages. However, near the threshold voltage for crossover to sequential
tunneling, the device becomes strongly nonideal. The (symmetrized)
current-charge cross-correlation vanishes for low frequencies, causing two
different definitions of detector nonideality to agree. Interpretations of
these findings are discussed.
