  We propose to monitor a qubit with a double-dot (DD) resonant-tunneling
detector, which can operate at higher temperatures than a single-dot detector.
In order to assess the effectiveness of this device, we derive rate equations
for the density matrix of the entire system. We show that the signal-to-noise
ratio can be greatly improved by a proper choice of the parameters and location
of the detector. We demonstrate that quantum interference effects within the DD
detector play an important role in the measurement. Surprisingly, these effects
produce a systematic measurement error, even when the entire system is in a
stationary state.
