  We calculate the noise spectrum of the output signal of a quantum detector
during continuous measurement of a two-level system (qubit). We generalize the
previous results obtained for the regime of high voltages (when $eV$ is much
larger than the qubit's energy level splitting $\Delta$) to the case of
arbitrary voltages and temperatures. When $V \sim \Delta$ the output spectrum
is essentially asymmetric in frequency, i.e., the output signal is no longer
classical. In the emission (negative frequency) part of the spectrum the peak
due to the qubit's coherent oscillations can be 8 times higher that the
background pedestal. For $V < \Delta$ and T=0 the coherent peaks do not appear
at all.
