Output spectrum of a measuring device at arbitrary voltage and temperature

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We calculate the noise spectrum of the electrical current in a quantum point contact which is used for continuous measurements of a two-level system (qubit). We generalize the previous results obtained for the regime of high transport voltages (when V is much larger than the qubit's energy level splitting B (we put e=ℏ=1)) to the case of arbitrary voltages and temperatures. When V B the background output spectrum is essentially asymmetric in frequency, i.e., it is no longer classical. Yet, the spectrum of the amplified signal, i.e., the two coherent peaks at ω=± B is still symmetric. In the emission (negative frequency) part of the spectrum the coherent peak can be 8 times higher than the background pedestal. Alternatively, this ratio can be seen in the directly measureable excess noise. For V < B and T=0 the coherent peaks do not appear at all. We relate these results to the properties of linear amplifiers.