Electrical quantum measurement of a two-level system at arbitrary voltage and temperature

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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 Δ) to the case of arbitrary voltages and temperatures. When V Δ 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 < Δ and T=0 the coherent peaks do not appear at all.