Output spectrum of a measuring device at arbitrary voltage and temperature
Alexander Shnirman, Dmitry Mozyrsky, Ivar Martin · Europhysics Letters (EPL) · 2004
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.