Output spectrum of a measuring device at arbitrary voltage and temperature

¹ Institut für Theoretische Festkörperphysik, Universität Karlsruhe D-76128 Karlsruhe, Germany
² Theoretical Division, Los Alamos National Laboratory Los Alamos, NM 87545, USA

Received: 14 November 2003
Accepted: 16 June 2004

Abstract

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 )) to the case of arbitrary voltages and temperatures. When , 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 , 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 and the coherent peaks do not appear at all. We relate these results to the properties of linear amplifiers.