Europhys. Lett.
Volume 67, Number 5, September 2004
Page(s) 840 - 846
Section Condensed matter: electronic structure, electrical, magnetic, and optical properties
Published online 01 August 2004
Europhys. Lett., 67 (5), pp. 840-846 (2004)
DOI: 10.1209/epl/i2003-10309-6

Output spectrum of a measuring device at arbitrary voltage and temperature

A. Shnirman1, D. Mozyrsky2 and I. Martin2

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 in final form 16 June 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 $e=\hbar=1$)) to the case of arbitrary voltages and temperatures. When $V\sim B$, 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 $\omega=\pm B$, 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.

74.50.+r - Tunneling phenomena; point contacts, weak links, Josephson effects.
03.65.Ta - Foundations of quantum mechanics; measurement theory.

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