2003/11/30 by A. Shnirman, Alexander Shnirman, Dmitry Mozyrsky +3
Computer Science · Physics and Astronomy · #Quantum Computing Algorithms and Architecture #Quantum Information and Cryptography #Quantum and electron transport phenomena #cond-mat.mes-hall
paper · pdf · doi:10.1209/epl/i2003-10309-6
published as Europhys. Lett., 67 (5), pp. 840-846 (2004) · 7 pages, 5 figures, the results generalized for arbitrary angle between the magnetic field and the observed component of the spin, minor corrections and typos
arxiv created 2004/06/03 · openalex publication_date 2004/08/25 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/30
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.