100 MHz regime to be realistic due to the exponential dependence of the tunneling rate on the potential bar- rier. Increasing the bias voltage V to values of a few mV the noise due to the classical two-level fluctuator starts to dominate and the crossover as depicted in Fig. 2 can be observed. We have determined the influence of a thermally driven two-level fluctuator on a point contact through the cal- culation of the full counting statistics of transported charge. Both, classical and quantum point contacts have been considered and extensions to multiple fluctuators have been discussed. In our analysis, we have made use of a mapping between correlation functions of classi- cal stochastic processes and simple time-ordered matrix We acknowledge fruitful discussions with Misha Suslov and financial support from the Pauli Center at ETH Zurich, the Swiss National Foundation, the Russian Foundation for Basic Research (11-02-00744-a), the pro- gram ‘Quantum physics of condensed matter’ of the RAS, and the Computer Company NIX (F1025/10-04). products. For the case of a quantum point contact, we have shown that the partitioning noise and the noise due the two-level fluctuator add up and the noise crosses over from sub- to super-Poissonian depending on the applied voltage bias. To extend the present formalism to the case of current correlators at finite frequencies, which provides additional insights into the dynamics of the two-level fluc- tuator, is an interesting problem for future studies. 8 Acknowledgments VII. CONCLUSION 1 B.L. Al’tshuler and B.Z. Spivak, Pis’ma Zh. Eksp. Teor. Fiz. 49 , 459 (1989) [JETP Lett. 49 , 527 (1989)]. 3 L.S. Levitov, H.W. Lee, and G.B. Lesovik, J. Math. Phys. 37 , 4845 (1996). 2 S. Feng, P.A. Lee, and A.D. Stone, Phys. Rev. Lett. 56 , 1960 (1986). 4 Yu.V. Nazarov and Ya.M. Blanter, Quantum Transport: Introduction to Nanoscience (Cambridge University Press, Cambridge, 2009). 5 G.B. Lesovik, Pis’ma Zh. Eksp. Teor. Fiz. 60 , 806 (1994) [JETP Lett. 60 , 820 (1994)]. 6 R.P. Feynman and F.L. Vernon, Jr., Ann. Phys. (NY) 24 , 118 (1963). 7 D.A. Bagrets and Yu.V. Nazarov, Phys. Rev. B 67 , 085316 (2003). 8 S. Gustavsson, R. Leturcq, B. Simoviˇ c, R. Schleser, T. Ihn, P. Studerus, K. Ensslin, D.C. Driscoll, and A.C. Gossard, Phys. Rev. Lett. 96 , 076605 (2006). 9 A.N. Jordan and E.V. Sukhorukov, Phys. Rev. Lett. 93 , 260604 (2004). 10 E.V. Sukhorukov, A.N. Jordan, S. Gustavsson, R. Leturcq, T. Ihn, and K. Ensslin, Nature Physics 3 , 243 (2007). 11 G. Falci, E. Paladino, and R. Fazio, in Quantum Phe- nomena in Mesoscopic Systems , edited by B.L. Al’tshuler, A. Tagliacozzo, and V. Tognetti (IOS Press, Amsterdam, 2003). 12 B. Abel and F. Marquardt, Phys. Rev. B 78 , 201302(R) (2008). 13 J. Schriefl, M. Clusel, D. Carpentier, and P. Degiovanni, Phys. Rev. B 72 , 035328 (2005). 14 J. Schriefl, Yu. Makhlin, A. Shnirman, and G. Sch¨ on, New J. Phys. 8 , 1 (2006). 15 E. Paladino, L. Faoro, G. Falci, and R. Fazio, Phys. Rev. Lett. 88 , 228304 (2002). 16 Yu.M. Galperin, B.L. Al’tshuler, J. Bergli, and D.V. Shant- sev, Phys. Rev. Lett. 96 , 097009 (2006). 17 J. Bergli, Yu.M. Galperin, and B.L. Al’tshuler, New J. Phys. 11 , 025002 (2009). 18 A. Peres, Phys. Rev. A 30 , 1610 (1984). 19 G.B. Lesovik, F. Hassler, and G. Blatter, Phys. Rev. Lett. 96 , 106801 (2006). 20 V.A. Khlus, Zh. Eksp. Teor. Fiz. 93 , 2179 (1987) [Sov. Phys. JETP 66 , 1243 (1987)]. 21 G.B. Lesovik, Pis’ma Zh. Eksp. Teor. Fiz. 49 , 513 (1989) [JETP Lett. 49 , 592 (1989)]. 22 H.A. Kramers, Physica (Utrecht) 4 , 284 (1940). 23 J. Zinn-Justin, Quantum Field Theory and Critical Phe- nomena , 4th edition (Clarendon press, Oxford, 2002). 24 C.W. Gardiner, Handbook of Stochastic Methods , 3rd edi- tion (Springer, Berlin, 2004). 25 R.P. Feynman and A.R. Hibbs, Quantum Mechanics and Path Integrals (McGraw-Hill Book Company, New York, 1965). 26 S. Pilgram, A.N. Jordan, E.V. Sukhorukov, and M. B¨ uttiker, Phys. Rev. Lett. 90 , 206801 (2003). 27 G. Kießlich, P. Samuelsson, A. Wacker, and E. Sch¨ oll, Phys. Rev. B 73 , 033312 (2006). 28 C.W. Groth, B. Michaelis, and C.W.J. Beenakker, Phys. Rev. B 74 , 125315 (2006). 29 P. Flajolet and R. Sedgewick, Analytic Combinatorics (Cambridge University Press, Cambridge, 2009), Part A. 30 C. Flindt, C. Fricke, F. Hohls, T. Novotn´ y, K. Netoˇ cn´ y, T. Brandes, and R.J. Haug, Proc. Natl. Acad. Sci. USA 106 , 10116 (2009). 31 F. Hassler, M.V. Suslov, G.M. Graf, M.V. Lebedev, G.B. Lesovik, and G. Blatter, Phys. Rev. B 78 , 165330 (2008). 32 Note that the time-dependence of the scattering matrix promotes the incoming state to higher energies. In order that the linear spectrum approximation is still valid the time dependence of T ( t ) should not be too abrupt. 33 B. K¨ ung, S. Gustavsson, T. Choi, I. Shorubalko, T. Ihn, S. Sch¨ on, F. Hassler, G. Blatter, and K. Ensslin, Phys. Rev. B 80 , 115315 (2009). 34 R. Hanson, L. P. Kouwenhoven, J. R. Petta, S. Tarucha, and L. M. K. Vandersypen, Rev. Mod. Phys. 79 , 1217 (2007).