Quasi-Langevin method for shot noise calculation in single-electron tunneling

It is shown that quasi-Langevin method can be used for the calculation of the shot noise in correlated single-electron tunneling. We generalize the existing Fokker-Plank-type approach and show its equivalence to quasi-Langevin approach. The advantage of the quasi-Langevin method is a natural possibility to describe simultaneously the high (``quantum'') frequency range.Comment: 13 pages (RevTeX), 1 figur

Error matrices in quantum process tomography

We discuss characterization of experimental quantum gates by the error matrix, which is similar to the standard process matrix $\chi$ in the Pauli basis, except the desired unitary operation is factored out, by formally placing it either before or after the error process. The error matrix has only one large element, which is equal to the process fidelity, while other elements are small and indicate imperfections. The imaginary parts of the elements along the left column and/or top row directly indicate the unitary imperfection and can be used to find the needed correction. We discuss a relatively simple way to calculate the error matrix for a composition of quantum gates. Similarly, it is rather straightforward to find the first-order contribution to the error matrix due to the Lindblad-form decoherence. We also discuss a way to identify and subtract the tomography procedure errors due to imperfect state preparation and measurement. In appendices we consider several simple examples of the process tomography and also discuss an intuitive physical interpretation of the Lindblad-form decoherence.Comment: 21 pages (slightly revised version

Continuous quantum measurement with observer: pure wavefunction evolution instead of decoherence

We consider a continuous measurement of a two-level system (double-dot) by weakly coupled detector (tunnel point contact nearby). While usual treatment leads to the gradual system decoherence due to the measurement, we show that the knowledge of the measurement result can restore the pure wavefunction at any time (this can be experimentally verified). The formalism allows to write a simple Langevin equation for the random evolution of the system density matrix which is reflected and caused by the stochastic detector output. Gradual wavefunction ``collapse'' and quantum Zeno effect are naturally described by the equation.Comment: 6 pages, 2 figure