70,561 research outputs found
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 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
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