36,675 research outputs found
Effects of uncertainties and errors on Lyapunov control
Lyapunov control (open-loop) is often confronted with uncertainties and
errors in practical applications. In this paper, we analyze the robustness of
Lyapunov control against the uncertainties and errors in quantum control
systems. The analysis is carried out through examinations of uncertainties and
errors, calculations of the control fidelity under influences of the
certainties and errors, as well as discussions on the caused effects. Two
examples, a closed control system and an open control system, are presented to
illustrate the general formulism.Comment: 4 pages, 5 figure
Nonmonotonic behavior of resistance in a superconductor-Luttinger liquid junction
Transport through a superconductor-Luttinger liquid junction is considered.
When the interaction in the Luttinger liquid is repulsive, the resistance of
the junction with a sufficiently clean interface shows nonmonotonic
temperature- or voltage-dependence due to the competition between the
superconductivity and the repulsive interaction. The result is discussed in
connection with recent experiments on single-wall carbon nanotubes in contact
with superconducting leads.Comment: Revtex4, 2 eps figure files, slightly revised from an earlier version
submitted to PRL on 2001.12.
Mixed Qubit Cannot Be Universally Broadcast
We show that there does not exist any universal quantum cloning machine that
can broadcast an arbitrary mixed qubit with a constant fidelity. Based on this
result, we investigate the dependent quantum cloner in the sense that some
parameter of the input qubit is regarded as
constant in the fidelity. For the case of constant , we establish the
optimal symmetric dependent cloner with a fidelity 1/2. It is also
shown that the optimal quantum cloning machine for pure qubits is also
optimal for mixed qubits, when is the unique parameter in the
fidelity. For general broadcasting of mixed qubits, the situation is
very different.Comment: 5 pages, Revte
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