25,268 research outputs found
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
Quantum interference in dirty d-wave superconductors
The local differential tunneling conductance on a Zn impurity in a disordered
d-wave superconductors is studied. Quantum interference between many impurities
leads to definitive quasiparticle spectra. We suggest that an elaborate
analysis on impurity-induced spectra with quantum interference effect included
may be able to pin down the sign and strength of the scattering potential of a
Zn impurity in low density limit. Numerical simulations calculated with
appropriately determined impurity parameters are in satisfactory agreement with
the observations from scanning tunneling microscopy (STM) experiments even in
subtle details
The temperature dependence of the local tunnelling conductance in cuprate superconductors with competing AF order
Based on the model with proper chosen parameters for describing
the cuprate superconductors, it is found that near the optimal doping at low
temperature (), only the pure d-wave superconductivity (SC) prevails and
the antiferromagnetic (AF) order is completely suppressed. At higher , the
AF order with stripe modulation and the accompanying charge order may emerge,
and they could exist above the SC transition temperature. We calculate the
local differential tunnelling conductance (LDTC) from the local density of
states (LDOS) and show that their energy variations are rather different from
each other as increases. Although the calculated modulation periodicity in
the LDTC/LDOS and bias energy dependence of the Fourier amplitude of LDTC in
the "pseudogap" region are in good agreement with the recent STM experiment
[Vershinin , Science {\bf 303}, 1995 (2004)], we point out that some of
the energy dependent features in the LDTC do not represent the intrinsic
characteristics of the sample
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