We propose some quantum logic gates by using SC-charge qubits coupled to
a resonator to study two types of quantum operation. By applying a classical
magnetic field with the flux, a simple rotation on the target qubit is generated.
Single and two-qubit gates of quantum logic gates are realized. Two-qubit joint
operations are firstly generated by applying a classical magnetic field with the
flux, and secondly by applying a classical magnetic field with the flux when
qubits are placed a quarter of the distance along the resonator.Ashort discussion
of fidelity is given to prove the success of the operations in implementing these
Quantum correlations, including entanglement and discord with its geometric
measure in a three-qubit Heisenberg XY chain, with phase decoherence, are
investigated when a nonuniform magnetic field is applied. When the qubits are initially
in an unentangled state, the nearest neighbor pairwise correlations are destroyed
by phase decoherence, but stationary correlations appear for next-to-neighbor qubits.
With an inhomogeneous magnetic field, the stationary correlations appear for nearest
neighbor qubits and they disappear for next-to-nearest neighbor qubits. But when the
qubits are initially in an entangled state, an inhomogeneous magnetic field can enhance
the stationary correlations of next-to-neighbor qubits, but it cannot do so for nearest
Control of purity and entanglement of two two-qubits dispersively coupled to a ¯eld with a reservoir are investi-
gated. Initially the qubits are entangled while the ¯eld is either in coherent state or statistical mixture of two coherent
states. For an alternative entanglement measure we calculate the negativity of the eigenvalues of the partially trans-
posed density matrix. A measure related to the mutual entropy namely, index of entropy is employed to measure the
entanglement. Its results agree well with the negativity. It is found that the entanglement and purity have strong
sensitivity to the phase damping.
We investigate the death of entanglement and the purity loss of a two qubits–field system in the
dispersive regime with a reservoir. For an alternative entanglement measure, we calculate the negativity
of the eigenvalues of a partially transposed density matrix and compare it with the mutual entropy. A
new measure related to the mutual entropy, namely, the index of entropy, is proposed to measure the
degree of entanglement, and this agrees well with the negativity. We found that the entanglement has
a strong sensitivity to the phase damping. The asymptotic behavior of the field states, the two qubits,
and the total system fall into a mixed state. We treat the phenomena of death of entanglement and
purity as they arise from the effect of phase damping.
Quantumness of the correlations between two qubits, coupled to a cavity field
whenever the system is open or closed, is investigated when sudden death (or birth) of the
entanglement occurs. It is found that quantum correlation not exists only in the entangled
state. It’s found that the dephasing parameters and the purity of the initial states play an
important role in the dynamics behaviors of the quantum correlations, including entanglement.
Quantumness of the correlations and entanglement, due to dephasing of the cavity, are
damped until the same their values which are contained in their initial states.
The quantum correlation (QC) of two qubits coupled to a thermal field is investigated based on
measurement-induced disturbance. It is found that QC does not exist only in the entangled
state. Due to the thermal parameter, QC is inhibited from vanishing and the phenomenon of
the death and birth of entanglement occurs. Due to the field-mode structure, QC is equal to
zero for separated qubits, whereas for a maximally entangled state, it has a periodic
asymptotic behavior which does not vanish. QC and entanglement differ when the qubit state
is initially in the separated or entangled state.
The analytical description of a superconducting qubit strongly coupled to a resonator with a thermal reservoir
is given. Link between nonlocality and entanglement is investigated, they can be manipulated and controlled
by any change in the thermal reservoir parameters. According to the Peres conjecture: if the state has
negative partial transposition, then the violation of Bell's inequality occurs. Under the effect of thermal noise,
the negative partial transposition is transformed to positive. Therefore, the state shows the phenomenon of
sudden death of both entanglement and nonlocality.
The exact solution of the master equation for the case of a high-Q cavity with atomic decay is found. We use
the negativity of the Wigner function (WF) as an indicator of nonclassicality. It is found that the negative values of
the field WF are very sensitive to any change in the damping parameter. The atomic spontaneous decay leads to the
simultaneous disappearance of both entanglement and nonclassicality of quantum states. Moreover, the purity of the
field states is completely lost.
An analytical solution to the master equation of a system describing a single quantum dot confined in a
single-mode microcavity, coupled to its environment, is found. The information loss of the phase space,
the purity and the relaxation process are investigated by using the Husimi distribution and its applications.
It is found that the spontaneous decay leads to the loss of information of the phase space and the purity.
Suggested indicator for the relaxation process is offered by using the Wehrl entropy.