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.
We study the loss of coherence of pure and mixed states for a nonlinear interaction between the multimode
field and a qubit. We calculate analytically partial entropies of the qubit and field subsystems,
which are used to measure the loss of coherence. We find that the loss of coherence loss is sensitive to
the initial field states, the number of modes, and the nonlinear-interaction parameter.
Analytical descriptions of the geometric phases (GPs) for the total
system and subsystems are studied for a current biased Josephson
phase qubit strongly coupled to a lossy LC circuit in the dispersive
limit. It is found that, the GP and purity depend on the damping
parameter which leads to the phenomenon of GP death. Coherence
parameter delays the phenomenon of a regular sequence of deaths
and births of the GP. The asymptotic behavior of the GP and the
purity for the qubit-LC resonator state closely follow that for the
qubit state, but however, for the LC circuit these asymptotic values
are equal to zero.
Death of entanglement between light and the vibrational motion of a single trapped ion in the
dispersive regime with a reservoir is investigated. It is found that with phase-damped cavity, the
purity of the light-motional states is lost forever, unlike the purity of the ion's internal states
which have regular patterns and they do not decay. The asymptotic behavior of the states of the
light, the ion-motional and the total system fall into a mixed state. The entanglement and purity
have strong sensitivity to the phase damping and the ionic distribution angle. The entanglement
sudden death has been treated as it arises from the e®ect of phase damping on mixed as well as
The dynamics of entanglement and mixedness of a superconducting qubit strongly coupled
to a cavity field induced by a cavity damping governed by a master equation are
investigated. It is found that, asymptotic decays as well as finite time disentanglement
depend on the parameter of the dissipation, which leads to the existence of the
entanglement sudden death.
A measure of nonclassicality of quantum states based on the negative values of the Wigner function
(WF) of a charge qubit-field system is proposed. It is found that, the negative values of the field WF are
very sensitive to any change in dissipation parameter. The dissipation leads to a long-time death for both
entanglement and nonclassicality, and also the coherence of the cavity state is lost completely.