The dynamics of two qubits ultra-strongly coupled with a quantum oscillator is investigated by the adiabatic approximation method. The evolution formula of the initial four Bell states are studied under the control mechanism of the coherent state of the quantum oscillator. The influential parameters for the preservation of the entanglement are the four parameters: the average number of the coherent state, the ultra-strong coupling strength, the ratio of two frequencies of qubit and oscillator, and the inter-interaction coupling of the two qubits. The novel results show that the appropriate choice of these parameters can enable this mechanism to be utilized to preserve the entanglement of the two qubits, which is initially in the state |I_0> of the four Bell states. We give two different schemes to choose the respective parameters to maintain the entangled state |I_0> almost unchanged. The results will be helpful for the quantum information process.
We investigate the dynamics of two qubits coupled with a quantum oscillator by using the adiabatic approximation method. We take account of the interaction between the qubits and show how the entanglement is affected by the interaction parameter. The most interesting result is that we can prolong the entanglement time or improve the entanglement degree by using an appropriate interaction parameter. As the generation and preservation of entanglement of qubits are crucial for quantum information processing, our research will be useful.
We explore the possibility of the controlled manipulation of the entanglement of two qubits with an external apparatus, the Rabi Hamiltonian. The novel results show that the initially entangled two qubits could have very high probability to stay unchanged by the coherent state of the photon with chosen parameters. If the inter-qubit coupling strength is negative, their entanglement could be kept for a much longer time by a relatively strong external quantum control with the suitable and novel choice of some parameters. Furthermore, their entanglement will not show any sudden death and revival. All these results are different from the previous studies and they are given with reasonable explanations. The maintaining of the entanglement of the two qubits with very high fidelity will be very helpful for the quantum information process.
The dynamics of entanglement and fidelity for a subsystem of two separate spin-
1∕2 qubits prepared in Bell states is investigated. One of the subsystem qubit labeled
A is under the influence of a Heisenberg
XY spin bath, while another one labeled
B is uncoupled with that. We discuss two cases: (i) the number of bath spins is infinite, $N\ensuremath{\rightarrow}\ensuremath{\infty}$, and (ii)
N is finite,
N=40. In both cases, the bath is initially prepared in a thermal equilibrium state. It is shown that the time dependence of the concurrence and the fidelity of the two subsystem qubits can be controlled by tuning the parameters of the spin bath, such as the anisotropic parameter, the temperature, and the coupling strength with qubit
A. It is interesting to find that the dynamics of the concurrence is independent of four different initial Bell states and that of the fidelity is divided into two groups.
We investigate the quantum entanglement dynamics of two coupled harmonic oscillators with a time-dependent interaction. Using the Lewis-Riesenfeld invariant method, we derive the exact analytical wave functions without any perturbative or adiabatic approximations and combine this with a phase-space analysis using the Wigner function to provide a complete description of the system's quantum state evolution. We obtain general expression form for the purity and the linear entropy
SL=1−P for arbitrary excitation numbers
(n,m), allows a systematic study of entanglement for a large class of quantum states. We show that the entanglement dynamics is very sensitive to the interplay between the detuning parameters
θ and
ϑ2, the frequency parameter
β0 and the coupling strength
ε: the increase of detuning takes the system from slow irregular oscillations to fast and regular periodic behavior, and the stronger coupling systematically enhances both the amplitude and the average value of the linear entropy. Most importantly, for the resonance case
ω1=ω2=1 and strong couplings
ε≈0.99, the system shows robust undamped synchronized periodic oscillations of the linear entropy for all quantum states considered, indicating preserved quantum coherence without saturation. Our findings demonstrate that linear entropy is a sensitive and practical entanglement witness, and we establish explicit analytical relations between the coupling parameters of the system and its entanglement properties, which are directly relevant to quantum information processing and the control of quantum correlations in continuous-variable systems.
Abstract Harnessing and protecting quantum entanglement from the detrimental effects of local environments is essential for quantum information and computation. With this aim, we present a simple scheme to preserve and control the quantum entanglement. The scheme consists of two coupled single-mode lossy cavities, each containing a qubit, which are in turn connected simultaneously with another two lossy cavities. We examine the protection of entanglement for initially maximal entangled state in both weak and strong coupling regimes. It is found that one can enhance the survival time of the entanglement expeditiously by adjusting the coupling strengths between the relevant cavities and increasing the number of auxiliary cavities. In addition, if the cavities without qubits do not lose photons then after certain damping oscillations the entanglement between two qubits becomes stable. Interestingly, the magnitude of the stable quantum entanglement depends not only on the hopping strengths but also on the number of contributing auxiliary cavities.