Abstract In this paper, we show that the cross-Kerr nonlinearity is induced by the ground-state decoherence in a Λ system even under the exact one- and two-photon resonance conditions, which is attributed to the fact that the dark-state resonance is broken slightly. By adiabatically eliminating the atomic variables, we find that the decoherence results in a frequency shift of cavity detuning and the two cavity fields experience cross-Kerr nonlinearity by choosing appropriate parameters. As a consequence, the internal nonlinear effects are responsible for the generation of light entanglement, which is verified by our numerical results. The present scheme demonstrates that the decoherence plays a positive role in controlling quantum entanglement, which may be useful for quantum information processing.
Abstract In this paper, we show that the cross-Kerr nonlinearity is induced by the ground-state decoherence in a Λ system even under the exact one- and two-photon resonance conditions, which is attributed to the fact that the dark-state resonance is broken slightly. By adiabatically eliminating the atomic variables, we find that the decoherence results in a frequency shift of cavity detuning and the two cavity fields experience cross-Kerr nonlinearity by choosing appropriate parameters. As a consequence, the internal nonlinear effects are responsible for the generation of light entanglement, which is verified by our numerical results. The present scheme demonstrates that the decoherence plays a positive role in controlling quantum entanglement, which may be useful for quantum information processing.
Entanglement dynamics of two identical non-interacting atoms (qubits) coupled individually with simultaneous classical and quantum fields are studied. The cavity field is filled with a nonlinear Kerr medium and initially prepared in a coherent state. The atoms are initially set up as a Bell-like pure state (BS). We present an approach for diagonalization of time-dependent nonlinear Hamiltonian of the system exactly. Connection between the change in the degree of entanglement and tomography of field state in phase space are also illustrated and interpreted. We demonstrate the possibility of atom–atom (qubit–qubit) entanglement optimization by suitably choosing initial interaction settings. Overall, we show that both classical driving amplitude and detuning as well as Kerr media and initial atomic states acts as the control parameters for the qubit–qubit entanglement. By adjusting of these parameters, accurately, entanglement can be enhanced noticeably and high degree of steady periodical entanglement can be generated. Moreover, starting with initial atomic BSs in presence of classical driving suppresses coherences randomness and considerably accompanied with (for specific values of detuning) slight decrease in their amplitudes. Furthermore, the addition of cross Kerr term suppresses degree of entanglement noticeably, where entanglement creation and enhancement could just be possible if cross Kerr effect is moved out from interaction. Our present approach promises the great advantage of being suitable for large quantum systems of various kinds of nonlinearities.
An entangled state, as an essential tool in quantum information processing, may be generated through the interaction between light and matter in cavity quantum electrodynamics. In this paper, we study the interaction between two two-level atoms and a two-mode field in an optical cavity enclosed by a medium with Kerr nonlinearity in the presence of a detuning parameter and Stark effect. It is assumed that the atom–field coupling and third-order susceptibility of the Kerr medium depend on the intensity of the light. In order to investigate the dynamics of the introduced system, we obtain the exact analytical form of the state vector of the considered atom–field system under initial conditions which may be prepared for the atoms (in a coherent superposition of their ground and upper states) and the fields (in a standard coherent state). Then, in order to evaluate the degree of entanglement between the subsystems, we investigate the dynamics of the entanglement by employing the entanglement of formation. Finally, we analyze in detail the influences of the Stark shift, the deformed Kerr medium, the intensity-dependent coupling, and also the detuning parameter on the behavior of this measure for different subsystems. The numerical results show that the amount of entanglement between the different subsystems can be controlled by choosing the evolved parameters appropriately.
We propose methods for generating Bell-type entangled coherent states for quantum information processing. The protocols are completed using linear optical elements, single-photon detectors, and cross-Kerr nonlinearity. We investigate the decoherence effect on our protocols; photon loss is found to affect only the probability, not the fidelity, of the generation process. The entanglement of the generated entangled states is also analyzed using concurrence. Finally, the feasibility of our scheme is discussed using the continuous-time cross-phase modulation model. In the weak nonlinearity region, on the condition that θ2|αp|2∼1, θ≪1, and σ2≪1, our scheme is applicable theoretically.
In this thesis, quantum coherent control of dynamic properties of double-dark-resonance atomic systems is pursued in detail. First we show our scheme for creating atomic coherent superposition states via the technique of stimulated Raman adiabatic passage in a Lambda-type four-level system. With the application of a control field, it is found that the presence of double dark states leads to two arbitrary coherent superposition states with equal amplitude but inverse relative phases, even though the condition of multiphoton resonance is not met. The suitable manipulation of the control field and detunings enables the proposed scheme to create any coherent superposition states one desired. Then, in the above mentioned double-dark-resonant system, a scheme for giant enhancement of the Kerr nonlinearity is put forward. Compared with that generated in a single-dark-resonance system, the Kerr nonlinearity can be enhanced by several orders of magnitude with vanishing linear absorption. We attribute this dramatic enhancement to the interaction of dark resonances. As another possible means for enhancement of Kerr nonlinearity, spontaneously generated coherence is proposed. Meanwhile, a theoretical investigation is carried out in a five-level tripod system for obtaining a high efficiency double-channel four-wave-mixing process. By manipulating the intensity of the two coupling fields, the conversion efficiency of four-wave-mixing can be controlled.
We study the nonlinear effect in a three-level atomic system in Λ configuration. As two or more fields are confined within the common electromagnetically induced transparency (EIT) window, they directly disturb EIT dark state at the same time, which leads to the self- and cross-Kerr nonlinear interactions. Our scheme is different from the existed schemes based on three-level or multilevel systems only existing single type of nonlinear interactions. In the common EIT windows, the self- and cross-Kerr nonlinearities are established with suppressed linear absorption and our scheme can be generalized polychromatic Kerr nonlinearity. Because the atoms are almost trapped in the ground states and the atom population becomes unchanged, the scheme is robust to the atomic spontaneous emission. Our scheme is a promising avenue for preparation of squeezing and entanglement .
In this paper, we explore the atomic population inversion and entanglement dynamics of a quantum system consisting of three identical two-level atoms interacting with a single-mode cavity field. The cavity is surrounded by a Kerr medium exhibiting f-deformed Kerr nonlinearity, and we consider a degenerate multi-photon transition in the nonlinear regime. The study systematically analyzes the effects of intensity-dependent coupling, detuning, Kerr nonlinearity, f-deformed Kerr nonlinearity, and multi-photon transition on the nonclassical properties of the system. Our results show that the Kerr and f-deformed Kerr nonlinearities significantly influence the atomic inversion dynamics, leading to phenomena such as collapse and revival, with distinct behaviors observed under varying detuning parameters. Furthermore, entanglement dynamics between the atoms and cavity mode are enhanced under intensity-dependent coupling and nonlinearity. The detuning parameter generally reduces entanglement but has a less pronounced effect in the two-photon transition case, where entanglement is notably enhanced. The study provides valuable insights into controlling quantum correlations in multi-atom systems, with potential applications in quantum information processing, computing, and communication.