Quantum treatment for two two-level atoms in interaction with an SU(1,1) quantum system

We consider the interaction between two identical two-level atoms prepared in superposition states and an SU(1, 1) quantum system prepared in the Perelemov coherent state. We determine the timedependent wave function through the Schrödinger equation for the resonance case, and, consequently, we obtain the density matrix. We consider the phenomenon of collapses and revivals of the atomic population inversion for different values of the parameters and show the coherent trapping. We investigate the entanglement in the system where we discuss the linear entropy for different values of the involved parameters and for some states. Finally we examine the second-order correlation function to distinguish between the classical and nonclassical behaviors. We show that the system is sensitive to the variation in both the Bargmann index k and the Perelomov coherent parameter μ, as well as the atomic phase parameters.     

Entropy squeezing and entanglement of the interaction between SU(1,1) and SU(2) quantum systems

The problem of the interaction between two quantum systems namely SU(1,1) and SU(2) is considered. Using the evolution operator technique, an exact solution of the wave function and consequently the density matrix are obtained. The entropy squeezing is examined and it has shown that, different values of the relative phase angle ? as well as the coupling parameter λ lead to different observation of the squeezing in the quadratures. In the meantime, we have shown that the entropy squeezing is also sensitive to the variation in the state angle θ, the detuning parameter Δ in addition to the excitation number m. Moreover, for a large value of the detuning parameter there is a weak entanglement between the atom and the quantum system and vice versa. Furthermore, we find that the Q-function is sensitive to the variation in the excitation number m in addition to the Bargmann index k where the nonclassical effect is pronounced for the even parity.     

Interaction of two two-level atoms with SU (1, 1 ) coherent states

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两个耦合二能级原子和双模真空场作用中的和压缩

研究了双模真空场与两个耦合二能级原子相互作用中的和压缩.并讨论了原子的初始相干性及原子间的耦合常数对该压缩的影响.     

Quantum discord between two moving two-level atoms

Considering a double JC model, this paper investigates the quantum discord dynamics of two isolated moving two-level atoms each interacting with a single-mode thermal cavity field, and studies the effect of the atomic motion and the field-mode structure on quantumdiscord. The results show that, on the one hand the quantum discord evolves periodically with time and the periods are affected by the atomic motion and the field-mode structure; on the other hand, the quantum discord still can capture the quantum correlation between the two atoms when the entanglement is zero. It is interesting to note that the quantum discord can be effectively preserved by controlling the field-mode structure parameter     

Anomalous interaction between phonons and IVAP-originated two-level systems

Summary The first-principle dynamics of IVAP-originated two-level systems is developed with reference to phonon-scattering and thermally activated processes. It is shown that the anomalous coupling term βδ, appearing in the phonon-IVAP interaction, originates from the elastic polarity ξ characteristic of the IVAP-like defects. The formal expressions for the quantities of interest, such as the resonant-scattering rate of phonons and the time-dependent density of TLS states, are given analytically. Since the tunnelling-atom theory is a limit case (β=0) of the present general approach, a point-by-point comparison with the standard formalism of tunnelling-originated TLS is performed. This work was partially supported by Ministero della Pubblica Istruzione.     

两个二能级原子与真空辐射场相互作用中的偶极平方压缩

研究了两个二能级原子与真空辐射场相互作用中原子的偶极平方压缩效应。通过数值计算,讨论了原子与辐射场之间耦合常数相对大小值以及原子的初始相干性对偶极平方压缩效应的影响     

Conditional quantum CNOT gate between two four-level atoms

We propose a scheme to implement a quantum controlled-NOT (CNOT) gate between two four-level atoms inside the detuned optical cavity. The system state is evolved inside the decoherence-free (DF) subspace through stimulated Raman processes, which yields the desired unitary evolution operation for the CNOT.Our scheme is immune to decoherence due to dissipation of cavity excitation and spontaneous emission from the excited atomic level.     

Peremolv SU(1,1)相干叠加态的二阶量子关联特性研究

本文首先讨论了Barut-GirardelloSU(1,1)相干叠加态在Bargmann指数k≠1/2时的二阶量子关联特性。发现随着Bargmann指数的增大,Barut-Girardello奇相干态呈现反关联的区域变大。然后提出并研究了PeremolovSU(1,1)相干叠加态。Peremolov奇相干态可呈现反关联,并且随着Bargmann指数的增加,反关联程度变弱,反关联区域变小。     

UNCORRELATED TWO-MODE SU(1,1) COHERENT STATES

We have constructed a new kind of two-mode bosonic realization of SU(1,1) Lie algebra, on the basis of which the SU(1,1) generalized coherent states in the two-mode Fock space are derived. These two-mode SU(1,1) coherent states, which are called uncorrelated two-mode SU(1,1) coherent states, include three special cases. For these states, we study the mean photon number distribution and their non-classical properties, which are photon anti-bunching, violations of Cauchy－Schwarz inequality and two-mode squeezing.     

Dynamics of a three level atom interacting with a detuned SU (1,1) quantum system

We study the stability of attractive atomic Bose-Einstein condensate and the macroscopic quantum many-body tunneling (MQT) in the anharmonic trap. We utilize correlated two-body basis function which keeps all possible two-body correlations. The anharmonic parameter (λ) is slowly tuned from harmonic to anharmonic. For each choice of λ the many-body equation is solved adiabatically. The use of the van der Waals interaction gives realistic picture which substantially differs from the mean-field results. For weak anharmonicity, we observe that the attractive condensate gains stability with larger number of bosons compared to that in the pure harmonic trap. The transition from resonances to bound states with weak anharmonicity also differs significantly from the earlier study of [N. Moiseyev, L.D. Carr, B.A. Malomed, Y.B. Band, J. Phys. B 37, L193 (2004)]. We also study the tunneling of the metastable condensate very close to the critical number N _cr of collapse and observe that near collapse the MQT is the dominant decay mechanism compared to the two-body and three-body loss rate. We also observe the power law behavior in MQT near the critical point. The results for pure harmonic trap are in agreement with mean-field results. However, we fail to retrieve the power law behavior in anharmonic trap although MQT is still the dominant decay mechanism.     

Dynamics and quantum entanglement of two-level atoms in de Sitter spacetime

In the framework of open quantum systems, we study the internal dynamics of both freely falling and static two-level atoms interacting with quantized conformally coupled massless scalar field in de Sitter spacetime. We find that the atomic transition rates depend on both the nature of de Sitter spacetime and the motion of atoms, interestingly the steady states for both cases are always driven to being purely thermal, regardless of the atomic initial states. This thermalization phenomenon is structurally similar to what happens to an elementary quantum system immersed in a thermal field, and thus reveals the thermal nature of de Sitter spacetime. Besides, we find that the thermal baths will drive the entanglement shared by the freely falling atom (the static atom) and its auxiliary partner, a same two-level atom which is isolated from external fields, to being sudden death, and the proper time for the entanglement to be extinguished is computed. We also analyze that such thermalization and disentanglement phenomena, in principle, could be understood from the perspective of table-top simulation experiment.     

Schr(o)dinger-cat态光场与耦合双原子相互作用系统的量子场熵演化

利用全量子理论,研究了Schr(o)dinger-cat态光场与耦合双能级原子相互作用系统在非共振情况下的量子场熵演化特性.结果表明,场-原子系统的量子场熵演化特性不仅与初始平均光子数n0、场与原子的耦合强度g、原子与原子之间的耦合强度ga以及失谐量△有着密切的关系,而且与原子的初始状态有关.     

Conditional quantum phase gate between two 3-state atoms

We propose a scheme for conditional quantum logic between two 3-state atoms that share a quantum data bus such as a single mode optical field in cavity QED systems, or a collective vibrational state of trapped ions. Making use of quantum interference, our scheme achieves successful conditional phase evolution without any real transitions of atomic internal states or populating the quantum data bus. In addition, it requires only common addressing of the two atoms by external laser fields.     

Time optimal control of two-level quantum systems

The objective of time-optimal control that helps to minimize relaxation losses, is the evolution of a quantum state from a given initial mixed state to a final target mixed state in minimum time. In this paper, we study a time-optimal control problem of the dynamic of a pure two-level system with unbounded control using Pontryagin's minimum principle and obtain the minimal time for some initial and final states. The results will apply to basically all qubit systems that one can consider such as NMR spectroscopy, trapped ions, superconducting qubits, etc. We also show that these results hold for pure states, and only the direction $\hat{n}$ is important in the evolution of a quantum state. In this work, the problem of computing minimum time to produce any unitary transformation $U_f\in SU(2)$ is reduced to finding the minimum time to steer the system from an initial to a final state.     

Thermal equilibration between two quantum systems

Two identical finite quantum systems prepared initially at different temperatures, isolated from the environment, and subsequently brought into contact are demonstrated to relax towards Gibbs-like quasiequilibrium states with a common temperature and small fluctuations around the time-averaged expectation values of generic observables. The temporal thermalization process proceeds via a chain of intermediate Gibbs-like states. We specify the conditions under which this scenario occurs and corroborate the quantum equilibration with two different models.