Study of squeezing in spin clusters

The conditions under which spin squeezing occurs in an asymmetric chain of spins are discussed. The time evolution of the system is calculated for different initial conditions. The effects of the use of spin coherent states to model the initial condition are analyzed.     

Qubit disentanglement in local squeezed reservoirs

We consider the influence of the local squeezed vacuum fields on two initially entangled two-qubit system. By considering the upper bound of entanglement under time evolution, we find that the decay of the quantum entanglement shows different behavior for different time scales (t?max{(2βA)⁻¹,(2βB)⁻¹}$t?\max \{{(2{\beta }_A)}^{\mathrm{-1}},{(2{\beta }_B)}^{\mathrm{-1}}\}$ and t?min{(2βA)⁻¹,(2βB)⁻¹}$t?\min \{{(2{\beta }_A)}^{\mathrm{-1}},{(2{\beta }_B)}^{\mathrm{-1}}\}$). The relative phase of the squeezing environment can also affect the entanglement dynamics profoundly.     

Entanglement and squeezing of multi-qubit systems using a two-axis countertwisting Hamiltonian with an external field

We study the squeezing and pairwise entanglement properties of multi-qubit spin systems, implemented by a two-axis countertwisting Hamiltonian, with and without an external field. We show that the addition of an external field, enforces stronger squeezing and entanglement, and also increases the time duration that these phenomena may be maintained.     

Stationary photon-atom entanglement and flow equation

A novel strategy for quantum-state engineering is presented by employing the method of flow equation. This strategy offers an optimal procedure for generating stationary entangled states of coupled systems of photons and atoms. To illustrate it, the Jaynes-Cummings model as a simple prototype example is analyzed to exhibit how stationary photon-atom entanglement is established. A nonlocal continuous unitary transformation is explicitly constructed and the associated positive operator-valued measures for the photons and atom are obtained. Then, flow of the entanglement entropy is analyzed. A comment is also made on implementing the unitary operation in the method.     

Preparation of Cluster States of Atomic Qubits in Cavity QED

We propose an optical scheme to generate cluster states of atomic qubits, with each trapped in separate optical cavity, via atom-cavity-laser interaction. The quantum information of each qubit is encoded on the degenerate ground states of the atom, hence the entanglement between them is relatively stable against spontaneous emission. A single-photon source and two classical fields are employed in the present scheme. By controlling the sequence and time of atom-cavity-laser interaction, we show that the atomic cluster states can be produced deterministically.     

Fresnel-Transform's Quantum Correspondence and Quantum Optical ABCD Law

Corresponding to the Fresnel transform there exists a unitary operator in quantum optics theory, which could be known the Fresnel operator (FO). We show that the multiplication rule of the FO naturally leads to the quantum optical ABCD law. The canonical operator methods as mapping of ray-transferABCD matrix is explicitly shown by the normally ordered expansion of the FO through the coherent state representation and the technique of integration within an ordered product of operators. We show that time evolution of the damping oscillator embodies the quantum optical ABCD law.     

Induced Rabi oscillations and revivals in the presence of dissipation

The “time reversal” dynamics of dissipative atom-cavity systems has been recently experimentally investigated. We model the experimental procedure using semiclassical approximations which allow for analytical results and to distinguish decoherence from damping effects. We show that the basic effect of decoherence is to drastically alter the envelope of the population inversion, mainly the amplitude. Damping, on the other hand, is responsible for the anticipation of the revival times and corrections to the fast frequencies in each envelope.     

Generation of Cluster-Type Entangled Coherent States via Cross-Kerr Nonlinearity

We propose an optical scheme for the generation of the cluster-type entangled coherent states in free travelling optical fields via cross-Kerr nonlinearity. The required resources for the generation are coherent state source, beam splitters, photodetectors, and Kerr media. We also discuss the implementation of the Hadamard gate operation for coherent states and the homodyne detection.     

Squeezing in a spin chain with site-dependent periodic and long-range interactions

Numerical and analytical results for the squeezing factor, ζ², in a pseudo-spin s-1/2 chain. The open chain is composed by N two-level atoms with site-dependent interactions. The time evolution of the squeezing factor is studied, as well as its dependence on the number of atoms and on the interactions. It is found that long-range interactions may optimize the degree of spin squeezing.     

Rotationally Invariant Proof of Bell’s Theorem Without Inequalities for 4k-Dimensional (k=1,2,...) Systems

We present rotationally invariant proof of Bell‘s theorem without inequalities for spin-(2N 1)/2system (N = 2k-1,k = 1, 2, 3, 4...) with only two particles, which is a generalization of Cabello‘s study [Phys. Rev. A 67 (2003)032107].     

Disentanglement of atom-photon via quantum interference in driven three-level atoms

In this paper, the effect of quantum interference on the entanglement of a driven V-type three-level atom and its spontaneous emission field was investigated by using the quantum entropy. The results indicate that, in the absence of quantum interference the atom and its spontaneous emission field are always entangled at the steady-state. But, in the presence of full quantum interference their steady-state entanglement depends on the atomic parameters. Specifically, with appropriate atomic parameters they can be entangled or disentangled at the steady-state. We realized that the steady-state entanglement is due to completely destructive nature of quantum interference. On the contrary, the steady-state disentanglement is due to instructive nature of quantum interference.     

Hawking–Unruh effect and the entanglement of two-mode squeezed states in Riemannian space–time

We consider the system of free scalar field, which is assumed to be a two-mode squeezed state from an inertial point of view. This setting allows the use of entanglement measure for continuous variables, which can be applied to discuss free and bound entanglement from the point of view from non-inertial observer.     

Effective field theory for dilute fermions with pairing

Effective field theory (EFT) methods for a uniform system of fermions with short-range, natural interactions are extended to include pairing correlations, as part of a program to develop a systematic Kohn-Sham density functional theory (DFT) for medium and heavy nuclei. An effective action formalism for local composite operators leads to a free-energy functional that includes pairing by applying an inversion method order by order in the EFT expansion. A consistent renormalization scheme is demonstrated for the uniform system through next-to-leading order, which includes induced-interaction corrections to pairing.     

Critical properties of entanglement in the Dicke model with the dipole-dipole interactions

In this paper, we investigate two aspects of entanglement properties of the ground state for the Dicke model with the dipole-dipole interaction between the atoms in the thermodynamic limit, and observe how they are affected by the quantum phase transition. The appearance of dipole-dipole interactions between the atoms does not change the maximum atom-field entanglement at the critical point, while it changes the maximum atom-atom entanglement at the critical point, and has an important influence on the atom-atom entanglement behavior.     

Entanglement and sudden death of two two-level atoms interacting with a cavity field in presence of degenerate parametric amplifier

In the presence of degenerate two-photon transitions the problem of the interaction between two two-level atoms and a single-mode is considered. Near resonance case, a closed form of the analytic solution for the wave function is obtained. The entanglement between an atom and field in the interacting system is studied by using the change in atomic and field entropies. The relationship between entropy changes and concurrence entanglement is discussed. Our results show that the behavior of the entropy change in agreement with the behavior of the concurrence to measure the entanglement between two subsystem structures.     

Preparation of Cluster States with Trapped Ions in Thermal Motion

A potential scheme is proposed for generating cluster states of many trapped ions in thermal motion, in which the effective Hamiltonian does not involve the external degree of freedom and thus the scheme is insensitive to the external state, allowing it to be thermal state. The required experimental techniques of the schemes are within the scope that can be obtained in the ion-trap setup.     

ABCD rule involved in the product of general SU(1,1) squeezing operators and quantum states’ Fresnel transformation

Base on our preceding paper [H.Y. Fan, H.L. Lu, Opt. Commun. 258 (2006) 51] we find that there exists ABCD rule involved in the product of general SU(1,1) squeezing operators, which can be considered as the quantum optics countpart of the ABCD rule in classical optics. Both one- and two-mode cases are considered, especially, the ABCD theorem embodied in the Fresnel transformation of two-mode number state is exhibited.     

Spin bath decoherence of quantum entanglement

We study an analytically solvable model for decoherence of a two spin system embedded in a large spin environment. As a measure of entanglement, we evaluate the concurrence for the Bell states (Einstein-Podolsky-Rosen pairs). We find that while for two separate spin baths all four Bell states lose their coherence with the same time dependence, for a common spin bath, two of the states decay faster than the others. We explain this difference by the relative orientation of the individual spins in the pair. We also examine how the Bell inequality is violated in the coherent regime. Both for one bath and two bath cases, we find that while two of the Bell states always obey the inequality, the other two violate the inequality at early times.     

Variance squeezing and information entropy squeezing via Bloch coherent states in two-level nonlinear spin models

The nonclassical squeezing effect emerging from a nonlinear coupling model (generalized Jaynes–Cummings model) of a two-level atom interacting resonantly with a bimodal cavity field via two-photon transitions is investigated in the rotating wave approximation. Various Bloch coherent initial states (rotated states) for the atomic system are assumed, i.e., (i) ground state, (ii) excited state, and (iii) linear superposition of both states. Initially, the atomic system and the field are in a disentangled state, where the field modes are in Glauber coherent states via Poisson distribution. The model is numerically tested against simulations of time evolution of the based Heisenberg uncertainty relation variance and Shannon information entropy squeezing factors. The quantum state purity is computed for the three possible initial states and used as a criterion to get information about the entanglement of the components of the system. Analytical expression of the total density operator matrix elements at t > 0 shows, in fact, the present nonlinear model to be strongly entangled, where each of the definite initial Bloch coherent states is reduced to statistical mixtures. Thus, the present model does not preserve the modulus of the Bloch vector.     

Local Modulation of Double Electromagnetically Induced Transparency Spectrum

The double electromagnetically induced transparency induced by two coupling fields can be realized in a fourlevel tripod-type atom. Such double transparency spectra can be locally modulated by using the weak coherent fields to perturb the coupling transitions. These investigations within this scheme can be independent of Doppler broadening by properly orienting these fieds.