Quantum entanglement between the two-mode fields and atomic entropy squeezing in the system of a moving atom interacting with two-mode entangled coherent field

This paper investigates the entropy squeezing of a moving two-level atom interacting with the two-mode entangled coherent field via two-photon transition by using an entropic uncertainty relation and the degree of entanglement between the two-mode fields by using quantum relative entropy.The results obtained from numerical calculation indicate that the squeezed period,the duration of entropy squeezing and the maximal squeezing can be controlled by appropriately choosing the intensity of the light field,the atomic motion and the field-mode structure.The atomic motion leads to the periodic recovery of the initial maximal degree of entanglement between the two-mode fields.Moreover,there exists a corresponding relation between the time evolution properties of the atomic entropy squeezing and those of the entanglement between the two-mode fields.

Entanglement Entropy Signature of Quantum Phase Transitions in a Multiple Spin Interactions Model

Through the Jordan-Wigner transformation,the entanglement entropy and ground state phase diagrams of exactly solvable spin model with alternating and multiple spin exchange interactions are investigated by means of Green's function theory.In the absence of four-spin interactions,the ground state presents plentiful quantum phases due to the multiple spin interactions and magnetic fields.It is shown that the two-site entanglement entropy is a good indicator of quantum phase transition (QPT).In addition,the alternating interactions can destroy the magnetization plateau and wash out the spin-gap of low-lying excitations.However,in the presence of four-spin interactions,apart from the second order QPTs,the system manifests the first order QPT at the tricritical point and an additional new phase called "spin waves",which is due to the collapse of the continuous tower-like low-lying excitations modulated by the four-spin interactions for large three-spin couplings.

Phase-controlled atom-photon entanglement in a three-level Λ-type closed-loop atomic system

We study the entanglement of dressed atom and its spontaneous emission in a three-level Λ-type closed-loop atomic system in a multi-photon resonance condition and beyond it.It is shown that the von Neumann entropy in such a system is phase-dependent,and it can be controlled by either the intensity or relative phase of applied fields.It is demonstrated that for the special case of the Rabi frequency of applied fields,the system is disentangled.In addition,we take into account the effect of Doppler broadening on the entanglement and it is found that a suitable choice of laser propagation direction allows us to obtain the steady state degree of entanglement(DEM) even in the presence of the Doppler effect.

The Scaling of Entanglement Entropy for One Spatial XXZ Spin Chain

We investigate the scaling of entanglement entropy for one spatial XXZ spin chain by using matrix product states to approximate ground states. The entanglement entropy scales logarithmically with a coefficient that is determined by the associated conformal field theory, the quantum phase transitions occurred between Large-D and Halde phase, Halde phase and Neel phase. The scaling relationship is given in this paper.

Symmetry and size effects on energy and entanglement of an exciton in coupled quantum dots

We study theoretically the essential properties of an exciton in vertically coupled Gaussian quantum dots in the presence of an external magnetic field. The ground state energy of a heavy-hole exciton is split into four energy levels due to the Zeeman effect. For the symmetrical system, the entanglement entropy of the exciton state can reach a value of 1. However, for a system with broken symmetry, it is close to zero. Our results are in good agreement with previous studies.

Calculation of Entanglement Entropy for Continuous-Variable Entangled State Based on General Two-Mode Boson Exponential Quadratic Operator in Fock Space

<Abstract>We obtain an explicit formula to calculate the entanglement entropy of bipartite entangled state of general two-mode boson exponential quadratic operator with continuous variables in Fock space.The simplicity and generality of our formula are shown by some examples.

Structure formation of entanglement entropy in a system of two superconducting qubits coupled with an LC-resonator

This paper investigates theoretically the evolutions of the entanglement entropy of a system of two coupled-charge-qubits interacting with an LC-resonator.It is found that when the initial states of the two qubits are prepared in a given superposition excited state,the evolution of the von Neumann entropy of the system depends significantly on the coupling strength between the two Josephson charge qubits.With the variation of the coupling strength,the evolution of the entanglement entropy of the system forms some structures,especially the periodically bistable properties,which are the first discovered for such a system to our knowledge.It is found that the relative entropy entanglement of the system is also sensitive to the variation of the coupling strength between the two charge qubits,some novel 'collective oscillations' of the relative entropy are found for the system.

Evolution of the field quantum entropy and entanglement in a system of multimode light field interacting resonantly with a two-level atom through N_j-degenerate N~Σ-photon process

The time evolution of the field quantum entropy and entanglement in a system of multi-mode coherent light field resonantly interacting with a two-level atom by de-generating the multi-photon process is studied by utilizing the Von Neumann re-duced entropy theory,and the analytical expressions of the quantum entropy of the multimode field and the numerical calculation results for three-mode field inter-acting with the atom are obtained. Our attention focuses on the discussion of the influences of the initial average photon number,the atomic distribution angle and the phase angle of the atom dipole on the evolution of the quantum field entropy and entanglement. The results obtained from the numerical calculation indicate that: the stronger the quantum field is,the weaker the entanglement between the quan-tum field and the atom will be,and when the field is strong enough,the two sub-systems may be in a disentangled state all the time; the quantum field entropy is strongly dependent on the atomic distribution angle,namely,the quantum field and the two-level atom are always in the entangled state,and are nearly stable at maximum entanglement after a short time of vibration; the larger the atomic dis-tribution angle is,the shorter the time for the field quantum entropy to evolve its maximum value is; the phase angles of the atom dipole almost have no influences on the entanglement between the quantum field and the two-level atom. Entangled states or pure states based on these properties of the field quantum entropy can be prepared.

Revised Geometric Measure of Entanglement for Multipartite and Continuous Variable Systems

基于我们建议的纠纷(RGME ) 的修订几何措施[J。Phys。A：数学。Theor。40 (2007 ) 3507 ] ，我们面对噪音和二模式的挤压的热状态包括 three-qubit GHZ 状态， W 状态，和概括 Smolin 状态(GSS ) 获得多国参加的状态的 RGME。而且，我们把他们的 RGME 与纠纷(GME ) 和纠纷(RE ) 的相对的熵的几何措施作比较。结果显示 RGME 是纠纷的一项适当措施。最后，我们定义是一个纠缠的单调的 Gaussian GME。

Entanglement and Energy for Vibrationally Localized States in Molecule CS2

von Neumann 熵描述的量纠纷的动力学为在分子 CS2 联合颤动的费密回声的局部性的状态被学习，在在拉长和弯曲模式之间的交往的精力被认为建立在纠纷和精力之间的一个连接的地方。纠纷为段局部性的状态与交往的精力揭示主导的反关联，这被显示出，当那为拐弯局部性的状态展出主导地积极的关联时。为 dislocalized 状态的纠纷和精力也被讨论。那些处于高维的状态为分子的量计算和量信息是有用的。

Entanglement Entropy of Reissner–Nordstrm Black Hole and Quantum Isolated Horizon

Based on the work of Ghosh and Pereze, who view the black hole entropy as the logarithm of the number of quantum states on the Quantum Isolated Horizon(QIH)§ the entropy of Reissner–Nordstr¨om black hole is studied.According to the Unruh temperature, the statistical entropy of quantum fields under the background of Reissner–Nordstr¨om spacetime is calculated by means of quantum statistics. In the calculations we take the integral from the position of QIH to infinity, so the obtained entropy is the entanglement entropy outside the QIH. In Reissner–Nordstr¨om spacetime it is shown that if only the position of QIH is properly chosen the leading term of logarithm of the number of quantum states on the QIH is equal to the leading term of the entanglement entropy outside the black hole horizon, and both are the Bekenstein–Hawking entropy. The results reveal the relation between the entanglement entropy of black hole and the logarithm of the number of quantum states.

Quantum phase transition and entanglement in Li atom system

By use of the exact diagonalization method, the quantum phase transition and en- tanglement in a 6-Li atom system are studied. It is found that entanglement appears before the quantum phase transition and disappears after it in this exactly solvable quantum system. The present results show that the von Neumann entropy, as a measure of entanglement, may reveal the quantum phase transition in this model.

Entanglement of linear cluster states in terms of averaged entropies

An entanglement measure,multiple entropy measures(MEMS) was proposed recently by using the geometric mean of partial entropies over all possible i-body combinations of the quantum system.In this work,we study the average subsystem von Neumann entropies of the linear cluster state and investigated the quantum entanglement of linear cluster states in terms of MEMS.Explicit results with specific particle numbers are calculated,and some analytical results are given for systems with arbitrary particle numbers.Compared with other example quantum states such as the GHZ states and W states,the linear cluster states are "more entangled" in terms of MEMS,namely their averaged entropies are larger than the GHZ states and W states.

Quantum entanglement of the binomial field interacting with a cascade three-level atom beyond the rotating wave approximation

In this paper, the quantum entanglement between a single mode binomial field and a cascade three-level atom is calculated mechanically without the rotating wave approximation. The numerical results indicate that the quantum entanglement at the first few periods is reduced notably due to the fact that the atom is initially in the superposition state. With increasing field parameter , the period of the entanglement evolution becomes obvious and the quantum decoherence phenomenon emerges in a short time.

Quantum Entropy of a Single Cooper-Pair Box Interacting with Two Electromagnetic Fields

A Hamiltonian which represents the interaction between a single Cooper-pair box and two quantized electromagnetic fields is considered in order to find new ways for quantum information. The wave function in Schrdinger picture is obtained. The evolution of the entropy of the box as a function of the scaled time is ploted to measure the entanglement between the box and the fields. It is found that the entanglement is sensitive to the detuning between the Josephson energy and the fields frequency, increasing the detuning can decrease the entanglement.

Constructing Entanglers in 2-Players–N-Strategies Quantum Game

In quantum games based on 2-player-N-strategies classical games, each player has a quNit (a normalized vector in an N-dimensional Hilbert space HN) upon which he applies his strategy (a matrix U∈SU(N)). The players draw their payoffs from a state . Here ?and J (both determined by the game’s referee) are respectively an unentangled 2-quNit (pure) state and a unitary operator such that ?is partially entangled. The existence of pure strategy Nash equilibrium in the quantum game is intimately related to the degree of entanglement of . Hence, it is practical to design the entangler J= J(β) to be dependent on a single real parameter β that controls the degree of entanglement of , such that its von-Neumann entropy SN(β) is continuous and obtains any value in . Designing J(β) for N=2 is quite standard. Extension to N>2 is not obvious, and here we suggest an algorithm to achieve it. Such construction provides a special quantum gate that should be a useful tool not only in quantum games but, more generally, as a special gate in manipulating quantum information protocols.

Quantum Entropy of a Cooper-Pair Box in a Single-Mode Cavity Field

A Cooper-pair box biased by a classical voltage and also irradiated by a squeezed state field is considered in order to find new ways to quantum communication and calculation. The quantum dynamics of the Cooper-pair box and the entanglement which is the core theoretics of quantum communication and calculation is investigated in this system, which is related to the squeezing parameter of the squeezed state. A model of Hamiltonian which represents the interaction between box and quantum field is introduced. Finally, the relationship between the entanglement and the squeezing parameter of the squeezed state is demonstrated.