Fidelity of Quantum Teleportation for Single-Mode Squeezed State Light

The fidelity of quantum teleportation of a single-mode squeezed state of light is calculated based on the general theory of quantum-mechanical measurement in the Schrodinger picture. It is shown that the criterion for the nonclassical state teleportation is different from that for coherent state. F = 1/2 is no longer the rigorous boundary between classical and quantum teleportation for a squeezed state of light. When the quantum entanglement of an Einstein-Podolsky-Rosen （EPR） beam used for teleportation and the parameters of the system are given, the fidelity depends on the squeezing of the input squeezed state. The higher the squeezing is, the smaller the fidelity is, and the lower the classical limitation of fidelity is. The dependence of the optimum gain for teleporting a squeezed vacuum state upon the EPR entanglement is also calculated. The results obtained provide important references for designing experimental systems of teleporting a non-classical state and judging the quality of the teleported quantum state.     

The dependence of fidelity on the squeezing parameter in teleportation of the squeezed coherent states

This paper investigates an analytical expression of teleportation fidelity in the teleportation scheme of a single mode of electromagnetic field.The fidelity between the original squeezed coherent state and the teleported one is expressed in terms of the squeezing parameter r and the quantum channel parameter(two-mode squeezed state) p.The results of analysis show that the fidelity increases with the increase of the quantum channel parameter p,while the fidelity decreases with the increase of the squeezing parameter r of the squeezed state.Thus the coherent state(r = 0) is the best quantum signal for continuous variable quantum teleportation once the quantum channel is built.     

Entanglement swapping of continuous variable using squeezed vacuum states

We present a realistic scheme for the entanglement swapping of continuous variable, in which a two-mode squeezed vacuum state serves as a quantum channel. The position sum and momentum difference of two local modes are measured. By taking the input entangled state also as a two-mode squeezed vacuum state, we investigate the average fidelity and the von Neumann entropy of the output state. The results show that the perfect teleportation can be achieved by increasing the squeezing of the quantum channel and that any nonzero squeezing in both the quantum channel and the input entangled state is sufficient to swap the entanglement.     

Quantum Mechanical Nature in Liquid NMR Quantum Computing

The quantum nature of bulk ensemble NMR quantum computing-the center of recent heated debate,is addressed.Concepts of the mixed state and entanglement are examined,and the data in a two-qubit liquid NMR quantum computation are analyzed.the main points in this paper are;i) Density matrix describes the “state“ of an average particle in an ensemble.It does not describe the state of an individual particle in an ensemble;ii) Entanglement is a property of the wave function of a microscopic particle(such as a molecule in a liquid NMR sample),and separability of the density matrix canot be used to measure the entanglement of mixed ensemble;iii) The state evolution in bulkensemble NMR quantum computation is quantum-mechanical;iv) The coefficient before the effective pure state density matrix,ε,is a measure of the simultaneity of the molecules in an ensemble,It reflets the intensity of the NMR signal and has no significance in quantifying the entanglement in the bulk ensemble NMR system.The decomposition of the density matrix into product states is only an indication that the ensemble can be prepared by an ensemble with the particles unentangeld.We conclude that effective-pure-state NMR quantum computation is genuine,not just classical simulations.     

Squeezing in a three-level Jaynes-Cummings model with strong field

Squeezing in three-level Jaynes-Cummings model with the strong initial coherent state is studied numerically. The dependence of the squeezing on the field intensity and time is analysed. The cases that the light field is initially in a Fock state and in a squeezed vacuum state are also discussed.     

An Intuitive Expression for Inseparability Condition of a Two-Mode Squeezed Vacuum State in a Thermal Environment

Considering the propagation of a two-mode optical field that is initially in a squeezed vacuum state in a thermalenvironment, we obtain an intuitive expression for inseparability condition of the two-mode mixed state which isgiven in the coherent state representation. This condition shows that the two modes have quantum entanglementif and only if the coefficient of the correlation term between the two modes is larger than that of the off-diagonalterm of each mode in the density matrix. We find that even if the quantum channel is dynamically coupled to thethermal environment, the fidelity for teleporting coherent states larger than 1/2 is still the criterion for quantum teleportation. We also show that the entanglement, squeezing and quantum teleportation conditions are alwaysconsistent with each other.     

Security of quantum key distribution using two-mode squeezed states against optimal beam splitter attack

For the beam splitter attack strategy against quantum key distribution using two-mode squeezed states, the analytical expression of the optimal beam splitter parameter is provided in this paper by applying the Shannon information theory. The theoretical secret information rate after error correction and privacy amplification is given in terms of the squeezed parameter and channel parameters. The results show that the two-mode squeezed state quantum key distribution is secure against an optimal beam splitter attack.     

Quantum Mechanical Nature in Liquid NMR Quantum Computing

The quantum nature of bulk ensemble NMR quantum computing the center of recent heated debate,is addressed. Concepts of the mixed state and entanglement are examined, and the data in a two-qubit liquid NMRquantum computation are analyzed. The main points in this paper are: i) Density matrix describes the “state“ of anaverage particle in an ensemble. It does not describe the state of an individual particle in an ensemble; ii) Entanglementis a property of the wave function of a microscopic particle (such as a molecule in a liquid NMR sample), and separabilityof the density matrix cannot be used to measure the entanglement of mixed ensemble; iii) The state evolution in bulk-ensemble NMRquantum computation is quantum-mechanical; iv) The coefficient before the effective pure state densitymatrix, e, is a measure of the simultaneity of the molecules in an ensemble. It reflects the intensity of the NMR signaland has no significance in quantifying the entanglement in the bulk ensemble NMR system. The decomposition of thedensity matrix into product states is only an indication that the ensemble can be prepared by an ensemble with theparticles unentangled. We conclude that effective-pure-state NMR quantum computation is genuine, not just classicalsimulations.     

Investigations into quantum correlation of coupled qubits in a squeezed vacuum reservoir

In this paper,we investigate the quantum correlation of coupled qubits which are initially in maximally entangled mixed states in a squeezed vacuum reservoir.We compare and analyze the effects of squeezed parameters on quantum discord and quantum concurrence.The results show that in a squeezed vacuum reservoir,the quantum discord and quantum concurrence perform with completely opposite behaviors with the change of squeezed parameters.Quantum discord survives longer with the increase of squeezed amplitude parameter,but entanglement death is faster on the contrary.The results also indicate that the classical correlation of the system is smaller than quantum discord in a vacuum reservoir,while it is bigger than quantum discord in a squeezed vacuum reservoir.The quantum discord and classical correlation are more robust than quantum concurrence in the two reservoir environments,which indicates that the entanglement actually is easily affected by decoherence and quantum discord has a stronger ability to avoid decoherence in a squeezed vacuum reservoir.     

Role of phase fluctuation and dephasing in the enhancing continuous variable entanglement of a two-photon coherent beat laser

A steady state analysis of the nonclassical features and statistical properties of the cavity radiation of a twophoton coherent beat laser is presented.Results show that the degree of two-mode squeezing,detectable entanglement and intensity of the cavity radiation can increase with the deviation of the phase fluctuations of the laser employed in preparing the atoms,but decrease with the increasing rate at which the induced coherence superposition decays.Although it is found that varying the phase fluctuations and dephasing can lead to modification in the quantum features and statistical properties of the radiation,it does not alter the similarity in the nature of the degree of entanglement detectable by the criteria following from Duan-Giedke-Cirac-Zoller and logarithmic negativity in a perceivable manner.Since the intensity and quantum features can be readily enhanced,this system is expected to be a viable source of a strong robust entangled (squeezed) light under various conditions.Moreover,comparison of the mean number of photon pairs with intensity difference shows that the chance of inciting a two-photon process can be enhanced by changing the rate of dephasing and phase fluctuations.     

A Lower Bound on the Entanglement in the Jaynes-Cummings Model

The entanglement between an atom and field is investigated by using the 3aynes-Cummings model. The initial atomic state is supposed in a mixed state and the field is in a squeezed state. The lower bound on the entanglement quantified by concurrence is calculated. It is found that the entanglement with the atom being initially in a mixed state can be larger than that with the atom being initially in a pure state. The entanglement is not a monotone function of the squeezing parameter r of the field and it achieves the maximum for certain r and then decreases with further increase of r.     

Manipulation of Quantum Nonlocality of the Single-Photon Entangled State via Beam Splitter

Quantum nonlocality of the single-photon entangled state emerging from a beam splitter with arbitrary controlling parameters is investigated by means of the parity measurement. The condition of the maximal violation of the Bell-CHSH inequality is obtained for the single-photon entangled state. Under the condition of the small coherent intensity J(J=0.1) and the phase difference γαβ of the two coherent displacements being opposite to the internal phase shift φ of the beam splitter (γαβ=-φ), we find that the quantum nonlocality depends only on the reflection coefficient R of the beam splitter, while φ plays an important role for selecting γαβ. The strongest quantum nonlocality can be observed for a 50:50 beam splitter. If R deviates from the intermediate value 0.5, the quantum nonlocality decreases gradually, even vanishes. Our results provide a method to observe and manipulate the quantum nonlocality of the single-photon entangled state by adjusting the parameters of the coherent displacements and the beam splitter.     

Preparation of the four-qubit cluster states in cavity QED and the trapped-ion system

This paper proposes a simple scheme to generate a four-atom entangled cluster state in cavity quantum electrodynamics. With the assistantce of a strong classical field the cavity is only virtually excited and no quantum information will be transferred from the atoms to the cavity during the preparation for a four-atom entangled cluster state, and thus the scheme is insensitive to the cavity field states and cavity decay. Assuming that deviation of laser intensity is 0.01 and that of simultaneity for the interaction is 0.01, it shows that the fidelity of the resulting four-atom entangled cluster state is about 0.9886. The scheme can also be used to generate a four-ion entangled cluster state in a hot trapped-ion system. Assuming that deviation of laser intensity is 0.01, it shows that the fidelity of the resulting four-ion entangled cluster state is about 0.9990. Experimental feasibility for achieving this scheme is also discussed.     

Design of quantum VQ iteration and quantum VQ encoding algorithm taking O（√N） steps for data compression

Vector quantization (VQ) is an important data compression method. The key of the encoding of VQ is to find the closest vector among N vectors for a feature vector. Many classical linear search algorithms take $O(N)$ steps of distance computing between two vectors. The quantum VQ iteration and corresponding quantum VQ encoding algorithm that takes $O(\sqrt N )$ steps are presented in this paper. The unitary operation of distance computing can be performed on a number of vectors simultaneously because the quantum state exists in a superposition of states. The quantum VQ iteration comprises three oracles, by contrast many quantum algorithms have only one oracle, such as Shor's factorization algorithm and Grover's algorithm. Entanglement state is generated and used, by contrast the state in Grover's algorithm is not an entanglement state. The quantum VQ iteration is a rotation over subspace, by contrast the Grover iteration is a rotation over global space. The quantum VQ iteration extends the Grover iteration to the more complex search that requires more oracles. The method of the quantum VQ iteration is universal.     

Characterization of squeezed states with controllable coherent light injection at sidebands

<正>The squeezed state was experimentally produced in the four wave mixing process for the first time thirty years ago[1].Its intrinsic nonclassical property has always attracted the attention of the scientists,and it has also presented an unpredictable application potential in quantum information processing[2-6]and quantum metrology[7-9].For gaining an insight into the quantum state,Bertrand et al.[10]introduced the concept of quantum tomography into quantum mechanics in 1987.And in 1997,Breitenbach et al.[11]presented the noise distribution of the squeezed states of light fields and reconstructed the quantum states by balanced homodyne detection(BHD).If the squeezed state light field has a relatively strong amplitude,BHD is not suitable.Consequently,other approaches have also been studied,such as self-tomography of the twin-beam state[12]and self-     

Formation mechanism of ethanol-water excimer

The fluorescent spectrum and the excitation spectrum were used to present the cluster molecular structure feature in ethanol-water solutions.Through analyzing the fluorescent characteristics of an excimer,it is proposed that the excimers are formed between the ethanol-water cluster molecules in the excited state and in the ground state.The fluorescent lifetime and the fluorescent intensity decay process give information about the photo-physical and photo-chemical processes of the formation and the dissociation of an excimer. The theoretical calculation and physical analysis coincide with the experimental results.The preliminary conclusion about the structure feature of ethanol-water cluster molecule is that it has a planar one like a sandwich.     

Role of Interactions in Electronic Structure of a Two-Electron Quantum Dot Molecule

We have studied a two-electron quantum dot molecule in a magnetic field. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate two lowest energy levels of the two-electron quantum dot molecule in a magnetic field. Our results show that the electron interactions are significant, as they can change the total spin of the two-electron ground state of the system by adjusting the magnetic field between S = 0 and S = 1. The energy difference AE between the lowest S = 0 and S = 1 states is shown as a function of the axial magnetic field. We found that the energy difference between the lowest S = 0 and S = 1 states in the strong-B S = 0 state varies linearly. Our results provide a possible realization for a qubit to be fabricated by current growth techniques.     

Quantum Effect in a Diode Included Nonlinear Inductance-Capacitance Mesoscopic Circuit

The mesoscopic nonlinear inductance-capacitance circuit is a typical anharmonie oscillator, due to diodes included in the circuit. In this paper, using the advanced quantum theory of mesoseopie circuits, which based on the fundamental fact that the electric charge takes discrete value, the diode included mesoscopic circuit is firstly studied. Schrodinger equation of the system is a four-order difference equation in p rep asentation. Using the extended perturbative method, the detail energy spectrum and wave functions axe obtained and verified, as an application of the results, the current quantum fluctuation in the ground state is calculated. Diode is a basis component in a circuit, its quantization would popularize the quantum theory of mesoscopie circuits. The methods to solve the high order difference equation are helpful to the application of mesoscopic quantum theory.     

A Comparative Study of Quantum and Classical Deletion

Here in this letter, we study the difference between quantum and classical deletion. We point out that the linear mapping deletion operation used in the impossibility proof for quantum systems applies also to classical system. The general classical deletion operation is a combined operation of measurement and transformation, i.e., first read the state and then transfer the state to the standard blank state. Though both quantum information and classical information can be deleted in an open system, quantum information cannot be recovered while classical information can be recovered.     

Teleportation of Squeezed Entangled State

Based on the coherent entangled state ｜a, x〉 we introduce the squeezed entangled state （SES）. Then we propose a teleportation protocol for the SES by using Einstein-Podolsky-Rosen entangled state ｜η〉 as a quantum channel. The calculation is greatly simplified by virtue of the Schmidt decompositions of both ｜a, x〉 and ｜η〉. Any bipartite states that can be expanded in terms of ｜a, x〉 may be teleported in this way due to the completeness of ｜a, x〉.