High Degree Of Entanglement Research Articles

Application of impedance spectroscopy to evaluate the effect of different setting accelerators on the developed microstructures of calcium phosphate cements

The main goal of the present study was to evaluate the effect of different setting accelerator agents on the developed microstructures of calcium phosphate cements (CPCs) by employing the impedance spectroscopy (IS) technique. Six compositions of CPCs were prepared from mixtures of commercial dicalcium phosphate anhydrous (DCPA) and synthesized tetracalcium phosphate (TTCP) as the solid phases. Two TTCP/DCPA molar ratios (1/1 and 1/2) and three liquid phases (aqueous solutions of Na(2)HPO(4), tartaric acid (TA) and oxalic acid (OA), 5% volume fraction) were employed. Initial (I) and final (F) setting times of the cement pastes were determined with Gillmore needles (ASTM standard C266-99). The hardened samples were characterized by X-ray powder diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and apparent density measurements. The IS technique was employed as a non-destructive tool to obtain information related to porosity, tortuosity and homogeneity of the cement microstructures. The formulation prepared from a TTCP/DCPA equimolar mixture and OA as the liquid phase presented the shortest I and F (12 and 20 min, respectively) in comparison to the other studied systems. XRD analyses revealed the formation of low-crystallinity hydroxyapatite (HA) (as the main phase) as well as the presence of little amounts of unreacted DCPA and TTCP after 24 h hardening in 100% relative humidity. This was related to the proposed mechanisms of dissolution of the reactants. The bands observed by FTIR allowed identifying the presence of calcium tartrate and calcium oxalate in the samples prepared from TA and OA, in addition to the characteristic bands of HA. High degree of entanglement of the formed crystals was observed by SEM in samples containing OA. SEM images were also correlated to the apparent densities of the hardened cements. Changes in porosity, tortuosity and microstructural homogeneity were determined in all samples, from IS results, when the TTCP/DCPA ratio was changed from 1/1 to 1/2. The cement formulated from an equimolar mixture of TTCP/DCPA and OA as the liquid phase presented setting times, degree of conversion to low-crystallinity HA and microstructural features suitable to be used as potential bone cement in clinical applications. The IS technique was shown to be a very sensitive and non-destructive tool to relate the paste composition to the developed microstructures. This approach could be very useful to develop calcium phosphate bone cements for specific clinical demands.

Cavity-assisted generation of entangled photons from a V-type three-level system

We propose a new type of entangled-photon generation from a V-type system in a microcavity. In contrast to the entangled-photon generation until now, all four Bell states can be freely generated from an identical cavity system by simply selecting applied-field polarizations and frequencies due to the excitation of dressed states in cavity quantum electrodynamics (CQED). CQED effects play a crucial role in providing a high degree of entanglement: (i) spectral filtering can be used to extract entangled photons due to the vacuum Rabi splitting and (ii) non-entangled co-polarized photons are strongly suppressed due to the photon blockade effect. The proposed system efficiently generates a high degree of entangled photons by moderately increasing the applied-field intensity.

Broadening the bandwidth of entangled photons: A step towards the generation of extremely short biphotons

We demonstrate a technique that allows full control of the bandwidth of entangled photons independently of the frequency band of interest and of the nonlinear crystal. We theoretically show that this technique allows generating nearly transform-limited biphotons with almost one octave of bandwidth hundreds of terahertz which corresponds to correlation times of just a few femtoseconds. The presented method becomes an enabling tool for attosecond entangled-photons quantum optics. The technique can also be used to generate paired photons with a very high degree of entanglement.

Entangled photons from quantum dot devices: efficiency of post‐selection

AbstractWe theoretically investigate the production of polarization‐entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. A biexciton radiatively decays through two intermediate exciton states, where polarization‐entangled photons are emitted if the two decay paths differ in polarization but are indistinguishable otherwise. This ideal performance is usually spoiled by the electron–hole exchange interaction splitting the intermediate exciton states by a small amount and consequently attaching a which‐path information to the photon frequencies. We discuss post‐selection schemes to mask this which‐path information to an outside observer. We show how spectral filtering and time shifts at a single photon level affect the photon state. Here the solid state environment plays a crucial role in the effective measurement of intermediate exciton states. Evaluating our analytical results with realistic quantum dot parameters we quantify the applicability of suggested protocols for solid‐state based quantum cryptography. Our results indicate, that a high degree of entanglement is only reached by spectral alignment of the exciton states. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamics of Bloch vectors and channel capacity of two non-identical charge qubits

We have considered a system of two superconducting charge qubits capacitively coupled to a microwave resonator. By means of the Bloch vectors and cross dyadic we quantify the degree of entanglement contained in the generated entangled state in different regimes. We show the important roles played by initial state settings, coupling constant and the mean photon number on generating entangled states with high degree of entanglement and high capacity.

Entangled‐photon generation from a quantum dot in cavity QED

AbstractWe theoretically study polarization‐entangled photon generation from a single quantum dot in a microcavity. Entangled‐photon pairs with singlet or triplet Bell states are generated in the resonant‐hyperparametric scattering via dressed states in the cavity QED.Although co‐polarized non‐entangled photons are also generated, the generation is dramatically suppressed in the strong‐coupling limit owing to the photon blockade effect. Finite binding energy of biexciton is also important for the generation of photon pairs with high degree of entanglement. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Detection of two-mode compression and degree of entanglement in continuous variables in parametric scattering of light

Generation of “twin beams” (of light with two-mode compression) in single-pass optical parametric amplifier (a crystal with a nonzero quadratic susceptibility) is considered. Radiation at the output of the nonlinear crystal is essentially multimode, which raises the question about the effect of the detection volume on the extent of suppression of noise from the difference photocurrent of the detectors. In addition, the longitudinal as well as transverse size of the region in which parametric transformation takes place is of fundamental importance. It is shown that maximal suppression of noise from difference photocurrent requires a high degree of entanglement of two-photon light at the outlet of the parametric amplifier, which is defined by Federov et al. [Phys. Rev. A 77, 032336 (2008)] as the ratio of the intensity distribution width to the correlation function width. The detection volume should be chosen taking into account both these quantities. Various modes of single-pass generation of twin beams (noncollinear frequency-degenerate and collinear frequency-nondegenerate synchronism of type I, as well as collinear frequency-degenerate synchronism of type II) are considered in connection with the degree of entanglement.

Entangled photon sources based on semiconductor quantum dots: The role of pure dephasing

We theoretically investigate the production of polarization-entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. To accomplish a high degree of entanglement, despite the exciton fine-structure splitting, one must either energetically align the two exciton states by means of external fields or erase the which-path information by postselection or time reordering of photons. Here we show that in the latter schemes the photon state becomes deteriorated through dephasing processes in the solid, and the degree of entanglement remains low.

Entanglement swapping between spacelike-separated atoms

We show a mechanism that projects a pair of neutral two-level atoms from an initially uncorrelated state to a maximally entangled state while they remain spacelike separated. The atoms begin both excited in a common electromagnetic vacuum, and the radiation is collected with a partial Bell-state analyzer. If the interaction time is short enough and a certain two-photon Bell state is detected after the interaction, a high degree of entanglement, even maximal, can be generated while one atom is outside the light cone of the other, for arbitrary large interatomic distances.

Spontaneous parametric down-conversion: Anisotropical and anomalously strong narrowing of biphoton momentum correlation distributions

We show that the wave packet of a biphoton generated via spontaneous parametric down-conversion is strongly anisotropic. Its anisotropic features manifest themselves very clearly in comparison with measurements performed in two different schemes: When the detector scanning plane is perpendicular or parallel to the plane containing the crystal optical axis and the laser axis. The first of these two schemes is traditional whereas the second one gives rise to such unexpected results, such as anomalously strong narrowing of the biphoton wave packet measured in the coincidence scheme and very high degree of entanglement. The results are predicted theoretically and confirmed experimentally.

Entanglement and Pancharatnam Phase for Two Two-Level Atoms Interacting with a Single Mode Field

A model of two 2-level atoms interacts with a single quantized electromagnetic field is considered. We study the effect of the mean photon number and the structure of the initial states of the two atoms on the dynamics of the atomic system from the separability point of view. It is found that, if we start from a product mixed atomic state, the probability of generating long living entangled states is increased as the mean photon number increases. Starting from excited atomic system in product state, one generates a more stable entangled states with high degree of entanglement. Also, the effect of the mean photon number on atomic system prepared initially in entangled states is investigated. It is found that the entangled state generated from the initially partial entangled states are more robust than those obtained from a maximum entangled state. The Pancharatnam phase for the separable and entangled states is studied under the effect of the mean photon number and the structure of the initial state. We find that for the separable states, the collapses decrease and the amplitude of the revivals is smaller than that for the entangled state, so there are long-living entangled phases. This property give us a great chances to store safely information in entangled state.

Stabilizing entanglement by quantum-jump-based feedback

We show that direct feedback based on quantum-jump detection can be used to generate entangled steady states. We present a strategy that is insensitive to detection inefficiencies and robust against errors in the control Hamiltonian. This feedback procedure is also shown to overcome spontaneous emission effects by stabilizing states with a high degree of entanglement.

An atom and a photon

The coherent control of single-photon emitters as, e.g., single ions or atoms, is a crucial element for mapping quantum information between light and matter. The possibility of generating entanglement between a photon and the emitter system provides an interface between matter-based quantum memories and photonic quantum communication channels, which is the essential resource for quantum repeaters and other future quantum information applications. To generate entangled atom-photon states, in our experiment, we store a single 87Rb atom in an optical dipole trap. The single-atom/single-photon character is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom allowed us to determine the mean kinetic energy of the atom corresponding to 105 μK. We describe a single-atom state analysis method which allowed us to characterize the entanglement between the atom and a single photon emitted in the spontaneous decay. We obtain an entanglement fidelity of 89% that clearly shows the high degree of entanglement in our system and potential for further applications in quantum communication.

Davies theory for reservoir-induced entanglement in a bipartite system

Two mutually noninteracting qubits with identical modest coupling to one and the same reservoir are considered. For a given Hamiltonian and uncorrelated initial state, the mathematically rigorous Davies theory of the weak-coupling and van Hove limit provides a unique Markovian quantum master equation where absolutely none of the usually made additional assumptions and further approximations are introduced. Due to completely positive time evolution also no artificial correlations can arise. Numerical solution of the Markovian master equation shows that the qubits become entangled. In a first short time-interval containing one single maximum of entanglement for reservoir temperature T = 0, different choices of uncorrelated initial states give rise to a remarkable emergence of entanglement of different degree. The quantitative evaluation is analysed in terms of a measure derived from Wootters concurrence. Selected results show that there are even states that acquire the possible maximum. Particularly those states will show a periodic type of ‘collapse and revival’ behaviour with exponentially decaying envelope at longer times. This has never been reported so far for noninteracting qubits as mediated by simultaneous coupling to an uncontrollable reservoir. Moreover, even selected uncorrelated mixed states of modest degree of mixture may show a similar behaviour, although less pronounced. For T > 0 states with high degree of entanglement at T = 0 in the first time-interval still show a gradually reduced value up to a few tenth of Kelvin but for T ⩾ 33 K no effects can be observed. Finally, initially entangled states will slowly lose their oscillatory degree, again with exponential envelope, as the bipartite system approaches its stationary final state.

Quantum entanglement in the SU(1, 1)-related coherent fields interacting with a moving atom

The entanglement properties of the SU(1, 1)-related coherent fields interacting with a moving atom via the two-photon transition are investigated. We discuss the quantum entanglement between the SU(1, 1)-related coherent fields and the moving atom by using the quantum reduced entropy and that between the SU(1, 1)-related coherent fields by using the quantum relative entropy. It is shown that we can obtain the maximal entangled state between the fields and the moving atom and the high degree of entanglement between the SU(1, 1)-related coherent fields can be preserved for some choiced system parameters.

Structure and Properties of Elastomeric Polypropylene from C2 and C2v-Symmetric Zirconocenes. The Origin of Crystallinity and Elastic Properties in Poorly Isotactic Polypropylene

A comparative analysis of the structure and mechanical properties of two elastomeric amorphous and semicrystalline polypropylenes samples, produced with C2v-symmetric and chiral C2-symmetric ansa-zirconocene catalysts, respectively, is presented. The poorly isotactic polypropylene (iam-PP) of medium molecular weight, prepared with the chiral C2-symmetric catalyst, slowly crystallizes, by aging at room temperature or by stretching, in a continuum of disordered modifications intermediate between α and γ forms of isotactic polypropylene. Structural and morphological transformations occur during stretching. Disordered α/γ modifications, containing a high fraction of perpendicular chains, present in the unoriented film, transform by stretching at high deformations into structures more similar to the α form with a high content of parallel chains. The development of the small level of crystallinity induces elastic properties. The small crystalline domains in the amorphous matrix, indeed, act as physical knots of the elastomeric lattice. This poorly isotactic iam-PP sample shows, instead, poor elastic properties and viscous flow at high deformations in the amorphous state, before crystallization, because of the not high molecular weight. The unique crystallization mode of the iam-PP sample explains the remarkable enhancement of elastic properties in low stereoregular polypropylene upon development of crystallinity. The fully amorphous high molecular weight atactic polypropylene (a-PP), prepared with the C2v-symmetric catalyst, presents elastic behavior only in a small deformation range (up to 300% strain) and experiences viscous flow of the chains at high deformations. The elasticity arises from the physical network generated by the high degree of entanglement of the polymeric chains generated by the high molecular weight of the sample. The poorly isotactic iam-PP sample, before crystallization, presents much poorer elastic properties than the a-PP sample because of the lower molecular weight. After crystallization, it shows better elastic properties in a large range of deformation with higher strength, notwithstanding the lower molecular weight, due to the presence of crystallinity.

Dynamic mechanical behavior of ultra-high molecular weight polyethylene irradiated with gamma rays

We have investigated the dynamic mechanical behavior of ultra-high molecular weight polyethylene (UHMWPE) irradiated with varying doses of gamma rays. A relaxation peak in the loss factor curve, which has not been reported previously in the literature, is observed at a temperature above the crystal melting temperature. The peak is unique to UHMWPE and appears to be related to the high degree of entanglement. Because the temperature and intensity of the peak are reduced by irradiation-induced chain scission and crosslinking, respectively, we believe that the peak is associated with disentanglement relaxation. The behavior of the storage modulus in the melt state agrees with the classical theory of rubber elasticity.

Experimental extraction of an entangled photon pair from two identically decohered pairs.

Entanglement is considered to be one of the most important resources in quantum information processing schemes, including teleportation, dense coding and entanglement-based quantum key distribution. Because entanglement cannot be generated by classical communication between distant parties, distribution of entangled particles between them is necessary. During the distribution process, entanglement between the particles is degraded by the decoherence and dissipation processes that result from unavoidable coupling with the environment. Entanglement distillation and concentration schemes are therefore needed to extract pairs with a higher degree of entanglement from these less-entangled pairs; this is accomplished using local operations and classical communication. Here we report an experimental demonstration of extraction of a polarization-entangled photon pair from two decohered photon pairs. Two polarization-entangled photon pairs are generated by spontaneous parametric down-conversion and then distributed through a channel that induces identical phase fluctuations to both pairs; this ensures that no entanglement is available as long as each pair is manipulated individually. Then, through collective local operations and classical communication we extract from the two decohered pairs a photon pair that is observed to be polarization-entangled.

Orientation of plane strain-compressed ultra-high-molecular-weight polyethylene

Ultra-high-molecular-weight polyethylene (UHMWPE) samples of molecular weight 6 million were compressed using a channel die and characterized by wide-angle and small-angle X-ray scattering. The results were compared to previous studies performed on high-density polyethylene (HDPE) under the same compression conditions. Most deformation mechanisms exhibited by UHMWPE were also observed in HDPE. However, the onset of crystallographic and morphological orientation occurred at lower compression ratios in UHMWPE than in HDPE. Greater resistance to deformation and more extensive strain recovery were also exhibited by UHMWPE. These differences in deformation behaviour of UHMWPE and HDPE were attributed to the larger number of tie molecules and higher degree of entanglement within the amorphous regions of UHMWPE—a direct effect of its very high molecular weight. Our results clearly demonstrate that crystallographic texturing in UHMWPE, especially molecular orientation along the flow direction, can be controlled by channel die compression. This renders the method suitable for production of oriented samples for future studies of possible relationships between crystallographic texture and wear behaviour.

The formation of nodular structures in the top layer of ultrafiltration membranes

AbstractThe formation of nodular structures in the top layer of ultrafiltration membranes is considered. A critical review of mechanisms described in the literature is given. Flat‐sheet poly(ether sulfone) membranes and hollow‐fiber poly(ether sulfone)/polyvinylpyrrolidone membranes were made by coagulation of a polymer solution in a nonsolvent medium under different circumstances. From these experiments, a number of empirical rules are found to describe the resulting morphology of the top layer. A new mechanism for the formation of a nodular structure is proposed. It is based on the small diffusion coefficient of the polymer molecules compared to the diffusion coefficient of solvent and nonsolvent combined with a high degree of entanglement of the polymer network. For unstable compositions, phase separation will proceed by growth in amplitude of concentration fluctuations. The rapid diffusional exchange of solvent for nonsolvent in the top layer leads to vitrification of the maxima of the concentration fluctuations that form the nodules. Complete disentanglement of the polymer chains between the nodules is not reached, which explains the small pores and the low porosity of ultrafiltration membranes. © 1994 John Wiley & Sons, Inc.