Degree Of Entanglement Research Articles

Entanglement enhancement from single-port and two-port feedback optical parametric amplifiers

Coherent feedback control has been proved to be an effective approach in entanglement enhancement of the entangled states produced by a nondegenerate optical parametric amplifier (NOPA). However, the present research mainly focuses on a two-port feedback NOPA (TFPA). In this study, a scheme of single-port feedback NOPA (SFPA) is proposed and investigated. For the ideal case, both SFPA and TFPA have the potential to realize an infinite degree of entanglement. For the practical case with feasible physical parameters of realistic systems, the features of entanglement enhancement for two schemes are compared. The effect of entanglement enhancement for SFPA is relatively weaker than that of TFPA, but has a loose phase-locking accuracy requirement. Our results may provide useful references on the control of nonclassical light fields.

Two-photon excitation with finite pulses unlocks pure dephasing-induced degradation of entangled photons emitted by quantum dots

Semiconductor quantum dots have emerged as an especially promising platform for the generation of polarization-entangled photon pairs. However, it was demonstrated recently that the two-photon excitation scheme employed in state-of-the-art experiments limits the achievable degree of entanglement by introducing which-path information. In this work, the combined impact of two-photon excitation and longitudinal acoustic phonons on photon pairs emitted by strongly-confining quantum dots is investigated. It is found that phonons further reduce the achievable degree of entanglement even in the limit of vanishing temperature due to phonon-induced pure dephasing and phonon-assisted one-photon processes, which increase the reexcitation probability. In addition, the degree of entanglement, as measured by the concurrence, decreases with rising temperature and/or pulse duration, even if the excitonic fine-structure splitting is absent and when higher electronic states are out of reach. Furthermore, in the case of finite fine-structure splittings, phonons enlarge the discrepancy in concurrence for different laser polarizations.

Enhancement of dynamical entanglement in a dispersive two-mode Jaynes–Cummings model via superposition of photon-added pair coherent state

In this paper, we introduce a new non-Gaussian state called superposition of photon-added pair coherent state (SPAPCS) by superposition addition of photons to two modes of a pair coherent state (PCS). It is demonstrated that the SPAPCS has an enhanced non-Gaussian property by testing its Wigner function when increasing the number of photons added to two modes of the PCS. We use the field as a SPAPCS to study a dynamical entanglement in a dispersive two-mode Jaynes–Cummings model in dissipation atom-field interaction. We detect the dynamical entanglement degrees of the global system and subsystems by using the linear entropy criteria. The obtained results show the dynamical entanglement degrees can be enhanced by the phase damping effect as well as the superposition adding photons to two modes of the field in SPAPCS.

Atom-photon entanglement in a four-level coherent media via structure light

In this paper, we proposed a new model controlling the quantum entropy in a four-level quantum system via optical vortex structure light. The four-wave mixing (FWM) processes in the medium cause it to become phase-dependent. Therefore, the degree of entanglement (DEM) can be controlled via relative phase between applied lights when the deriving light becomes plane wave. However, for optical vortex of the deriving light, the spatial control of the DEM is possible via orbital angular momentum (OAM) of light. Our findings may have potential applications in the OAM-dependent devices in quantum information science.

Enhanced entanglement and quantum teleportation of two-mode squeezed vacuum state via multistage non-Gaussian operations

The improvement of quantum features and processes, prominent as entanglement and quantum teleportation, of the two-mode squeezed vacuum state (TMSVS) in the small squeezed parameter domain is of great importance. Therefore, it has attracted the attention of scientists in the field of quantum information in recent years. In this paper, we introduce a new optical scheme to enhance the degree of entanglement and quantum teleportation fidelity of such a state using ordinary quantum devices including beam splitters and photon-number-resolving detectors. The scheme allows us to sequentially perform non-Gaussian operations, namely, photon addition, photon subtraction, quantum optical catalysis, and combinations of these techniques, into both modes of the TMSVS. The investigated results show that the degree of entanglement and fidelity in the novel two-mode non-Gaussian state is enhanced, especially in the small regions of the squeezed parameter compared with the original TMSVS and non-Gaussian states constructed from it in previous works. They also indicate that the enhanced zones recede to the small squeezed parameter areas with increasing the number of stages of the non-Gaussian effect.

Non-classical correlations of accelerated observers interacting with a classical stochastic noise beyond the single mode approximation

This paper investigates the behaviour of the quantum correlations for an accelerated two-qubit system during its interaction with a classical stochastic field, utilizing the Wigner function and concurrence. The non-classical behaviour is indicated by negative values of the Wigner function, while the degree of entanglement is demonstrated by the concurrence. To consider acceleration and interaction with a common or independent environment, we discard the single-mode approximation and utilize the Unruh construction of the quantum field mode in our analysis. Our results suggest that coherence suppression is caused by the acceleration effect, noise strength, and noise frequency. Through an examination of quantum concurrence, this study analyses the level of quantum entanglement that corresponds positively with negative values of the Wigner function. The findings indicate that coherence degradation in the initial system is reduced when observers interact independently with their environments, as opposed to interacting with a common environment. Additionally, the acceleration of both observers has an impact on coherence reduction. During system evolution, the Wigner function displays collapse and reappearance behaviour, while quantum entanglement undergoes local collapse and revival phenomena. Notably, common noise situations exhibit more rapid variation in both the Wigner function and entanglement compared to independent noise configurations.

Mechanical Properties and Swelling Behaviors of Ultrathin Chemically Cross-Linked Polybutadiene Films

With the advancement of nanoscience and nanotechnology, understanding the unique mechanical properties of ultrathin polymer films is becoming increasingly important. We report the mechanical properties and swelling behaviors of ultrathin chemically cross-linked polybutadiene films. Upon reducing the film thickness, the Young’s modulus increased, and an unusual stress-relaxation behavior was observed. After stress relaxation, the fully relaxed modulus was significantly higher than that expected from the amount of added cross-linker and the entanglement degree. The out-of-plane swelling of the ultrathin films prepared on a substrate was comparable to that in the bulk, whereas the in-plane swelling of films floating on water was significantly suppressed. Such suppression of the in-plane deformation may be attributed to the effects of interface-induced orientations of the chains and the chain segments trapped at the interfaces acting as pseudo-cross-linking points. This study contributes to an understanding of the physical properties of ultrathin cross-linked elastomer films.

Generating six pairs of bandwidth-expanded entangled sideband modes via time delay compensation.

Quantum entanglement is an important pillar of quantum information processing. In addition to the entanglement degree, the bandwidth of entangled states becomes another focus of quantum communication. Here, by virtue of a broadband frequency-dependent beam splitter, we experimentally demonstrate six pairs of independent entangled sideband modes with maximum entanglement degree of 8.1 dB. Utilizing a time delay compensation scheme, the bandwidth of independent entangled sideband modes is expanded to dozens of megahertz. This work provides a valuable resource to implement efficient quantum information processing.

Quantum Language Model With Entanglement Embedding for Question Answering.

Quantum language models (QLMs) in which words are modeled as a quantum superposition of sememes have demonstrated a high level of model transparency and good post-hoc interpretability. Nevertheless, in the current literature, word sequences are basically modeled as a classical mixture of word states, which cannot fully exploit the potential of a quantum probabilistic description. A quantum-inspired neural network (NN) module is yet to be developed to explicitly capture the nonclassical correlations within the word sequences. We propose a NN model with a novel entanglement embedding (EE) module, whose function is to transform the word sequence into an entangled pure state representation. Strong quantum entanglement, which is the central concept of quantum information and an indication of parallelized correlations among the words, is observed within the word sequences. The proposed QLM with EE (QLM-EE) is proposed to implement on classical computing devices with a quantum-inspired NN structure, and numerical experiments show that QLM-EE achieves superior performance compared with the classical deep NN models and other QLMs on question answering (QA) datasets. In addition, the post-hoc interpretability of the model can be improved by quantifying the degree of entanglement among the word states.

Polarization Entanglement from Parametric Down-conversion with an LED Pump

Spontaneous parametric down-conversion (SPDC) is a reliable platform for entanglement generation. Routinely, a coherent laser beam is an essential prerequisite for pumping the nonlinear crystal. Here we break this barrier to generate polarization-entangled photon pairs by using a commercial light-emitting diode (LED) source to serve as the pump beam. This effect is counterintuitive, as the LED source is of extremely low spatial coherence, which is transferred during the down-conversion process to the biphoton wave function. However, the type-II phase matching condition naturally filters the specific frequency and wavelength of LED light exclusively to participate in SPDC such that localized polarization Bell states can be generated, regardless of the global incoherence over the full transverse plane. In our experiment, we characterize the degree of LED light-induced polarization entanglement in the standard framework of the violation of Bell's inequality. We achieve the Bell value $S=2.33\ifmmode\pm\else\textpm\fi{}0.097$, obviously surpassing the classical bound $S=2$ and thus witnessing the quantum entanglement. Our work can be extended to prepare polarization entanglement by using other natural light sources, such as sunlight and biolight, which holds promise for electricity-free quantum communications in outer space.

Interplay of Particle Size and Temperature on Low-Entanglement Ultrahigh-Molecular-Weight Polyethylene Sintering in Blended Compositions: Analysis of Entanglement and Crystal Structure

The present study focuses on improving the entanglement reconstruction of low-entanglement ultrahigh-molecular-weight polyethylene (UHMWPE) through the incorporation of small particles into large particles in different sintering temperature environments. 10 wt % of small particles was added to the large particles, and the resulting entanglement degree of sintered products was examined through isothermal annealing experiments, high-temperature tensile tests, and rheological measurements. To gain insights into the entanglement formation process, the McLeish theory was employed to analyze the rheological results, and the chain dynamics of the bulk and interface domains in the particles were calculated. The analysis revealed that the addition of small particles significantly enhanced entanglement formation in both the interface and bulk domains of the particles. Regarding the characterization of the crystalline structures, it is found that the establishment of interphase has a positive correlation with entanglement construction, especially at a high sintering temperature. The tailored dimension of particle sizes and sintering temperatures result in optimized entanglement density and interphase content, leading to a substantial improvement in the tensile strength and breaking elongation of the material.

Dynamics of quantum-memory assisted entropic uncertainty of a two-spin Heisenberg XXX model under the intrinsic decoherence effect

In this study, the quantum-memory assisted entropic uncertainty (QM-EU) and entanglement dynamics of the two-qubit Heisenberg XXX chain have been explored in the presence of intrinsic decoherence. The effect of the x-component of Dzyaloshinskii-Moriya (DM) and Kaplan-Shekhtman-Entin-Wohlman-Aharony (KSEA) interactions has been considered. The generation and preservation of quantum memory and entanglement have been examined for various values of the DM, KSEA, spin-spin, and spin coupling strengths. The uncertainty negatively affects the entanglement and both have anti-correlation. The absence and presence of intrinsic decoherence prevail in differing impacts on the dynamics of the system. In the first case, prolonged entanglement preservation, uncertainty suppression, and oscillatory dynamics have been observed. Moreover, in order to achieve the best-prolonged entanglement preservation and relative reduction of the entropic uncertainty, we have analyzed several parameter settings. We find that the effects of raising the DM, KSEA, and spin-spin interaction individually and simultaneously are different. The individual and simultaneous increase of the DM, KSEA, and spin-spin interaction parameters control the degree of entanglement, entropic uncertainty, and primarily the dynamics of the system.

Molecular dynamics simulation of stretch-induced crystallization of star polymers as compared to their linear counterparts

Abstract The linear and star polyethylene during static crystallization and stretch-induced crystallization has been investigated by molecular dynamics simulations. The findings demonstrate that the branching point of the star polymer system does not participate in crystallization, and the crystallization ability of the segments near the branching point and at the end of the chains is inferior. Due to the existence of branching points, the mobility and conformational extension of chain segments are weak, and the entanglement degree is higher than that of linear systems. For stretch-induced crystallization, stretching promotes the extension of molecular chains and arranges them along the stretching direction. The crystal nucleation and growth in linear and star polymer systems are significantly faster than in static crystallization. The mobility of the chain segments close to the branching point is partially enhanced by stretching, while the branching point still substantially affects the chain conformation and segment orientation. It is worth mentioning that we have verified some crucial results that cannot be observed in the experiments at the microscopic scale.

Novel phases in rotating Bose-condensed gas: vortices and quantum correlation

We present the exact diagonalization study of rotating Bose-condensed gas interacting via finite-range Gaussian potential confined in a quasi-2D harmonic trap. The system of many-body Hamiltonian matrix is diagonalized in given subspaces of quantized total angular momentum to obtain the lowest-energy eigenstate employing the beyond lowest-Landau-level approximation. In the co-rotating frame, the quantum mechanical stability of angular momentum states is discussed for the existence of phase transition between the stable states of interacting system. Thereby analyzing the von Neumann entanglement entropy and degree of condensation provide the information about quantum phase correlation in the many-body states. Calculating the conditional probability distribution, we further probe the internal structure of quantum mechanically stable and unstable states. Much emphasis is put on finding the spatial correlation of bosonic atoms in the rotating system for the formation and entry of singly quantized vortices, and then organizing into canonical polygons with and without a central vortex at the trap center. Results are summarized in the form of a movie depicting the vortex patterns having discrete p-fold rotational symmetry with p = 2,3,4,5,6.

Facilely prepare high-performance loose nanofiltration membranes through regulation of polymer chain entanglements

The membrane forming process was fundamentally affected by the thermodynamics of polymer solution, the core of which involved the governance of polymer chain entanglements in the solution. In this work, a high-performance loose nanofiltration (LNF) membrane with high water permeance and high separation accuracy was facilely fabricated by the regulation of polymer chain entanglements through only changing CaCO3 nanoparticle (nano-CaCO3) size in the casting solution. The degree of polymer chain entanglements was characterized by the critical entanglement viscosity of the casting solution. The solution viscosity was markedly increased with the decrease of nano-CaCO3 size and then significantly reduced when nano-CaCO3 disappeared. While the mutual diffusion rate between the non-solvent (acidic water) and the solvent was increased to bring about rapid phase inversion with increasing the viscosity of the casting solution. The high solution viscosity and rapid phase inversion, which was a unconventional phenomenon, leaded to the formation of LNF membranes with outstanding performance. The LNF membranes obtained had a pore diameter of 1.4 nm (MWCO = 1625 Da) and a high pure water permeance of 98 L·m−2·h−1·bar−1, accompanying an excellent separation accuracy of small molecule dyes and NaCl mixtures (the rejection is 99.9% for eriochrome black T (EBT, Mw = 461 Da) and 4.5% for NaCl). This work provides general guidance for the structural regulation of high-performance separation membranes.

Pseudo-Qutrit Formed by Two Interacting Identical Spins (s = 1/2) in a Variable External Magnetic Field.

An analytical solution is obtained for the problem of two interacting, identical but separated spin 1/2 particles in a time-dependent external magnetic field, in a general case. The solution involves isolating the pseudo-qutrit subsystem from a two-qubit system. It is shown that the quantum dynamics of a pseudo-qutrit system with a magnetic dipole-dipole interaction can be described clearly and accurately in an adiabatic representation, using a time-dependent basis set. The transition probabilities between the energy levels for an adiabatically varying magnetic field, which follows the Landau-Majorana-Stuckelberg-Zener (LMSZ) model within a short time interval, are illustrated in the appropriate graphs. It is shown that for close energy levels and entangled states, the transition probabilities are not small and strongly depend on the time. These results provide insight into the degree of entanglement of two spins (qubits) over time. Furthermore, the results are applicable to more complex systems with a time-dependent Hamiltonian.

Collapse-revival and stability of magnon-superconducting qubit entanglement in a tripartite hybrid cavity system

In this paper we pay our attention to the generation and manipulation of bipartite entanglement in a hybrid cavity system which is comprised of cavity microwave photons, a magnon and a superconducting qubit. To achieve the purpose of paper, at first, the time-independent effective Hamiltonian of the system is derived with considering some assumptions, and then the system solution is obtained via solving the Lindblad master equation. At last, the bipartite magnon-superconducting qubit entanglement is quantitatively analyzed using negativity, as a suitable measure of bipartite entanglement. The numerical simulations obviously show that, by adjusting system parameters, collapse-revival behavior of the entanglement can be apparently observed, and by appropriate choice of thermal occupation photon number and cavity decay rate, quasi-stable, even stable with significant degrees of entanglement are accessible. Our results indicate that, such a hybrid cavity system considered here can be used for studying large-scale quantum phenomena which has a crucial role in quantum information science and technologies.

Scattering solution of an interacting Hamiltonian for the electronic control of molecular spin qubits

We theoretically study how a scattered electron can entangle molecular spin qubits (MSQs). This requires solving the inelastic transport of a single electron through a scattering region described by a tight-binding interacting Hamiltonian. We accomplish this using a Green's-function solution. We can model realistic physical implementations of MSQs by parametrizing the tight-binding Hamiltonian with first-principles descriptions of magnetic anisotropy and exchange interactions. We find that, for two-MSQ systems with inversion symmetry, projective measurement of the spin degree of freedom of the scattered electron offers probabilistic control of the degree of entanglement between the MSQs.

Chain Dynamics of Partially Disentangled UHMWPE around Melting Point Characterized by 1H Low-Field Solid-State NMR.

Melts of ultrahigh molecular weight polyethylene (UHMWPE) entangled significantly, suffering processing difficulty. In this work, we prepared partially disentangled UHMWPE by freeze-extracting, exploring the corresponding enchantment of chain mobility. Fully refocused 1H free induction decay (FID) was used to capture the difference in chain segmental mobility during the melting of UHMWPE with different degrees of entanglement by low-field solid-state NMR. The longer the polyethylene (PE) chain is in a less-entangled state, the harder the process of merging into mobile parts after detaching from crystalline lamella during melting. 1H double quantum (DQ) NMR was further used to obtain information caused by residual dipolar interaction. Before melting, the DQ peak appeared earlier in intramolecular-nucleated PE than in intermolecular-nucleated PE because of the strong constraints of crystals in the former one. During melting, less-entangled UHMWPE could keep disentangled while less-entangled high density polyethylene (HDPE) could not. Unfortunately, no noticeable difference was found in DQ experiments between PE melts with different degrees of entanglement after melting. It was ascribed to the small contribution of entanglements compared with total residual dipolar interaction in melts. Overall, less-entangled UHMWPE could reserve its disentangled state around the melting point long enough to achieve a better way of processing.

Entanglement complexity of the Rokhsar-Kivelson-sign wavefunctions

Entanglement comes in different forms, some more complex than others. In this paper we study the transitions of entanglement complexity in an exemplary family of states---the Rokhsar-Kivelson-sign wavefunctions---whose degree of entanglement is controlled by a single parameter. This family of states is known to feature a transition between a phase exhibiting volume-law scaling of entanglement entropy and a phase with subextensive scaling of entanglement, reminiscent of the many-body-localization transition of disordered quantum Hamiltonians. We study the singularities of the Rokhsar-Kivelson-sign wavefunctions and their entanglement complexity across the transition using several tools from quantum information theory: fidelity metric, entanglement spectrum statistics, entanglement entropy fluctuations, stabilizer R\'enyi entropy, and the performance of a disentangling algorithm. Across the whole volume-law phase the states feature universal entanglement spectrum statistics. Yet a ``superuniversal'' regime appears for small values of the control parameter in which all metrics become independent of the parameter itself, the entanglement entropy as well as the stabilizer R\'enyi entropy appear to approach their theoretical maximum, the entanglement fluctuations scale to zero as in output states of random universal circuits, and the disentangling algorithm has essentially null efficiency. All these indicators consistently reveal a complex pattern of entanglement. In the sub-volume-law phase, on the other hand, the entanglement spectrum statistics is no longer universal, entanglement fluctuations are larger and exhibiting a nonuniversal scaling, and the efficiency of the disentangling algorithm becomes finite. Our results, based on model wavefunctions, suggest that a similar combination of entanglement scaling properties and of entanglement complexity features may be found in high-energy Hamiltonian eigenstates---a very strong candidate being offered by the many-body localization transition of disordered lattice-spin models.