High Entanglement Research Articles

Experimental demonstration of high fidelity entanglement distribution over decoherence channels via qubit transduction.

Quantum coherence and entanglement, which are essential resources for quantum information, are often degraded and lost due to decoherence. Here, we report a proof-of-principle experimental demonstration of high fidelity entanglement distribution over decoherence channels via qubit transduction. By unitarily switching the initial qubit encoding to another, which is insensitive to particular forms of decoherence, we have demonstrated that it is possible to avoid the effect of decoherence completely. In particular, we demonstrate high-fidelity distribution of photonic polarization entanglement over quantum channels with two types of decoherence, amplitude damping and polarization-mode dispersion, via qubit transduction between polarization qubits and dual-rail qubits. These results represent a significant breakthrough in quantum communication over decoherence channels as the protocol is input-state independent, requires no ancillary photons and symmetries, and has near-unity success probability.

Broadband indistinguishability from bright parametric downconversion in a semiconductor waveguide

Parametric downconversion (PDC) in semiconductor Bragg-reflection waveguides (BRW) is routinely exploited for photon-pair generation in the telecommunication range. Contrary to many conventional PDC sources, BRWs offer possibilities to create spectrally broadband but nevertheless indistinguishable photon pairs in orthogonal polarizations that simultaneously incorporate high frequency entanglement. We explore the characteristics of co-propagating twin beams created in a type-II ridge BRW. Our PDC source is bright and efficient, serving as a benchmark of its performance and justifies its exploitation for further use in quantum photonics. We then examine the coalescence of the twin beams and investigate the effect of their inevitable multi-photon contributions on the observed photon bunching. Our results show that BRWs have a great potential for producing broadband indistinguishable photon pairs as well as multi-photon states.

A New Multipartite Entanglement Measure for Arbitrary n-qudit Pure States

In this paper, we propose a multipartite entanglement measure for arbitrary pure states, which is presented based on reduced density matrices of multi-qudit pure states. We review some multipartite entanglement measures based on density matrices. This is helpful for us to introduce a new good entanglement measure, which is vanishing if and only if a state is separable, invariant under local unitary transformations and non-increasing under local operations assisted by classical communication. We apply our entanglement measure for some explicit examples. It demonstrates that our entanglement measure is practical and convenient for computation. It can also distinguish the relatively high entanglement and the maximal entanglement. In short, our entanglement measure is good at characterizing multipartite entanglement.

Scheme for purifying a general mixed entangled state and its linear optical implementation

We propose a scheme for purification of a general mixed entangled state. In this scheme, we start from a large number of general mixed entangled states and end up, after local operation and classical communication, with a smaller number of Bell diagonal states with higher entanglement. In particular, the scheme can purify one maximally entangled state from two entangled pairs prepared in a class of mixed entangled state. Furthermore we propose a linear optical implementation of the present scheme with polarization beam splitters and photon detectors.

Mechanisms of Chain Reentanglement during the Sintering of UHMWPE Nascent Powder: Effect of Molecular Weight

Nascent powders of ultrahigh molecular weight polyethylene (UHMWPE) with different molecular weights in the range 0.6 < M-v < 10.5 million g/mol have been processed by means of sintering. The interface consolidation or particle welding was carried out under pressure at various temperatures above the melting point and for various durations. Tensile drawing experiments performed above the melting point enabled to study the role of chain interdiffusion through the particle interface. The melting explosion phenomenon was demonstrated to occur for each molecular weight. The higher is M-v, the greater is the efficiency of the phenomenon. But this does not allow a homogenization of the entanglement network of the sintered sample. Indeed, heterogeneity of deformation between the grain and the interface clearly appears during tensile tests above the melting temperature. The differences in mechanical behavior in the molten state between the molecular weights seem exacerbated by crystallization under tension. After melting explosion, end-chain diffusion as well as sideways motions are probably involved in the healing of very long chains polymer interfaces to reach a sufficiently high entanglement density. This latter phenomenon seems thermally activated.

Ring-banded spherulites of six-arm star-shaped poly(ε-caprolactone) with different arm length via CO2

The lamellae structure and the ring-banded features of the concentric ring-banded spherulites grown from six-arm star-shaped poly(e-caprolactone) (PCL) with different arm length under CO2 condition were investigated by polarized optical microscope, atomic force microscopy, and scanning electronic microscopy in this paper. Experimental results indicated that the concentric ring-banded spherulites with alternative periodically ridge and valley bands with continuous edge-on lamellae formed under the condition of CO2. The band space got narrower, and the number of the ring bands increased in PCL with the increasing pressure due to the plasticization effect of CO2. The band space of the six-arm star-shaped PCL with longer arm length was wider than that of the six-arm star-shaped PCL with shorter arm length whereas the number of ring bands was fewer. Because of the longer arm length which resulted in the higher entanglement, the chain mobility of the star-shaped PCL with longer arm length decreased markedly. The different entanglement resulting from the different arm length structure was considered to be the primary factors influencing the characteristic crystallization of the six-arm star-shaped PCL.

Characterization of conical intersection in a cis–trans isomerization through nonclassicality and entanglement

In this paper we have characterized the conical intersection, an important topological feature of potential energy surfaces in a in cis–trans isomerization, in terms of the nonclassicality and quantum entanglement which are shown to be controlled by the torsion angle as a molecular parameter. In this context we have provided a quantitative measure of entanglement in terms of Wigner function matrix and compared with other standard measures, namely, von Neumann entropy and partial transpose of joint density matrix. It is shown that the entanglement in the ground state maximizes as the angle reaches the conical intersection point. The highly nonclassical nature of the conical intersection is shown in terms of the significant amount of squeezing for long ranges of the torsion angle and vibronic coupling. The region of high entanglement at and around the conical intersection is connected to the spectroscopically ‘dark’ time window recently detected in ultrafast transient optical spectroscopy experiments in molecular systems with torsional motion which is associated with the presence of conical intersection.

Effects of chain entanglement on liquid-liquid phase separation behavior of LCST-type polymer blends: Cloud point and decomposition rate

By preparing homogenous blend samples with different degrees of chain entanglement, we report an anomalous contribution of chain entanglement to phase separation temperature and rate of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends presenting a typical lower critical solution temperature (LCST) behavior. The meltmixed PMMA/SMA blends with a higher chain entanglement density present a lower cloud point (Tc) and shorter delay time, but lower phase separation rate at the given temperature than solution-cast ones, suggesting that for the polymer blends with different condensed state structure, thermodynamically more facilitation to phase separation (lower Tc) is not necessarily equivalent to faster kinetics (decomposition rate). The experimental results indicate that the lower Tc of melt-mixed sample is ascribed to smaller concentration fluctuation wavelength (Λm) induced by higher entanglement degree, while higher entanglement degree in melt-mixed sample leads to a confined segmental dynamics and consequently a slower kinetics (decomposition rate) dominated by macromolecular diffusion at a comparable quench depth. These results reveal that the chain packing in polymer blends can remarkably influence the liquid-liquid phase separation behavior, which is a significant difference from decomposition of small molecular mixtures.

Effect of the dispersion on multipartite continuous-variable entanglement in optical parametric amplifier**Project supported by the State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China (Grant No. KF201401) and the National Natural Science Foundation of China (Grant No. 11404084).

Based on the quantum fluctuations, we adopt the method of generalized V1 criterion to investigate multipartite entanglement characteristics in an optical parametric amplification system with the consideration of dispersion. The nonlinear interaction becomes strong because of the existence of dispersion coefficient σ . Considering the influence of dispersion factor σ , with increasing the pump parameter μ , the value of minimum variance V1 decreases and the squeezing curve nearly equals 1/(1 + μ ). The larger particle number N results in a smaller variance and higher entanglement.

Enhancement of discord and entanglement with detuning for two coupled quantum dots in a driven cavity

We study the quantum discord for a system of two identical coupled quantum dots interacting with quantized cavity field in the presence of cavity as well as dot decay and detuning. The cavity is externally driven by a coherent light. These results are compared with the entanglement of the quantum dots in various parameter regimes in which system may or may not show bistability. We show that the discord in the steady state is nonzero for any nonzero cavity field amplitude. The system has higher discord in the upper branch of the bistability curve where the entanglement is zero. We also find many other interesting results including high discord and entanglement in the presence of detuning, a phenomenon which we further examine by approximating the density matrix in the appropriate limit.

Experimental generation of 84 dB entangled state with an optical cavity involving a wedged type-II nonlinear crystal

Entangled state of light is one of the essential quantum resources in quantum information science and technology. Especially, when the fundamental principle experiments have been achieved in labs and the applications of continuous variable quantum information in the real world are considered, it is crucial to design and construct the generation devices of entangled states with high entanglement and compact configuration. We have designed and built an efficient and compact light source of entangled state, which is a non-degenerate optical parametric amplifier (NOPA) with the triple resonance of the pump and two subharmonic modes. A wedged type-II KTP crystal inside the NOPA is used for implementing frequency-down-conversion of the pump field to generate the optical entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. The EPR entangled state of light with quantum correlations of 8.4 dB for both amplitude and phase quadratures are experimentally produced by a single NOPA under the pump power of 75 mW.

Long-range Cooper pair splitter with high entanglement production rate.

Cooper pairs in the superconductor are a natural source of spin entanglement. The existing proposals of the Cooper pair splitter can only realize a low efficiency of entanglement production, and its size is constrained by the superconducting coherence length. Here we show that a long-range Cooper pair splitter can be implemented in a normal metal-superconductor-normal metal (NSN) junction by driving a supercurrent in the S. The supercurrent results in a band gap modification of the S, which significantly enhances the crossed Andreev reflection (CAR) of the NSN junction and simultaneously quenches its elastic cotunneling. Therefore, a high entanglement production rate close to its saturation value can be achieved by the inverse CAR. Interestingly, in addition to the conventional entangled electron states between opposite energy levels, novel entangled states with equal energy can also be induced in our proposal.

Protecting quantum entanglement at finite temperature by the weak measurements

According to the map of a qubit, a scheme for protecting entanglement of two-qubit by the weak measurements at finite temperature is proposed. Since the choices of the channel parameters and initial states are very different for different weak measurement strengths, two local unitary equivalent initial entangled states |ψ&gt; and |φ&gt; are chosen. Weak measurements are performed when two initial entangled states go through the generalized amplitude damping channel, and the analytical expressions of getting maximum concurrence entanglement Cr and weak measurement parameters m and n can be obtained by performing an overall optimization for four weak measurement parameters. What is more, the relationship between the weak measurement parameters and the channel parameters is further explored. Theoretical results show the entanglement protection project based on weak measurements can effectively enhance the entanglement and even prevent the sudden death of entanglement in some cases. When the channel parameter r is fixed, for different values of parameter p, the concurrence is centered at p = 0.5, and the weak measurement parameters of the maximum concurrence entanglement are the same as those for initial state |ψ&gt;, while they are different for initial state |φ&gt;. Under the condition of different values of r, for the fixed p and initial state |ψ&gt; or |φ&gt;, the weak measurement parameters remain constant as the entanglement reaches the maximum and the concurrence decreases with the increase of parameter r. Through the analysis of channel parameters, higher entanglement can be obtained by choosing appropriate channel parameters and initial state.

Influence of Molecular Weight Average, Degree of Crystallinity, and Viscosity of Different Polyamide PA12 Powder Grades on the Microstructures of Laser Sintered Part

Laser Sintering (LS) allows functional parts to be produced in a wide range of powdered materials using a dedicated machine, and is thus gaining popularity within the field of rapid prototyping. It offers the user the ability to optimise part design in order to meet customer requirements with few manufacturing restrictions. A problem with LS is that sometimes the surface of the parts produced displays a texture similar to that of the skin of an orange (the so-called “orange peel” texture). The main aim of this research is to develop a methodology of controlling the input material properties of PA12 powder that will ensure consistent and good quality of the fabricated parts. Melt Flow Rate (MFR) and Gel permeation chromatography (GPC) were employed to measure the flow viscosity and molecular weight distributions of Polyamide PA12 powder grades. The experimental results proved that recycle PA12 powder with higher melt viscosity polymer has a higher entanglement with a longer molecule chain causes a higher resistance to flow which cause poor and rough surface finished on laser sintered part.

Entanglement Between Qubits Interacting with Thermal Field

We have investigated the entanglement between two dipole coupled two-level artificial atoms (superconducting qubits, ion, spins etc.). The model, in which only one atom is trapped in an lossless cavity and interacts with single-mode thermal field, and the other one can be spatially moved freely outside the cavity has been carried out. We have considered the effect of the atomic coherence on the entanglement behavior. We have shown that a thermal field might cause high entanglement between the atoms both for coherent and incoherent initial atomic states only for small values of cavity mean photon number. We have also derived that the degree of entanglement is weakly dependent on the strength of dipole-dipole interaction for coherent initial states. In the considered model the atoms would get entangled even when both atoms are initially in the excited state.

Topological energy storage of work generated by nanomotors.

Most macroscopic machines rely on wheels and gears. Yet, rigid gears are entirely impractical on the nano-scale. Here we propose a more useful method to couple any rotary engine to any other mechanical elements on the nano- and micro-scale. We argue that a rotary molecular motor attached to an entangled polymer energy storage unit, which together form what we call the "tanglotron" device, is a viable concept that can be experimentally implemented. We derive the torque-entanglement relationship for a tanglotron (its "equation of state") and show that it can be understood by simple statistical mechanics arguments. We find that a typical entanglement at low packing density costs around 6kT. In the high entanglement regime, the free energy diverges logarithmically close to a maximal geometric packing density. We outline several promising applications of the tanglotron idea and conclude that the transmission, storage and back-conversion of topological entanglement energy are not only physically feasible but also practical for a number of reasons.

Atmospheric continuous-variable quantum communication

We present a quantum communication experiment conducted over a point-to-point free-space link of 1.6 km in urban conditions. We study atmospheric influences on the capability of the link to act as a continuous-variable (CV) quantum channel. Continuous polarization states (that contain the signal encoding as well as a local oscillator (LO) in the same spatial mode) are prepared and sent over the link in a polarization multiplexed setting. Both signal and LO undergo the same atmospheric fluctuations. These are intrinsically auto-compensated which removes detrimental influences on the interferometric visibility. At the receiver, we measure the Q-function and interpret the data using the framework of effective entanglement (EE). We compare different state amplitudes and alphabets (two-state and four-state) and determine their optimal working points with respect to the distributed EE. Based on the high entanglement transmission rates achieved, our system indicates the high potential of atmospheric links in the field of CV quantum key distribution.

Fracture mechanism of amorphous polymers at strain fields

Owing to the wide application of polymeric materials, understanding the fracture mechanism of amorphous polymers at strain fields is a fundamentally important challenge. In this work, we use molecular dynamics simulations to investigate the uniaxial deformation of amorphous polyethylene and further monitor the polyethylene fracture process induced by stretching. Results indicate that the polyethylene systems with chain lengths of 600-800 united atoms exhibit the fracture behavior at a temperature T < 200 K and the strain of 1.0. Further study shows that in the stretching process, the disentanglement and orientation of chains lead to the formation of small cavities in the middle region of the system, and the small cavities subsequently form a large hole, causing the fracture of the whole system. Definitely, the fracture is determined by the two factors of mobility and entanglement of chains. The polyethylene systems with a high chain mobility or a high chain entanglement do not fracture. Finally, a schematic diagram is put forward to illustrate the fracture behavior.

Optimal quench for distance-independent entanglement and maximal block entropy

We optimize a quantum walk of multiple fermions following a quench in a spin chain to generate near-ideal resources for quantum networking. We first prove a useful theorem mapping the correlations evolved from specific quenches to the apparently unrelated problem of quantum state transfer between distinct spins. This mapping is then exploited to optimize the dynamics and produce large amounts of entanglement distributed in very special ways. Two applications are considered: the simultaneous generation of many Bell states between pairs of distant spins (maximal block entropy) and high entanglement between the ends of an arbitrarily long chain (distance-independent entanglement). Thanks to the generality of the result, we study its implementation in different experimental setups using present technology: nuclear magnetic resonance, ion traps, and ultracold atoms in optical lattices.

Fully quantum approach to optomechanical entanglement

The radiation pressure induced coupling between an optical cavity field and a mechanical oscillator can create entanglement between them. In previous works this entanglement was treated as that of the quantum fluctuations of the cavity and mechanical modes around their classical mean values. Here we provide a fully quantum approach to optomechanical entanglement, which goes beyond the approximation of classical mean motion plus quantum fluctuation, and applies to arbitrary cavity drive. We illustrate the real-time evolution of optomechanical entanglement under drive of arbitrary detuning to show the existence of high, robust and stable entanglement in blue detuned regime, and highlight the quantum noise effects that can cause entanglement sudden death and revival.