High Degree Of Entanglement Research Articles

Improving Quantum Optimization Algorithms by Constraint Relaxation

Quantum optimization is a significant area of quantum computing research with anticipated near-term quantum advantages. Current quantum optimization algorithms, most of which are hybrid variational-Hamiltonian-based algorithms, struggle to present quantum devices due to noise and decoherence. Existing techniques attempt to mitigate these issues through employing different Hamiltonian encodings or Hamiltonian clause pruning, but they often rely on optimistic assumptions rather than a deep analysis of the problem structure. We demonstrate how to formulate the problem Hamiltonian for a quantum approximate optimization algorithm that satisfies all the requirements to correctly describe the considered tactical aircraft deconfliction problem, achieving higher probabilities for finding solutions compared to previous works. Our results indicate that constructing Hamiltonians from an unconventional, quantum-specific perspective with a high degree of entanglement results in a linear instead of exponential number of entanglement gates instead and superior performance compared to standard formulations. Specifically, we achieve a higher probability of finding feasible solutions: finding solutions in nine out of nine instances compared to standard Hamiltonian formulations and quadratic programming formulations known from quantum annealers, which only found solutions in seven out of nine instances. These findings suggest that there is substantial potential for further research in quantum Hamiltonian design and that gate-based approaches may offer superior optimization performance over quantum annealers in the future.

A source of entangled photons based on a cavity-enhanced and strain-tuned GaAs quantum dot

A quantum-light source that delivers photons with a high brightness and a high degree of entanglement is fundamental for the development of efficient entanglement-based quantum-key distribution systems. Among all possible candidates, epitaxial quantum dots are currently emerging as one of the brightest sources of highly entangled photons. However, the optimization of both brightness and entanglement currently requires different technologies that are difficult to combine in a scalable manner. In this work, we overcome this challenge by developing a novel device consisting of a quantum dot embedded in a circular Bragg resonator, in turn, integrated onto a micromachined piezoelectric actuator. The resonator engineers the light-matter interaction to empower extraction efficiencies up to 0.69(4). Simultaneously, the actuator manipulates strain fields that tune the quantum dot for the generation of entangled photons with corrected fidelities to a maximally entangled state up to 0.96(1). This hybrid technology has the potential to overcome the limitations of the key rates that plague QD-based entangled sources for entanglement-based quantum key distribution and entanglement-based quantum networks.

Enhancement of quantum entanglement between twin beams via a four-wave mixing process with double feedback

Two-port feedback has been theoretically studied and proved to be effective in enhancing the degree of entanglement of the output beams for a four-wave mixing process. Here, to further consider the loss effects and phase delays, we use a model that is closer to practical implementation. By reasonably tuning the feedback coefficients, we find that a higher degree of entanglement and higher power of the output beams can be achieved, and a loose requirement for phase-locking accuracy can be obtained. This scheme may have promising applications in practical quantum computation and quantum communication.

Tunable Continuous‐Variable Tripartite Entanglement via Simultaneous and Ordinal Cascaded Nonlinear Processes

AbstractMultipartite quantum entanglement plays a vital role in both fundamental science and quantum applications. The cascaded four‐wave mixing (FWM) process is an effective method to prepare multipartite entanglement, however, the physical nature of entanglement based on different cascading paths, i.e., simultaneous cascaded FWM (SC‐FWM) and ordinal cascaded FWM (OC‐FWM), has not yet been conclusively determined. In this article, tunable continuous‐variable (CV) triple‐mode entanglement is proposed to be generated by using both the SC‐FWM and OC‐FWM schemes. The simulation results reveal that the absorption/dispersion properties of the two nonlinear processes can be efficiently tuned by varying the optical parameters (i.e., the photon detuning and the nonlinear susceptibility), resulting in triple‐mode CV entanglement with tunable properties. Compared with the OC‐FWM scheme, the SC‐FWM scheme has a broader entanglement bandwidth and a higher degree of entanglement, where the tripartite entanglement region is 4.38% larger than that of the OC‐FWM scheme. Furthermore, these results indicate that the SC‐FWM scheme has a more compact and stable mode structure with better entanglement performance. Such tunable optical triple‐mode entanglement may find applications in specific quantum communication protocols and pave the way for implementing and manipulating multichannel quantum networks.

Analysis of quantum properties of two-mode coupled Harmonic Oscillator based on the entangled state representation

The quantum oscillator model plays a significant role in quantum optics and quantum information and has been one of the hot research topics in related fields. Inspired by the single-mode linear harmonic oscillator and the two-mode entangled state representation, this paper constructs a two-mode coupled harmonic oscillator. Different from the quantum transformation method used in previous literature, this paper directly uses the entangled state representation to solve its energy eigenvalues ​​and eigenfunctions easily. Compared with the one-mode harmonic oscillator, the energy eigenvalues ​​and eigenfunctions of this two-mode coupled harmonic oscillator are continuous.<br>Using the matrix theory of quantum operators, we derive the transformation and inverse transformation of the time evolution operator corresponding to the two-mode coupled harmonic oscillator. In addition, using the entangled state representation, the specific form of the time evolution of the two-mode vacuum state under the action of the oscillator is obtained. Through the analysis of quantum fidelity, it is found that the fidelity of the output quantum state decreases with the increase of the oscillator frequency, and the fidelity eventually tends to zero with the increase of time.<br>When analyzing the orthogonal squeezing properties of the output quantum state, this type of two-mode oscillator does not have the orthogonal squeezing effect, but instead has a strong quantum dissipation effect. This conclusion is further verified by the quasi-probability distribution Q function of the quantum state phase space. Therefore, the two-mode coupled harmonic oscillator has a major reference value in quantum control such as quantum decoherence and quantum information transmission.<br>Like the two-mode squeezed vacuum state, the photon distribution of the output quantum light field corresponding to the two-mode harmonic oscillator presents a super-Poisson distribution, and the photons exhibit a strong anti-bunching effect. Using the three-dimensional discrete plot of the photon number distribution, the super-Poisson distribution and quantum dissipation effect of the output quantum state are intuitively demonstrated.<br>Finally, the SV entanglement criterion is used to determine that the output quantum state has a high degree of entanglement. Further numerical analysis shows that the degree of entanglement increases with the action time and the oscillator frequency.<br>In summary, the two-mode coupled harmonic oscillator constructed in this paper can be used to prepare highly entangled quantum states through a complete quantum dissipation process. This provides theoretical support for the experimental preparation of quantum entangled states based on dissipative mechanisms.

Indistinguishability-assisted two-qubit entanglement distillation

Production of quantum states exhibiting a high degree of entanglement out of noisy conditions is one of the main goals of quantum information science. Here, we provide a conditional yet efficient entanglement distillation method which functions within the framework of spatially localized operations and classical communication. This method exploits indistinguishability effects due to the spatial overlap between two identical qubits in distinct sites and encompasses particle statistics imprint. We derive the general conditions for the maximum entanglement distillation out of mixed states. As applications, we give a thorough description of distilled entanglement and associated success probability starting from typical noisy states, such as thermal Gibbs states and Werner states. The influence of local temperatures and of a noise parameter is discussed, respectively, in these two cases. The proposed scheme paves the way towards quantum repeaters in composite networks made of controllable identical quantum particles.

High efficiency grating couplers for strain tunable GaAs quantum dot based entangled photon sources

The on-chip integration of single photon and entangled photon emitters such as epitaxially grown semiconductor quantum dots into photonic frameworks is a rapidly evolving research field. GaAs quantum dots offer high purity and a high degree of entanglement due to, in part, exhibiting very small fine structure splitting along with short radiative lifetimes. Integrating strain-tunable quantum dots into nanostructures enhances the quantum optical fingerprint, i.e., radiative lifetimes and coupling of these sources, and allows for on-chip manipulation and routing of the generated quantum states of light. Efficient out-coupling of photons for off-chip processing and detection requires carefully engineered mesoscopic structures. Here, we present numerical studies of highly efficient grating couplers reaching up to over 90% transmission. A 2D Gaussian mode overlap of 83.39% for enhanced out-coupling of light from within strain-tunable photonic nanostructures for free-space transmission and single-mode fiber coupling is shown. The photon wavelength under consideration is 780 nm, corresponding to the emission from GaAs quantum dots resembling the 87Rb D2 line. The presented numerical study helps implement such sources for applications in complex quantum optical networks.

Entangled photon-pair source using a wedge-shaped nonlinear crystal

Entangled photon pairs constitute a crucial resource in quantum technologies. The current technique for preparing photon pairs involves performing spontaneous parametric down-conversion (SPDC) in bulk nonlinear crystals. The increased prominence of quantum technologies has led to a growing interest in high performance entangled photon-pair sources. However, the birefringence walk-off caused by nonlinear crystals affects the quality of the entangled photon pairs generated in SPDC. This letter presents an innovative approach for producing entangled photon pairs using a wedge-shaped nonlinear crystal to compensate for the birefringence walk-off. The proposed scheme features a basic entanglement source path, which can be applied to other methods of preparing entangled photon pairs. An experiment demonstrated that this method can produce high-quality entangled photon pairs with a high degree of entanglement, paving the way for the development of practical quantum communication, sensing, and computation.

Strong coupling in the entanglement dynamics of two qubits interacting with a graphene nanodisk

We investigate the entanglement dynamics of two qubits interacting with a graphene nanodisk using the macroscopic quantum electrodynamics method. By modifying the free-space decay rate of each qubit, we study the coupling strength between the nanoparticle and the qubits. We find that as the free-space decay rate increases, the decaying Rabi oscillations featured in the qubit population dynamics change to complex non-Markovian dynamical population evolution. This is also reflected on the concurrence, which at weak or moderate light–matter coupling conditions, attains values up to 0.5, while as the coupling conditions become stronger, larger values are also transiently observed. Our findings indicate that graphene nanostructures can provide a platform for the realization of high degree of entanglement in the strong coupling regime at the nanoscale, essential for quantum technology applications.

Implementation of Photonic Phase Gate and Squeezed States via a Two-Level Atom and Bimodal Cavity

We propose a theoretical model for realizing a photonic two-qubit phase gate in cavity QED using a one-step process. The fidelity and probability of success of the conditional quantum phase gate is very high in the presence of cavity decay. Our scheme only employs one two-level atom, and thus is much simpler than other schemes involving multi-level atoms. This proposal can also be applied to generate two-mode squeezed states; therefore, we give three examples, i.e., the two-mode squeezed vacuum state, two-mode squeezed odd coherent state, and two-mode squeezed even coherent state, to estimate the variance of Duan’s criterion when taking into account cavity decay. It is shown that the variance is smaller than 2 for the three squeezed states in most cases. Furthermore, we utilize logarithmic negativity to measure the entanglement, and find that these squeezed states have very high degrees of entanglement.

Constraining the parameter space of a quantum spin liquid candidate in applied field with iterative optimization

The quantum spin liquid (QSL) state is an exotic state of matter featuring a high degree of entanglement and lack of long-range magnetic order in the zero-temperature limit. The triangular antiferromagnet YbMgGaO4 is a candidate QSL host, and precise determination of the Hamiltonian parameters is critical to understanding the nature of the possible ground states. However, the presence of chemical disorder has made directly measuring these parameters challenging. Here we report neutron scattering and magnetic susceptibility measurements covering a broad range of applied magnetic field at low temperature. Our data shows a field-induced crossover in YbMgGaO4, which we reproduce with complementary classical Monte Carlo and Density Matrix Renormalization Group simulations. Neutron scattering data above and below the crossover reveal a shift in scattering intensity from M to K points and, collectively, our measurements provide essential characteristics of the phase crossover that we employ to strictly constrain proposed magnetic Hamiltonian parameters despite the chemical disorder. Constrained exchange parameters further suggest the material's proximity to the QSL state in the clean limit. More broadly, our approach demonstrates a means of pursuing QSL candidates where Hamiltonian parameters might otherwise be obscured by disorder.

A new approach to produce polystyrene monoliths by gelation and capillary shrinkage

Polymeric monoliths are of great interest in a variety of applications. A new gelation approach to produce a mechanically stable polystyrene (PS) gel directly from its microemulsion is reported. To produce a PS gel, the as-prepared microemulsion is first demulsified by adding selected water-miscible organic solvents. The small PS latex particles liberated from the surfactant are assembled into a piece of bulk material at an appropriate temperature with a high degree of entanglement of the polymer chains. It is found that the d2T/η value is an important parameter to evaluate the gelation ability of the organic solvents and helps determine the gelation conditions. Finally, PS monoliths are obtained by capillary drying and their pore structures can be effectively tuned by changing the gelation time and the amount of solvent exchanged with water. This allows the controlled preparation of bulk PS artefacts with densities in the range of 0.06 to 1.14 g cm−3. This simple method of PS monolith production avoids the use of shaping tools or chemical templates, needs less energy, and is a promising alternative approach to design either integrated porous or compact polymer materials.

Coherent control of quantum and entanglement dynamics via periodic modulations in optomechanical semiconductor resonator coupled to quantum-dot excitons

We systematically study the influence of simultaneously modulating the input laser intensity and quantum-dot (QD) resonance frequency on the mean-field dynamics, fluctuation energy transfer and entanglement in a optomechanical semiconductor resonator embedded with a QD. We show that the modulation and the hybrid system can be engineered to attain the desired mean-field values and control the fluctuation energy transfer and the entanglement among the various degrees of freedom. A remarkably high degree of entanglement can be generated by modulating only the QD frequency. The interplay between the two modulations leads to a decrease in the entanglement. Switching on the modulation leads to a transition from low stationary to large dynamical entanglement. This investigation provides novel strategies to coherently control data signal transfer and storage in quantum information processing networks.

Ductile keratin/deacetylated chitin composites with nanoparticle-induced formation of ordered and entangled structures

Highly ductile and wet-durable bio-composites from keratin reinforced by chitin-nanoparticles with engineered deacetylation have been developed. Though featuring good biodegradability and biocompatibility, poultry-feather-extracted keratin-based composites usually suffered from poor ductility and moisture stabilities because of poor interfacial properties, limited recovery of ordered protein structures and crosslinkages. In this work, chitin-nanoparticles with engineered deacetylation were used to reinforce keratin. Via engineered deacetylation, chitin-nanoparticles not only improved interfacial interactions but also formed crosslinkages with keratin. As a result, ordered keratin structures with high degree of entanglement in composites were substantially formed without addition of any coupling agents. Furthermore, engineered deacetylation for chitin could adjust the wet performance of composites via control of hydrophilicity of matrix and reinforcements. Via control of regularity and hydrophilicity of molecules by chitin-nanoparticles, mechanical properties and wet performance of keratin-based composites can be substantially improved. Reinforced keratin composites had a 230%, 260%, 540% increase in breaking strain, breaking stress and ductility, respectively as well as 94% weight retention after immersed in water for 1 week.

Phase separation of a Bose-Bose mixture: Impact of the trap and particle-number imbalance

We explore a few-body mixture of two bosonic species confined in quasi-one-dimensional parabolic traps of different length scales. The ground state phase diagrams in the three-dimensional parameter space spanned by the harmonic length scale ratio, inter-species coupling strength and particle number ratio are investigated. As a first case study we use the mean-field ansatz (MF) to perform a detailed analysis of the separation mechanism. It allows us to derive a simple and intuitive rule predicting which of the immiscible phases is energetically more favorable at the miscible-immiscible phase boundary. We estimate the critical coupling strength for the miscible-immiscible transition and perform a comparison to correlated many-body results obtained by means of the Multi-Layer Multi-Configuration Time Dependent Hartree method for bosonic mixtures (ML-X). At a critical ratio of the trap frequencies, determined solely by the particle number ratio, the deviations between MF and ML-X are very pronounced and can be attributed to a high degree of entanglement between the components. As a result, we evidence the breakdown of the effective one-body picture. Additionally, when many-body correlations play a substantial role, the one-body density is in general not sufficient for deciding upon the phase at hand which we demonstrate exemplarily.

Entangling Macroscopic Light States by Delocalized Photon Addition.

We present a scheme, based on the delocalized heralded addition of a single photon, to entangle two or more distinct field modes, each containing arbitrary light states. A high degree of entanglement can in principle endure light states of macroscopic intensities and is expected to be particularly robust against losses. We experimentally establish and measure significant entanglement between two identical weak laser pulses containing up to 60 photons each.

Antiferromagnetic magnons as highly squeezed Fock states underlying quantum correlations

Employing the concept of two-mode squeezed states from quantum optics, we demonstrate a revealing physical picture for the antiferromagnetic ground state and excitations. Superimposed on a N{\'e}el ordered configuration, a spin-flip restricted to one of the sublattices is called a sublattice-magnon. We show that an antiferromagnetic spin-up magnon is comprised by a quantum superposition of states with $n+1$ spin-up and $n$ spin-down sublattice-magnons, and is thus an enormous excitation despite its unit net spin. Consequently, its large sublattice-spin can amplify its coupling to other excitations. Employing von Neumann entropy as a measure, we show that the antiferromagnetic eigenmodes manifest a high degree of entanglement between the two sublattices, thereby establishing antiferromagnets as reservoirs for strong quantum correlations. Based on these novel insights, we outline strategies for exploiting the strong quantum character of antiferromagetic (squeezed-)magnons and give an intuitive explanation for recent experimental and theoretical findings in antiferromagnetic magnon spintronics.

Wear Resistance Mechanism of Ultrahigh-Molecular-Weight Polyethylene Determined from Its Structure–Property Relationships

Although it is generally believed that the extraordinarily high molecular weight plays a great role in the excellent wear resistance of ultrahigh-molecular-weight polyethylene (UHMWPE), the mechanism behind this effect remains poorly understood. In this study, we investigated the wear behavior of UHMWPE with respect to its microstructures, measured in terms of its crystallinity, lamellar thickness and crystallite size, entanglement, interphase fractions, and surface roughness. From these structure–property relationships, we conclude that the high wear resistance of UHMWPE can be attributed to its higher degree of entanglement and its high fraction content of interphase domains; the other parameters were either not significant or not relevant. Accordingly, we propose a mechanism in which the more greatly entangled molecular chain networks of UHMWPE protect the surface and subsurface layers from damage under stresses during sliding wear.

Spin-orbit excitons in CoO

CoO has an odd number of electrons in its unit cell, and therefore is expected to be metallic. Yet, CoO is strongly insulating owing to significant electronic correlations, thus classifying it as a Mott insulator. We investigate the magnetic fluctuations in CoO using neutron spectroscopy. The strong and spatially far-reaching exchange constants reported in [Sarte et al. Phys. Rev. B 98 024415 (2018)], combined with the single-ion spin-orbit coupling of similar magnitude [Cowley et al. Phys. Rev. B 88, 205117 (2013)] results in significant mixing between $j_{eff}$ spin-orbit levels in the low temperature magnetically ordered phase. The high degree of entanglement, combined with the structural domains originating from the Jahn-Teller structural distortion at $\sim$ 300 K, make the magnetic excitation spectrum highly structured in both energy and momentum. We extend previous theoretical work on PrTl$_{3}$ [Buyers et al. Phys. Rev. B 11, 266 (1975)] to construct a mean-field and multi-level spin exciton model employing the aforementioned spin exchange and spin-orbit coupling parameters for coupled Co$^{2+}$ ions on a rocksalt lattice. This parameterization, based on a tetragonally distorted type-II antiferromagnetic unit cell, captures both the sharp low energy excitations at the magnetic zone center, and the energy broadened peaks at the zone boundary. However, the model fails to describe the momentum dependence of the excitations at high energy transfers, where the neutron response decays faster with momentum than the Co$^{2+}$ form factor. We discuss such a failure in terms of a possible breakdown of localized spin-orbit excitons at high energy transfers.

Molecular Weight Dependence of the Viscosity of Highly Entangled Polyisobutylene

An unsolved problem in the viscoelastic properties of entangled linear polymers is the viscosity–molecular weight relationship. Experiments indicate a power law, η0 ∝ Ma, with a = 3.5 ± 0.2, whereas the theoretical prediction for “pure reptation” is an exponent equal to 3.0. This discrepancy is ascribed to dynamic modes that compete with reptation so that a cubic dependence on M is observed only at sufficiently high molecular weights. That is, almost all existing experimental data are for polymers with too few entanglements to exhibit η0 ∝ M3. To address this issue, we carried out experiments on polyisobutylene having as many as 430 entanglements per chain. Because reentanglement of polymer melts prepared from solution takes extraordinarily long times (another unsolved problem in rheology), annealing was carried out for 5 years prior to the viscosity measurements. We determined that at all temperatures, η0 was less than the extrapolation assuming the M3.5 dependence of lower molecular weight polyisobutylene. This result is consistent with the idea that only at sufficiently high degrees of entanglement is pure reptation the operative mechanism, whereby linear flexible-chain polymers exhibit the predicted behavior. Our work is in good agreement with an earlier study on polybutadiene [Colby, R. H.; Fetters, L. J.; Graessley, W. W. Melt viscosity-molecular weight relationship for linear polymers. Macromolecules 1987, 20, 2226–2237.].