High Degree Of Entanglement Research Articles

Coherence and degree of time-bin entanglement from quantum dots

We report on the generation of time-bin entangled photon pairs from a semiconductor quantum dot via pulsed resonant biexciton generation. Based on theoretical modeling we optimized the duration of the excitation pulse to minimize the laser-induced dephasing and increase the biexciton-to-background single exciton occupation probability. This results in a high degree of entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state. Theoretical simulations also indicate a power dependent nature of the dephasing during the laser excitation that limits the coherence of the excitation process.

Quantum analysis and experimental investigation of the nondegenerate optical parametric oscillator with unequally injected signal and idler

We developed a quantum analysis of the nondegenerate optical parametric oscillator (NOPO) with unequally injected signal and idler. Both the steady-state output field and the two-mode quantum correlation spectrum are investigated under the condition of different injected idler-to-signal ratios (ISRs) and the relative phase between the pump and the injected seed. It is found that when the seed is injected through the output coupler, the NOPO allows for the robust generation of two-mode quantum entanglement even if the relative phase is free running and the ISR is as high as 0.7. At the specific relative phase of zero, a high degree of entanglement can exist across a whole range of ISRs. An experimental study of the NOPO with unequal seeds is presented, and the observed results verify the theoretical predictions.

DYNAMICS OF TWO-LEVEL ATOMS INTERACTING WITH THERMAL FIELD FOR ENTANGLED INITIAL ATOMIC STATES

In this paper the entanglement dynamics for system of two two-level atoms interacting with a mode of thermal electromagnetic field in lossless cavity has been investigated. Initial atomic states are the entangled Bell type states. Using the full set of eigenvectors of the Hamiltonian of the considered model we have derived the exact solution for the density matrix for the whole system. On its basis the reduced atomic density matrix and Peres - Horodetcki parameter have been calculated. Calculating of entanglement parameter shows the possibility of high degree of entanglement even for large intensity of thermal field. Thus there is a possibility of maintenance and control over the degree of entanglement.

On the concept of cryptographic quantum hashing

In the letter we define the notion of a quantum resistant (-resistant) hash function which consists of a combination of pre-image (one-way) resistance (ε-resistance) and collision resistance (δ-resistance) properties.We present examples and discussion that supports the idea of quantum hashing. We present an explicit quantum hash function which is ‘balanced’, one-way resistant and collision resistant and demonstrate how to build a large family of quantum hash functions. Balanced quantum hash functions need a high degree of entanglement between the qubits. We use a phase transformation technique to express quantum hashing constructions, which is an effective way of mapping hash states to coherent states in a superposition of time-bin modes. The phase transformation technique is ready to be implemented with current optical technology.

Complete temporal mode analysis in pulse-pumped fiber-optical parametric amplifier for continuous variable entanglement generation.

Mode matching plays an important role in measuring the continuous variable entanglement. For the signal and idler twin beams generated by a pulse pumped fiber optical parametric amplifier (FOPA), the spatial mode matching is automatically achieved in single mode fiber, but the temporal mode property is complicated because it is highly sensitive to the dispersion and the gain of the FOPA. We study the temporal mode structure and derive the input-output relation for each temporal mode of signal and idler beams after decomposing the joint spectral function of twin beams with the singular-value decomposition method. We analyze the measurement of the quadrature-amplitude entanglement, and find mode matching between the multi-mode twin beams and the local oscillators of homodyne detection systems is crucial to achieve a high degree of entanglement. The results show that the noise contributed by the temporal modes nonorthogonal to local oscillator may be much larger than the vacuum noise, so the mode mis-match can not be accounted for by merely introducing an effective loss. Our study will be useful for developing a source of high quality continuous variable entanglement by using the FOPA.

Bell states and entanglement dynamics on two coupled quantum molecules

This work provides a complete description of entanglement properties between electrons inside coupled quantum molecules, nanoestructures which consist of two quantum dots. Each electron can tunnel between the two quantum dots inside the molecule, being also coupled by Coulomb interaction. First, it is shown that Bell states act as a natural basis for the description of this physical system, defining the characteristics of the energy spectrum and the eigenstates. Then, the entanglement properties of the eigenstates are discussed, shedding light on the roles of each physical parameters on experimental setup. Finally, a detailed analysis of the dynamics shows the path to generate states with a high degree of entanglement, as well as physical conditions associated with coherent oscillations between separable and Bell states.

Strain‐Induced Alignment Mechanisms of Carbon Nanotube Networks

Random networks comprised of millimeter‐long multi‐walled carbon nanotubes (CNTs) have shown unique microstructure change mechanisms under uniaxial strain. These networks can be modified into highly aligned microstructures from strain‐induced plastic deformation. Applying a treatment consisting of an uncured resin as a load transfer enhancement medium leads to a dramatically increased degree of alignment and final mechanical properties of the CNT networks. The structural evolution of the CNT networks includes different modes: de‐bundling, elongation to reduce waviness, sliding friction, and packing for self‐assembly into large bundles. The high ductility of the treated networks, which allows the network to achieve high degrees of strain‐induced alignment is mainly because the extra high aspect ratios of the individual CNT and their bundles as well as enhanced load transfer. High aspect ratio causes high degrees of entanglement and locking points between the nanotubes in the random network, which are critical to provide adequate nanotube to nanotube load transfer for ductile deformation and lead to substantially increased CNT alignment during mechanical stretching. The classical strain strengthening mechanisms used in metals and polymers such as strain hardening and crystallization of long molecular chains are discussed and compared to CNT network deformation mechanisms. The CNT network strain hardening parameter n value is as high as 0.65, over three times that of annealed low‐carbon steel and more than four times of polycarbonate plastics. Strength coefficient K values for the CNT network also show high values up to roughly 450 MPa, comparable to that of annealed magnesium alloys. The results show how the high degree of alignment of CNT networks and strain strengthening can be achieved through simple uniaxial strain and load transfer medium.

On-demand generation of indistinguishable polarization-entangled photon pairs

An on-demand source of indistinguishable and entangled photon pairs is a fundamental component for different quantum information applications such as optical quantum computing, quantum repeaters, quantum teleportation and quantum communication. Parametric down-conversion and four-wave mixing sources of entangled photons have shown high degrees of entanglement and indistinguishability but the probabilistic nature of their generation process also creates zero or multiple photon pairs following a Poissonian distribution. This limits their use in complex algorithms where many qubits and gate operations are required. Here we show simultaneously ultra-high purity (g^(2)(0) < 0.004), high entanglement fidelity (0.81 +/- 0.02), high two-photon interference non-post selective visibilities (0.86 +/- 0.03 and 0.71 +/- 0.04) and on-demand generation of polarization-entangled photon pairs from a single semiconductor quantum dot. Through a two-photon resonant excitation scheme, the biexciton population is deterministically prepared by a Pi-pulse. Applied on a quantum dot showing no exciton fine structure splitting, this results in the deterministic generation of indistinguishable entangled photon pairs.

Entangling an optical cavity and a nanomechanical resonator beam by means of a quantum dot

The stationary continuous-variable entanglement between an optical cavity and a nanomechanical resonator beam is generated by their common interaction with a quantum dot. To deal with the quantum dot which is modeled as a two-level system, we do not use the low excitation limit approximation to bosonize the spin operators, but we keep them in the quantum Langevin equations. We linearize the quantum Langevin equations reasonably and investigate the stationary continuous-variable entanglement in detail, and finally we show that a high degree of entanglement can be achieved for experimentally feasible parameters.

Quantification of Mixed-State Entanglement in a Quantum System Interacting with Two Time-Dependent Lasers

We give a theoretical description of two time-dependent laser beams in the Λ scheme using a unitary transformation method and a trapped three-level ion. We extend earlier investigations aimed at finding the three types of density matrices. We present figures showing that the entanglement degree accelerates due to the time-dependent interaction and the second-order terms of the Lamb–Dicke parameter η(t). Our results explain that the time-dependent ionic–phononic quantum system is observed at a higher degree of entanglement for three optimum times; these are, respectively, 16.5, 110, and 220 fs. These optimum entangled states can be modified for the structure of black holes in a probabilistic Universe.

Entanglement-based shape memory polyurethanes: Synthesis and characterization

In this paper, we report on the shape memory behavior of a family of hardblock-free multiblock thermoplastic polyurethanes (TPUs) consisting of poly(ε-caprolactone) (PCL) and poly(ethylene glycol) (PEG). Due to high molecular weight (Mn > 200 kDa), high degrees of entanglement were achieved. Instead of conventional “hard” blocks, entanglements served as the physical crosslinks in this system, slowing stress relaxation (suspending flow) above the melting temperatures of the soft blocks long enough to allow facile elastic deformation for shape fixing. Moreover, highly deformed samples (>800%) imparted by plastic deformation at room temperature completely recovered their as-cast shape within 1 min when heating above the transition temperature. Upon tensile deformation, the constituent chains and domains of both PCL and PEG phases became oriented, while heating-induced recovery reversed this orientation, as evidenced by wide-angle and small-angle X-ray diffraction studies. The observed large recoverable deformation and fast actuation make these materials strong candidates for applications in such medical devices as self-tightening sutures.

Everything is entangled

We show that big bang cosmology implies a high degree of entanglement of particles in the universe. In fact, a typical particle is entangled with many particles far outside our horizon. However, the entanglement is spread nearly uniformly so that two randomly chosen particles are unlikely to be directly entangled with each other – the reduced density matrix describing any pair is likely to be separable.

Effects of molecular entanglement on molecular dynamics and phase-separation kinetics of poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) blends

Poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends with various compositions were prepared through solution casting and melt blending. Two preparation routes, solution casting and melt blending, were used to achieve different degrees of molecular entanglement in the samples with solution casting giving rise to a lower degree of entanglement. Therefore, the effect of molecular entanglement on molecular dynamics and phase-separation kinetics of PMMA/SMA blends was investigated by using broadband dielectric spectroscopy and small-angle laser light scattering (SALLS). Molecular entanglement is found to have a pronounced effect on the α-relaxation process. The glass transition temperature (Tg) is related to the degree of entanglement and a higher degree of entanglement can result in a higher Tg which shifts to a higher temperature after annealing. The relaxation time (τ) of the α-relaxation process is lower for lower degrees of entanglement. Neither the dynamics nor the distribution width of the β-relaxation process is affected by degree of entanglement, regardless of the blend composition. The kinetics of phase-separation by spinodal decomposition (SD) in PMMA/SMA blends are however significantly influenced by the degrees of entanglement with decomposition rate being higher at lower degrees of entanglement.

High degree of entanglement and nonlocality of a two-photon state generated at 532 nm

In the last years the attention of the scientific community on the generation of entangled states has constantly increased both for their importance in the foundation of quantum mechanics and for their application in the quantum computation and communication field. To these aims high quality of generated states is required. A standard procedure to produce entangled photons pairs is spontaneous down conversion process in nonlinear crystals. In this paper we report preparation of quantum entangled states using CW laser at 266 nm pumping the standard Kwiat’s source. We have been able to generate the full set of Bell’s states with very high purity, fidelity and Concurrence which have been estimated using standard tomography procedure. To proof the high degree of achieved entanglement, we performed a non-locality test obtaining a high violation of the CHSH inequality.

Production of bright entangled photons from moving optical boundaries

We discuss a mechanism of generating two separable beams of light with high degree of entanglement in momentum using a fast and sharp optical boundary. Three regimes of light generation are identified depending on the number of resonant interactions between the optical perturbation and the electromagnetic field. The intensity of the process is discussed in terms of the relevant physical parameters: variation of refractive index and apparent velocity of the optical boundary. Our results suggest a different class of generation entangled light robust against thermal degradation by exciting zero point fluctuations using parametric resonant optical modulations.

Detecting highly entangled states with a joint qubit readout

A single-channel joint readout is used to analyze highly entangled two-qubit states in a circuit quantum electrodynamics architecture. The measurement model for the readout is fully characterized, demonstrating a large sensitivity to two-qubit correlations. We quantify the high degree of entanglement by measuring a violation of the Clauser-Horne-Shimony-Holt inequality with a value of $2.61\ifmmode\pm\else\textpm\fi{}0.04$, without optimizing the preparation of the two-qubit state. In its present form, this joint readout can resolve improvements to the fidelity of two-qubit operations and be extended to three or four qubits.

Ground state entanglement in one-dimensional translationally invariant quantum systems

We examine whether it is possible for one-dimensional translationally invariant Hamiltonians to have ground states with a high degree of entanglement. We present a family of translationally invariant Hamiltonians {Hn} for the infinite chain. The spectral gap of Hn is Ω(1/poly(n)). Moreover, for any state in the ground space of Hn and any m, there are regions of size m with entanglement entropy Ω(min{m,n}). A similar construction yields translationally invariant Hamiltonians for finite chains that have unique ground states exhibiting high entanglement. The area law proven by Hastings [“An area law for one dimensional quantum systems,” J. Stat. Mech.: Theory Exp. 2007 (08024)] gives a constant upper bound on the entanglement entropy for one-dimensional ground states that is independent of the size of the region but exponentially dependent on 1/Δ, where Δ is the spectral gap. This paper provides a lower bound, showing a family of Hamiltonians for which the entanglement entropy scales polynomially with 1/Δ. Previously, the best known such bound was logarithmic in 1/Δ.

Effect of feedback control on the entanglement evolution

We study the effect of feedback control on the entanglement evolution of two spins in a dissipative cavity governed by the Lindblad master equation. By numerically solving the master equation, we show that the entanglement can be controlled by the feedback based on the quantum jumps of the field in a leaking cavity. With the feedback added to the spins, the stable states with high degree of entanglement can be obtained in absence of the spontaneous decay of the spins, and the entanglement can also be generated for a period in presence of spontaneous decay of the spins. All the controlled entangled states are closely related to the initial states.

Entangled States Generation of Two Atoms Interacting with a Cavity

We classify different classes of entangled states arise in a two-qubit system. Some of these classes are of Bell's state types, while others are of the Werner's state types. The degree of entanglement is quantified for different values of the atomic and the cavity parameters. We show that it is possible to generate entangled state with high degree of entanglement by controlling the detuning and the number of photon inside the cavity.

Entangled photons from a strongly coupled quantum dot-cavity system

A quantum dot strongly coupled to a photonic crystal has been recently proposed as a source of entangled photon pairs [R. Johne et al., Phys. Rev. Lett. 100, 240404 (2008)]. The biexction decay via intermediate polariton states can be used to overcome the natural splitting between the exciton states coupled to the horizontally and vertically polarized light modes, so that high degrees of entanglement can be expected. We investigate theoretically the features of realistic dot-cavity systems, including the effect of the different oscillator strength of excitons resonances coupled to the different polarizations of light. We show that in this case, an independent adjustment of the cavity resonances is needed in order to keep a high entanglement degree. We also consider the case when the biexciton-exciton transition is also strongly coupled to a cavity mode. We show that a very fast emission rate can be achieved allowing the repetition rates in the THz range. Such fast emission should however be paid for by a very complex tuning of the many strongly coupled resonances involved and by a loss of quantum efficiency. Altogether a strongly coupled dot-cavity system seems to be very promising as a source of entangled photon pairs.