Scheme For Generation Of Entanglement Research Articles

Generation of multipartite entangled states based on a double-longitudinal-mode cavity optomechanical system.

An optomechanical system is a promising platform to connect different "notes" of quantum networks. Therefore, entanglements generated from it is also of great importance. In this paper, the parameter dependence of optomechanical and optical-optical entanglements generated from the double-longitudinal-mode cavity optomechanical system are discussed and two quadrapartite entanglement generation schemes based on such a system are proposed. Furthermore, 2N and 4N-partite entangled states of optical modes can be obtained by coupling N cavities that used in the above two schemes with N-1 beamsplitters, respectively. Certain ladder or linear entanglement structures are included in the finally obtained entangled state, which are important for its application in one-way quantum computing.

On safe post-selection for Bell tests with ideal detectors: Causal diagram approach

Reasoning about Bell nonlocality from the correlations observed in post-selected data is always a matter of concern. This is because conditioning on the outcomes is a source of non-causal correlations, known as aselection bias, rising doubts whether the conclusion concerns the actual causal process or maybe it is just an effect of processing the data. Yet, even in the idealised case without detection inefficiencies, post-selection is an integral part of experimental designs, not least because it is a part of the entanglement generation process itself. In this paper we discuss a broad class of scenarios with post-selection on multiple spatially distributed outcomes. A simple criterion is worked out, called theall-but-oneprinciple, showing when the conclusions about nonlocality from breaking Bell inequalities with post-selected data remain in force. Generality of this result, attained by adopting the high-level diagrammatic tools of causal inference, provides safe grounds for systematic reasoning based on the standard form of multipartite Bell inequalities in a wide array of entanglement generation schemes, without worrying about the dangers of selection bias. In particular, it can be applied to post-selection defined by single-particle events in each detection chanel when the number of particles in the system is conserved.

Optimizing practical entanglement distillation

The goal of entanglement distillation is to turn a large number of weakly entangled states into a smaller number of highly entangled ones. Practical entanglement distillation schemes offer a tradeoff between the fidelity to the target state, and the probability of successful distillation. Exploiting such tradeoffs is of interest in the design of quantum repeater protocols. Here, we present a number of methods to assess and optimize entanglement distillation schemes. We start by giving a numerical method to compute upper bounds on the maximum achievable fidelity for a desired probability of success. We show that this method performs well for many known examples by comparing it to well-known distillation protocols. This allows us to show optimality for many well-known distillation protocols for specific states of interest. As an example, we analytically prove optimality of the distillation protocol utilized within the Extreme Photon Loss (EPL) entanglement generation scheme, even in the asymptotic limit. We proceed to present a numerical method that can improve an existing distillation scheme for a given input state, and we present an example for which this method finds an optimal distillation protocol. An implementation of our numerical methods is available as a Julia package.

Multiphoton Knill-Laflamme-Milburn states generated by nonlinear optics

The Knill-Laflamme-Milburn (KLM) entangled state, proposed by Knill et al. in Nature409, 46 (2001)NATUAS0028-083610.1038/35051009, has unique advantages for linear optics quantum information processing tasks, because it is able to decrease the error rate. In this paper, we propose scalable schemes to generate the multi-qubit KLM entangled states with single-photon resources and weak cross-Kerr nonlinearity theoretically. We first construct the two-qubit KLM state, through which the three- and other n-qubit KLM states are generated. For the schemes using weak cross-Kerr nonlinearity, P homodyne measurement has a higher success rate than X homodyne measurement but with narrow applications since |αeiθ⟩ and |αe−iθ⟩ are distinguished from each other, which is undesirable for most of the entanglement generation schemes. To avoid this disadvantage, Jin et al. [Phys. Rev. A75, 054302 (2007)PLRAAN1050-294710.1103/PhysRevA.75.054302] introduced the coherent state superposition (CSS), c(|α⟩+|−α⟩), to generate a Greenberger-Horne-Zeilinger state but with a rapid oscillation term, which needs to be ignored. Through designing the optical path uniquely and choosing the phase of cross-Kerr nonlinearity suitably, our P homodyne measurement and CSS probe mode-based schemes to generate the KLM state avoid the needless term. The present scalable schemes have high total success probabilities, high fidelities even in consideration of decoherence, and may find potential applications in the future with the further development of nonlinear optics.

Experimental pre-assessing of two-mode entanglement in Gaussian state mixing

We suggest and demonstrate a method to assess entanglement generation schemes based on mixing of Gaussian states at a beam splitter (BS). Our method is based on the fidelity criterion and represents a tool to analyze the effect of losses and noise before the BS in both symmetric and asymmetric channels with and without thermal effects. More generally, our scheme allows one to pre-assess entanglement resources and to optimize the design of BS-based schemes for the generation of continuous variable entanglement.

Long-distance entanglement distribution using individual atoms in optical cavities

Individual atoms in optical cavities can provide an efficient interface between stationary qubits and flying qubits (photons), which is an essentiel building block for quantum communication. Furthermore, cavity assisted controlled-not (CNOT) gates can be used for swapping entanglement to long distances in a quantum repeater setup. Nonetheless, dissipation introduced by the cavity during the CNOT may increase the experimental difficulty in obtaining long-distance entanglement distribution using these systems. We analyse and compare a number of cavity-based repeater schemes combining various entanglement generation schemes and cavity assisted CNOT gates. We find that a scheme, where high-fidelity entanglement is first generated in a two-photon detection scheme and then swapped to long distances using a recently proposed heralded CZ-gate exhibits superior performance compared to the other schemes. The heralded gate moves the effect of dissipation from the fidelity to the success probability of the gate thereby enabling high-fidelity entanglement swapping. As a result, high-rate entanglement distribution can be achieved over long distances even for low cooperativities of the atom-cavity systems. This high-fidelity repeater is shown to outperform the other cavity-based schemes by up to two orders of magnitude in the rate for realistic parameters and large distances (1000 km).

Virtual-photon-induced entanglement with two nitrogen-vacancy centers coupled to a high-Q silica microsphere cavity

We propose a potentially practical scheme for efficient generation of entanglement with two nitrogen-vacancy centers (NVC) coupled to a whispering-gallery mode cavity. By virtue of the virtual-photon-excitation, the entanglement with two separate NVC can be produced in a deterministic way. The required operations are very close to the capabilities of current experimental techniques. The effects of decoherence induced by the cavity decay and the atomic spontaneous decay are also investigated.

Robust-to-loss entanglement generation using a quantum plasmonic nanoparticle array

We introduce a scheme for generating entanglement between two quantum dots using a plasmonic waveguide made from an array of metal nanoparticles. We show that the scheme is robust to loss, enabling it to work over long distance plasmonic nanoparticle arrays, as well as in the presence of other imperfections such as the detuning of the energy levels of the quantum dots. The scheme represents an alternative strategy to the previously introduced dissipative driven schemes for generating entanglement in plasmonic systems. Here, the entanglement is generated by using dipole-induced interference effects and detection-based postselection. Thus, contrary to the widely held view that loss is major problem for quantum plasmonic systems, we provide a robust-to-loss entanglement generation scheme that could be used as a versatile building block for quantum state engineering and control at the nanoscale.

Optimal entanglement manipulation via coherent-state transmission

We derive an optimal bound for arbitrary entanglement manipulation based on the transmission of a pulse in coherent states over a lossy channel followed by local operations and unlimited classical communication (LOCC). This stands on a theorem to reduce LOCC via a local unital qubit channel to local filtering. We also present an optimal protocol based on beam splitters and a quantum nondemolition (QND) measurement on photons. Even if we replace the QND measurement with photon detectors, the protocol outperforms known entanglement generation schemes.

Quantum communication through open-ended quantum networks

We study a one-dimensional fibre-coupled cavity array with an open-end boundary condition, where each cavity is doped with a two-level atom, and obtain explicit expressions for the dynamics evolution of the array with arbitrary number of nodes in the far off-resonant condition. It is shown that distant state transfer and entanglement generation schemes may be implemented through this quantum network and an arbitrary state is transferred with high fidelity for the array with specific number of nodes. The entanglement transfer through both single array and double independent arrays is also examined.

Effective dynamics for two-atom entanglement and quantum information processing by coupled cavity QED systems

We propose a scheme for deterministic generation of entanglement embodied by two \(\Lambda \)-type atoms distributed in two coupled cavities. We study such a system in the dispersive atom-field interactions, where the dynamics of the system operates through the virtual population of both the atomic excited states and the photonic states in the cavities (plus the fiber). We verify the validity of the dynamics, and moreover, study the influences of the decoherence due to the spontaneous emission and photon leakage. We also apply the dynamics for realizing quantum state transfer and quantum phase gates.

Two Schemes for Generation of Entanglement for Vibronic CollectiveStates of Multiple Trapped Ions

We propose two schemes to prepare entanglement for the vibroniccollective states of multiple trapped ions. The first scheme aimsto generating multipartite entanglement for vibrational modes oftrapped ions, which only requires a single laser beam tuned to theionic carrier frequency. Our scheme works in the mediatedexcitation regime, in which the corresponding Rabi frequency isequal to the trap frequency. Beyond their fundamental importance,these states may be of interest for experimental studies ondecoherence since the present scheme operates in a fast way. Thesecond scheme aims to preparing the continuous variable multimodemaximally Greenberger–Horne–Zeilinger state.The distinct advantage is that the operationtime is only limited by the available laser intensity, not by theinherent mechanisms such as off-resonant excitations. This makesit promising to obtain entanglement of multiple coherent andsqueezing states with desired amplitudes in a reasonable time.

Generation of Multipartite Ionic Entangled States only bySingle-Qubit Measurements via Linear Optical Elements

We propose a scheme for generation of multipartite ionicGreenberger–Horne–Zeilinger (GHZ) states only by single-qubitmeasurements. Our scheme not only does not need jointmeasurements but also avoids the difficulty of synchronizing thearrival time of the two scattered photons, which is faced by theprevious schemes. Therefore our entanglement generation scheme canbe implemented more easily than the schemes based on atomicinterference in experiment.

Robust Entanglement through Macroscopic Quantum Jumps

We propose an entanglement generation scheme that requires neither the coherent evolution of a quantum system nor the detection of single photons. Instead, the desired state is heralded by a macroscopic quantum jump. Macroscopic quantum jumps manifest themselves as a random telegraph signal with long intervals of intense fluorescence (light periods) interrupted by the complete absence of photons (dark periods). Here we show that a system of two atoms trapped inside an optical cavity can be designed such that a dark period prepares the atoms in a maximally entangled ground state. Achieving fidelities above 0.9 is possible even when the single-atom cooperativity parameter is as low as 10 and when using a photon detector with an efficiency as low as eta=0.2.

Generation of pure ionic entangled states via linear optics

In this paper, we propose a scheme to generate two-ion maximally entangled states from either pure product states or mixed states using linear optics. Our new scheme is mainly based on the ionic interference. Because the proposed scheme can generate pure maximally entangled states from mixed states, we denote it as a purificationlike generation scheme. The scheme does not need a Bell state analyzer as the existing entanglement generation schemes do, it also avoids the difficulty of synchronizing the arrival time of the two scattered photons faced by the existing schemes, thus the proposed new entanglement generation scheme can be implemented more easily in practice.

Efficient generation of graph states for quantum computation

We present an entanglement generation scheme which allows arbitrary graph states to be efficiently created in a linear quantum register via an auxiliary entangling bus (EB). The dynamical evolution of the EB is described by an effective non-interacting fermionic system undergoing mirror-inversion in which qubits, encoded as local fermionic modes, become entangled purely by Fermi statistics. We discuss a possible implementation using two species of neutral atoms stored in an optical lattice and find that the scheme is realistic in its requirements even in the presence of noise.

Scheme for on-resonance generation of entanglement in time-dependent asymmetric two-qubit-cavity systems

We present an efficient scheme for the controlled generation of pure two-qubit states possessing any desired degree of entanglement and a prescribed symmetry. This is achieved in two-qubit-cavity QED systems (e.g., cold-trapped ions and flying atoms) via on-resonance ion- or atom-cavity couplings, which are time dependent and asymmetric, yielding a trapping vacuum state condition which does not arise for identical couplings. A duality in the role of the coupling ratio yields states with a given concurrence but opposing symmetries. Both the trapping state condition and the resulting entanglement power are robust against decoherence channels.

Scheme for Generation of Entanglement among Bimodal Cavities

We present a scheme for generation of an entangled state in manyspatially separated bimodal cavity modes via cavity quantumelectrodynamics. A V-type three-level atom, initially prepared in acoherent superposition of its excited states, successively passesthrough both the bimodal cavities. If the atom is measured in its ground stateafter leaving the last cavity, an entangled state of many cavity modescan be generated. The conditions to generate the maximally entangledstate with unity probability are worked out.

Impurity scattering induced entanglement of ballistic electrons.

We show how entanglement between two conduction electrons is generated in the presence of a localized magnetic impurity embedded in an otherwise ballistic conductor of special geometry. This process is a generalization of beam-splitter mediated entanglement generation schemes with a localized spin placed at the site of the beam splitter. Our entangling scheme is unconditional and robust to randomness of the initial state of the impurity. The entangled state generated manifests itself in noise reduction of spin-dependent currents.