Preparation Of Entangled States Research Articles

Two-qutrit maximally entangled states prepared via adiabatic passage in ion-trapped system

This paper considers a scheme for the preparation of two-qutrit entangled states via adiabatic passage in ion-trapped system. In the proposal, the two three-level V-type ions are initially cooled to the ground states and need not be separately addressed. Moreover, only the ionic states act as memory and the system evolves in the dark space during the whole procedure, which makes the system robust against the decoherence and the fluctuation of the laser pulse.

Preparation of genuine Yeo-Chua entangled state and teleportation of two-atom state via cavity QED

We first propose a scheme for preparing the genuine Yeo-Chua 4-qubit entangled state via cavity QED. Using the genuine Yeo-Chua atomic state, we further propose a cavity QED scheme for teleporting an arbitrary two-atom state. In two schemes the large-detuning is chosen and the necessary time is designed to be much shorter than Rydberg-atom’s lifespan. Both schemes share the distinct advantage that cavity decay and atom decay can be neglected. As for the interaction manipulation, our preparation scheme is more feasible than a recent similar one. Compared with the Yeo and Chua’s scheme, our teleportation scheme has significantly reduced the measuring difficulty.

An Efficient and Economic Scheme for Remotely Preparing a Multi-Qudit State via a Single Entangled Qudit Pair

We propose some schemes for remote preparation of arbitrary high-dimensional equatorial entangled state via a single bipartite high-dimensional entangled state as quantum channel. We firstly present the remote preparation of bipartite three- and d-dimensional equatorial entangled state by using a single entangled qutrit and qudit pair, respectively, and then directly generalize the schemes to multipartite case. The cases of the quantum channel being non-maximally two-qutrit and two-qudit entangled state are also considered, respectively. In these schemes the required resources are single-particle projective measurement, appropriate local unitary operation, auxiliary particle, and high-dimensional C-NOT operation. It is shown that the entanglement resource and classical communication cost are both greatly reduced in our schemes.

Unconditional preparation of multipartite continuous-variable entangled states among remote parties

We demonstrate that the n-partite continuous-variable entanglement can be unconditionally prepared among n parties that share no common past, from n two-mode squeezed states. Both GHZ-like and cluster-like states can be generated for any nonzero squeezing in the entangled sources. An application of the resulting multipartite entangled state to a teleportation network is illustrated.

Preparation of multipartite entangled states and one-step implementation of 1→M economical phase-covariant quantum anti-cloning in cavity QED

The scheme for preparing a multipartite entangled state and implementing 1→M economical phase-covariant quantum anti-cloning only by one step are proposed. By using the symmetric and asymmetric interactions between N atoms and the cavity, N-particle Greenberger–Horne–Zeilinger (GHZ) and W states are prepared, respectively. Furthermore, a scheme of 1→M anti-cloning is also proposed through preparing the W state. The decoherence from atomic spontaneous emission is negligible due to the excited state of the atom being adiabatically eliminated under large detuning. The scheme is insensitive to the cavity decay due to the cavity only being virtually excited. The experimental feasibility of the present scheme is also discussed.

Remote Preparation of Multipartite Equatorial Entangled States in High Dimensions with Three Parties

A scheme for probabilistic remotely preparing N-particle d-dimensional equatorial entangled states via entangled swapping with three parties is presented. The quantum channel is composed of N – 1 pairs of bipartite d-dimensional non-maximally entangled states and a tripartite d-dimension non-maximally entangled state. It is shown that the sender can help either of the two receivers to remotely prepare the original state, and the N-particle projective measurement and the generalized Hadamard transformation are needed in this scheme. The total success probability and classical communication cost are calculated.

High efficient scheme for remote state preparation with cavity QED

In this paper, a scheme is proposed for remote state preparation (RSP) with cavity quantum electrodynamics (QED). In our scheme, two observers share two-atom nonmaximally entangled state as quantum channels and can realize remote preparation of state of an atom. We also propose a generalization for remote preparation of N-atom entangled state by (N+1)-atom GHZ-like state (N ≥ 2). By this scheme, one single-atom projective measurement is enough for the RSP of a qubit or N-atom entangled state, and the probability of success for RSP is unity. Furthermore, we have considered the case where observers use W-like state as quantum channels to realize RSP of a qubit. We compare our scheme with existing ones.

Gaussian Quantum Marginal Problem

The quantum marginal problem asks what local spectra are consistent with a given spectrum of a joint state of a composite quantum system. This setting, also referred to as the question of the compatibility of local spectra, has several applications in quantum information theory. Here, we introduce the analogue of this statement for Gaussian states for any number of modes, and solve it in generality, for pure and mixed states, both concerning necessary and sufficient conditions. Formally, our result can be viewed as an analogue of the Sing-Thompson Theorem (respectively Horn’s Lemma), characterizing the relationship between main diagonal elements and singular values of a complex matrix: We find necessary and sufficient conditions for vectors (d 1,..., d n ) and (c 1,..., c n ) to be the symplectic eigenvalues and symplectic main diagonal elements of a strictly positive real matrix, respectively. More physically speaking, this result determines what local temperatures or entropies are consistent with a pure or mixed Gaussian state of several modes. We find that this result implies a solution to the problem of sharing of entanglement in pure Gaussian states and allows for estimating the global entropy of non-Gaussian states based on local measurements. Implications to the actual preparation of multi-mode continuous-variable entangled states are discussed. We compare the findings with the marginal problem for qubits, the solution of which for pure states has a strikingly similar and in fact simple form.

Mesoscopic entanglement of noninteracting qubits using collective spontaneous emission

We describe an experimentally straightforward method for preparing an entangled W state of at least 100 qubits. Our repeat-until-success protocol relies on detection of single photons from collective spontaneous emission in free space. Our method allows entanglement preparation in a wide range of qubit implementations that lack entangling qubit–qubit interactions. We give detailed numerical examples for entanglement of neutral atoms in optical lattices and of nitrogen-vacancy centres in diamond. The simplicity of our method should enable preparation of mesoscopic entangled states in a number of physical systems in the near future.

Photon number projection using non-number-resolving detectors

Number-resolving photo-detection is necessary for many quantum optics experiments, especially in the application of entangled state preparation. Several schemes have been proposed for approximating number-resolving photo-detection using non-number-resolving detectors. Such techniques include multi-port detection and time-division multiplexing. We provide a detailed analysis and comparison of different number-resolving detection schemes, with a view to creating a useful reference for experimentalists. We show that the ideal architecture for projective measurements is a function of the detector's dark count and efficiency parameters. We also describe a process for selecting an appropriate topology given actual experimental component parameters.

Using biharmonic laser pumping for preparation of pure and entangled multiexciton states in clusters of resonantly interacting fluorescent centres

Use of a biharmonic laser pumping for preparation of pure and entangled multiexciton states in dimers and tetramers of resonantly interacting fluorescent particles is analysed. Special emphasis is given to the preparation of all possible pure exciton states and their maximally entangled Bell states. The general results are illustrated using as an example the pair and quartet centres of neodymium ions in calcium fluoride ( M- and N-centres), where all necessary experimental information concerning the interactions and decoherence is available, and experimental preparation of Bell vacuum–single exciton and vacuum–biexciton states has been recently demonstrated. These results can be easily rescaled for the cases of quantum dots and dye molecules. Numerical results are compared with the analytical results obtained for a particular case of the biharmonic excitation of dimers. Excellent agreement between these approaches is demonstrated.

Scalable preparation of multiple-particle entangled states via the cavity input-output process

We propose schemes for generating multiple-atom entangled states and a multiple-photon Greenberger-Horne-Zeilinger state, respectively, based on the input-output relation of the cavity. The numerical simulations show that produced multiple-particle entangled states have high fidelity even if the atoms are not localized in the Lamb-Dicke regime. Some practical quantum noises, such as atomic spontaneous emission and output coupling inefficiency, only decrease the success probability but exert no influence on the fidelity of prepared multiple-particle entangled states. The successful probabilities of our protocols approach unity in the ideal case. In addition, no need for individually addressing keeps the schemes easy to implement from the experimental point of view.

A simple scheme for conditional generation of macroscopic entangled states using χ(2) nonlinearity

Experimental arrangement consisting of spontaneous parametric down converter with type-I phase matching (SPDCI) and a beam splitter with unitary Hadamard transformation is proposed for conditional preparation of macroscopic entangled states. The given method requires no photon number resolving detection to generate most probable macroscopic entangled state. We calculate the amount of entanglement stored in the macroscopic entangled states and develop a method to increase the amount of entanglement stored in the state.

A simple scheme with coupled down converters with type-I phase matching as resource for conditional preparation of macroscopic entangled states

It is shown that macroscopic entangled states can be generated using an experimental arrangement consisting of coupled spontaneous parametric down-converters with type-I phase matching (SPDCI) pumped simultaneously by optical fields in coherent state and two beam splitters. Two beam splitters in auxiliary generated modes are used to conditionally prepare macroscopic entangled states in output pumping modes of the studied system. Identification of two macroscopic entangled states is produced by use of photon number resolving detection. In contrast to all previous schemes, our scheme does not need Kerr-type nonlinear interaction and is purely based on second-order susceptibility of the crystal which is stronger for the Kerr nonlinearity. We calculate concurrence of the states as a measure of the amount of entanglement stored in the states and present analysis concerning ‘separation’ between components forming studied entangled states.

Quantum communication cost of preparing multipartite entanglement

We study the preparation and distribution of high-fidelity multiparty entangled states via noisy channels and operations. In the particular case of Greenberger-Horne-Zeilinger and cluster states, we study different strategies using bipartite or multipartite purification protocols. The most efficient strategy depends on the target fidelity one wishes to achieve and on the quality of transmission channel and local operations. We show the existence of a crossing point beyond which the strategy making use of the purification of the state as a whole is more efficient than a strategy in which pairs are purified before they are connected to the final state. We also study the efficiency of intermediate strategies, including sequences of purification and connection. We show that a multipartite strategy is to be used if one wishes to achieve high fidelity, whereas a bipartite strategy gives a better yield for low target fidelity.

Parity meter for charge qubits: An efficient quantum entangler

We propose a realization of a charge parity meter based on two double quantum dots alongside a quantum point contact. Such a device is a specific example of the general class of mesoscopic quadratic quantum measurement detectors previously investigated by Mao et al. [Phys. Rev. Lett. 93, 056803 (2004)]. Our setup accomplishes entangled state preparation by a current measurement alone, and allows the qubits to be effectively decoupled by pinching off the parity meter. Two applications of the parity meter are discussed: the measurement of Bell's inequality in charge qubits and the realization of a controlled NOT gate.

Conditional preparation of X (2) macroscopic entangled states

Two experimental arrangements consisting of coupled spontaneous parametric down-converters with type-I phase matching pumped simultaneously by a powerful optical field in a coherent state through a balanced beam splitter and linear optical elements are proposed for conditional preparation of macroscopic entangled states in output pumping modes of the studied system. Successful generation of the macroscopic entangled state in the pumping modes is unambiguously heralded by coincident detection of two photons in the generated signal and idler modes of the system. We calculate the amount of entanglement and probabilities of successfully observing the X (2) macroscopic entangled states in the total wavefunction. We show that the proposed schemes can be used to obtain a new type of macroscopic entangled states.

Preparation and Decoherence of Two-Atom Entangled States in aDissipative Cavity

We present a scheme for generating four pairs of two-atomEinstein–Podolsky–Rosen (EPR) states using the simultaneousinteraction of the two atoms with a single-mode cavity field under alarge detuning condition. The influence of cavity dissipation on theprepared EPR states is investigated by means of the superoperatormethod and the state fidelity. It is shown that some kinds of theprepared EPR states are robust against cavity dissipation and theintensity of the field, and maintain their entanglement invariance, andthe others are fragile and completely destroyed by the action of cavitydissipation and the intensity of the field in the long-time limit.Decoherence time of the fragile entangled states is extremely small fora typical cavity-QED experimental data.

Quantum teleportation with qubits

An overview of experiments on quantum teleportation with qubits is presented. Issues are analyzed concerning Bell-state measurement and entangled-state preparation. Unitary transformations are discussed that are required in the teleportation of an unknown or a known state.

Efficient scheme for multipartite entanglement and quantum information processing with trapped ions

In this paper, based on the recent experiment by Roos et al. [Science 304, 1478 (2004)], a theoretical scheme is proposed to create the multipartite entanglement of many trapped ions and implement a two-qubit quantum phase gate between two ions in ion trap. In the scheme, the ion is illuminated by a single laser tuned to the first lower vibrational sideband. We also show that the scheme can be used to directly transfer information between two ions. The scheme has the advantage that it does not use the vibrational mode as the data bus and only requires a single resonant interaction. Thus the scheme is very simple and the quantum dynamics operation can be realized at a high speed. In view of the decoherence mechanism, the simplification for the entangled state preparation and experimental implementation of quantum logic operation may become crucial.