Nguyen Ba An

Joint remote preparation of a general two-qubit state

We propose a set of novel protocols to remotely prepare a general two-qubit state by joint actions of two separate people who independently share the classical knowledge of the state, without and with quantum control of a supervisor. Various quantum resources are used as the quantum channel. Although the execution complexity differs for different protocols, the total success probability is protocol independent.

Greenberger-Horne-Zeilinger-type and W-type entangled coherent states: Generation and Bell-type inequality tests without photon counting

We study Greenberger-Horne-Zeilinger-type (GHZ-type) and W-type three-mode entangled coherent states. Both types of entangled coherent states violate Mermins version of the Bell inequality with threshold photon detection (i.e., without photon counting). Such an experiment can be performed using linear optics elements and threshold detectors with significant Bell violations for GHZ-type entangled coherent states. However, to demonstrate Bell-type inequality violations for W-type entangled coherent states, additional nonlinear interactions are needed. We also propose an optical scheme to generate W-type entangled coherent states in free-traveling optical fields. The required resources for the generation are a single-photon source, a coherent state source, beam splitters, phase shifters, photodetectors, and Kerr nonlinearities. Our scheme does not necessarily require strong Kerr nonlinear interactions; i.e., weak nonlinearities can be used for the generation of the W-type entangled coherent states. Furthermore, it is also robust against inefficiencies of the single-photon source and the photon detectors.

Shortcuts to adiabatic passage for population transfer and maximum entanglement creation between two atoms in a cavity

We use the approach of “transitionless quantum driving” proposed by Berry to construct shortcuts to the population transfer and the creation of maximal entanglement between two � -type atoms based on the cavity quantum electronic dynamics system. An effective Hamiltonian is designed by resorting to an auxiliary excited level, a classical driving field, and an extra cavity field mode to supplement or substitute the original reference Hamiltonian, and steer the system evolution along its instantaneous eigenstates in an arbitrarily short time, speeding up the rate of population transfer and creation of maximal entanglement between the two atoms inside a cavity. Numerical simulation demonstrates that our shortcuts are robust against the decoherences caused by atomic spontaneous emission and cavity photon leakage.

Genuine multiqubit entanglement and controlled teleportation

We construct a genuine (2N+1)-qubit entangled state to perform controlled teleportation of an arbitrary N-qubit state. The constructed state is a complementarity to the genuine 2N-qubit entangled state constructed by Yeo and Chua for N=2 [Phys. Rev. Lett. 96, 060502 (2006)] and by Chen, Zhu, and Guo for any N [Phys. Rev. A 74, 032324 (2006)]. We also quantify the entanglement of the state and classify it with the well-known GHZ and W states by means of the recently proposed generalized global entanglement and the associated auxiliary measures [Phys. Rev. A 74, 022314 (2006)]. Our study is of general importance with respect to exploring and exploiting the genuine multiqubit entanglement.

COLLECTIVE REMOTE STATE PREPARATION

We show that any M ≥ 2 distant parties who independently share the complete classical knowledge of a secret qubit state can collectively prepare the state at another remote location. Two distinct schemes for such a task are proposed: one via a single (M + 1)-partite GHZ-type state and the other via M EPR-type pairs. Analytical expressions of the total success probability are derived explicitly for both the schemes. Of interest is the M-dependence of both the success probability and the receivers action. We also construct simple quantum circuits for the two-qubit operators whose execution is necessary to accomplish the schemes.

Joint remote preparation of four-qubit cluster-type states revisited

We revisit the protocols proposed recently (Zhan et al 2011 J. Phys. B: At. Mol. Opt. Phys. 44 095501) for joint remote preparation of four-qubit cluster-type states. We not only point out errors in those protocols but also make considerable improvements. Our protocols, for both the cases of real and complex coefficients of the state to be prepared, consume much less quantum resource as well as classical communication cost.

Deterministic joint remote preparation of an arbitrary three-qubit state via Einstein?Podolsky?Rosen pairs with a passive receiver

In this work, a useful protocol for deterministic joint remote preparation of an arbitrary three-qubit state using only Einstein–Podolsky–Rosen pairs is put forward. Our protocol, in clear contrast with a previous one, works for a receiver who, due to limited local equipments, is unable to carry out any measurements and controlled-NOT gates as well as to send classical communication. Therefore, the practical merit of our protocol is that it is applicable to situations in which the receiver has insufficient resource to meet the demand.

Entanglement measure and dynamics of multiqubit systems: non-Markovian versus Markovian and generalized monogamy relations

In this paper, we first present a simple measure for multiqubit entanglement based on the strategy of bipartite cuts and the measure of negativity. Then, we establish generalized monogamy inequalities and associated partition-dependent residual entanglement (PRE) accounting for arbitrary partitions of a multiqubit system. By virtue of the defined quantities, we investigate the entanglement dynamics of a system of N qubits, either in the Greenberger–Horne–Zeilinger (GHZ)-type state or in the W state, interacting with N independent reservoirs in both Markovian and non-Markovian regimes. We observe entanglement revivals of qubits at instantaneous points of disappearance or after a finite interval of abrupt vanishing due to the memory effect of non-Markovian reservoirs. We also follow the whole entanglement evolution in terms of the PRE to demonstrate the process of transition between the bipartite entanglement of all possible bipartitions and the multipartite entanglement. In particular, we show that the change in time of entanglement formats differs qualitatively for the GHZ-type and W states.

On conditions for atomic entanglement sudden death in cavity QED

In this paper, we study the entanglement dynamics of atoms coupled to cavity fields. We consider conditions characterized not only by the purity of the atomic initial state but also by the cavity photon number. By studying three different models within the framework of cavity QED, we show that the so-called atomic entanglement sudden death always occurs if initially the atomic state is sufficiently impure and/or the cavity photon number is nonzero.

Economical and feasible controlled teleportation of an arbitrary unknown N-qubit entangled state

We propose a new quantum protocol to teleport an arbitrary unknown N-qubit entangled state from a sender to a fixed receiver under the control of M (M < N) controllers. In comparison with other existing protocols, ours is more economical and more feasible. The quantum resource required is just M Greenberger–Horne–Zeilinger trios plus (N − M) Einstein–Podolsky–Rosen pairs. The techniques required are only N Bell measurements by the sender, a von Neumann measurement by a controller and N single-qubit transformations by the receiver. The rule for the receiver to reconstruct the desired state is derived explicitly in the most general case.