International Journal of Quantum Information

We have investigated the quantum discord (QD) of the thermal density matrix of spin-1/2 Heisenberg chains with Dzyaloshinskii–Moriya (DM) interaction. With fermionization technique, we study the mutual effect of DM interaction and the external magnetic field on the QD and the entanglement. Our analysis implies that the DM interaction can enhance the QD while the external magnetic field will shrink the QD. By a comparison between the entanglement and the QD, we find that the QD is more robust to the temperature and to the external magnetic field than the entanglement of formation (EoF) in the sense that the EoF takes a zero value while the QD does not for high temperature and strong external magnetic field. This point confirms the conclusion that there exist some separable states containing non-zero QD.

We discuss a problem of parameter estimation for quantum two-level system, qubit system, in presence of unknown phase parameter. We analyze trade-off relations for mean square errors (MSEs) when estimating relevant parameters with separable measurements based on known precision bounds; the symmetric logarithmic derivative (SLD) Cramér–Rao (CR) bound and Hayashi–Gill–Massar (HGM) bound. We investigate the optimal measurement which attains the HGM bound and discuss its properties. We show that the HGM bound for relevant parameters can be attained asymptotically by using some fraction of given n quantum states to estimate the phase parameter. We also discuss the Holevo bound which can be attained asymptotically by a collective measurement.

The use of the B_n group operators is developed for implementing entangling processes for large n-qubits systems (n > 2). By operating with the B_n group operators on the computational basis of states, entangled states are obtained with properties analogous to those of the 2-qubits Bell states. Tracing any qubit from the 3-qubits B_n entangled states, we get mixed states which are separable with zero concurrence, and correspondingly with only classical correlations. These properties are generalized to larger B_n entangled states (n > 3). The properties of the B_n entangled states are analyzed by the use of Hilbert–Schmidt (HS) decompositions.

The entanglement dynamics of two identical atoms coupled to a single-mode cavity is considered. Based on the reduced non-perturbative quantum master equation method, the entanglement evolution of the two atoms with spontaneous emission under the quantum-jump-based feedback control is investigated beyond the conventional rotating-wave approximation (RWA). The research of entanglement evolution beyond RWA is a significant problem because of its importance to the field of strong coupling. The result of numerical simulations shows that an appropriate feedback control can prolong the entanglement time and increase the entanglement amplitude. The effect of synchronous control on two atoms is superior to that of one control.

We present a scheme for transferring quantum state between atom and cavity field in Jaynes–Cummings model (JC) in the aid of spin-echo-like technique. It is based on the facts that the atom in a cavity can induce the generation of modified coherent states (MCSs), which can be shown to be macroscopically distinguishable, and the anti-commutation relation between the Hamiltonian and the z-component Pauli matrix. We show that this scheme is applicable for a class of cavity field states. The application on two-cavity system provides an alternative scheme for preparation of non-local superpositions of quasi-classical light states. Numerical simulation shows that the proposed schemes are efficient.

Entanglement-assisted quantum error correcting code (EAQECC) is a generalization of standard stabilizer quantum code. Maximal entanglement EAQECCs can achieve the EA-hashing bound asymptotically. In this work, we give elementary recursive constructions of quaternary zero radical codes with dual distance three for all n ≥ 4. Consequently, good maximal entanglement EAQECCs of minimum distance three for such length n are obtained. Almost all of these EAQECCs are optimal or near optimal according to the EA-quantum Hamming bound.

We present a scheme to implement a quantum gate for two-atom trapped in distant cavities connected via an optical fiber. In the whole process, the atomic system, the cavity modes and the fiber are not excited ensuring that the operation is insensitive to atomic spontaneous emission, cavity decay and the fiber loss. This scheme is significant for distributed and scalable quantum computation.

We study the dynamics of four-qubit W state under various noisy environments by solving analytically the master equation in the Lindblad form in which the Lindblad operators correspond to the Pauli matrices and describe the decoherence of states. Also, we investigate the dynamics of the entanglement using the lower bound to the concurrence. It is found that while the entanglement decreases monotonically for Pauli-Z noise, it decays suddenly for other three noises. Moreover, by studying the time evolution of entanglement of various maximally entangled four-qubit states, we indicate that the four-qubit W state is more robust under same-axis Pauli channels. Furthermore, three-qubit W state preserves more entanglement with respect to the four-qubit W state, except for the Pauli-Z noise.

The rate region for transmission of classical information over a quantum broadcast channel and an inner bound for the rate of transmission of private classical information over a single-user quantum channel have been achieved in the previous paper. In this work, we extended the proofs of them to obtain an achievable private classical capacity for a two receivers quantum broadcast channel.

We give the analytic expression for the information-theoretical symmetric discord of one type of two-qubit X states which depend on only four real parameters. As an application, we study the symmetric discord in a model, which shows that the numerical and analytic results are well consistent.