International Journal of Quantum Information

We construct quantum gates entanglers for different classes of multipartite states. In particular, we construct entangler operators for W and GHZ classes of multipartite states based on the construction of the concurrence classes. We also discuss in detail these two classes of the quantum gates entanglers for three-partite states.

The minimal time, T^Shor, in which a one-way quantum computer can execute Shor's algorithm is derived. In the absence of an external magnetic field, this quantity diverges at very small temperatures. This result coincides with that of Anders et al. obtained simultaneously to ours but using thermodynamical arguments. Such divergence contradicts the common belief that it is possible to do quantum computation at low temperatures. It is shown that in the presence of a weak external magnetic field, T^Shor becomes a quantized quantity which vanishes at zero temperature. Decoherence is not a problem because T^Shor/τ_dec < 10^-9, where τ_dec is decoherence time.

We study eavesdropping in quantum key distribution with the six state protocol, when the signal states are mixed with white noise. This situation may arise either when Alice deliberately adds noise to the signal states before they leave her lab, or in a realistic scenario where Eve cannot replace the noisy quantum channel by a noiseless one. We find Eve's optimal mutual information with Alice, for individual attacks, as a function of the qubit error rate. Our result is that added quantum noise reduces Eve's mutual information more than Bob's.

An experimentally feasible scheme for implementing quantum dense coding in cavity QED is proposed. In the scheme a W-class state can first be prepared by letting the atoms interact simultaneously with a highly detuned cavity mode. Then the states coded on this W-class state can be exactly distinguished by detecting atomic states. The scheme is insensitive to the cavity field, and the quantum dense coding can be realized in a simple way.

We propose a feasible scheme for the quantum teleportation of tripartite entangled coherent state in terms of linear optical devices such as beam splitters, phase shifters and photo detectors. The scheme is based on the bipartite maximally entangled coherent state and the tripartite entangled coherent state with bipartite maximal entanglement as quantum channels. It shows that for an appreciable mean number of photons equal to 2, the total minimum of average fidelity for an arbitrary tripartite entangled state is 0.981684.

Properties and structure of the 15-dimensional parameter space of four-state density matrices are examined using the SU(4) generator expansion. Appropriate classification of one-, two- and three-parameter density matrices is obtained, based on the sameness of the characteristic polynomial of density matrices belonging to a given type. It is found that in the one parameter case of 15 different density matrices only three distinct types exist, while in the two parameter case 105 different density matrices group into 11 distinct types. In the three parameter case appropriate classification of 455 different density matrices into 44 types is determined. Two- and three-dimensional cross sections of the space of generalized Bloch vectors are illustrated by randomly drawing matrices for several types of density matrices, providing some insight into the intricate and complex structure of the space of density matrices for a four-state system. Positions of the representative points corresponding to the pure states are found for all types. Global properties of observables are determined by generating, by the Monte Carlo sampling method, and averaging over nearly all density matrices pertaining to a given type.

In this paper we propose a four-party scheme for a sender to achieve the tripartition of his/her arbitrary single-qubit quantum information among three recipients via an asymmetric four-qubit W state as quantum channels. In the scheme, if and only if, the three recipients cooperate together, they can perfectly retrieve the sender's quantum information by performing first two 2-qubit collective unitary operations and then a single-qubit unitary operation. The scheme is symmetric with respect to the reconstruction for any recipient can conclusively recover the quantum information with the other two's helps.

We consider a semiconductor quantum well in the excitation system. Applying the pertinent Hamiltonian, we investigate the second-order intensity correlation function and the entanglement properties between the cavity and exciton mode. It is found that nonclassical (antibunching) effect and sudden death effect occur in our system.

A scheme of realizing controlled dense coding via generalized measurement was presented. In this protocol, the supervisor can control the entanglement between the sender and the receiver and then the average amount of information transmitted from the sender to the receiver by only adjusting measurement angle θ. It is shown that when the quantum channel was a GHZ state, the entanglement and the average amount of information are determined by supervisor's measurement angle θ only; whereas when the quantum channel was a GHZ-class state, those are determined not only by supervisor's measurement angle θ but also the minimal coefficient of the GHZ-class state.

We construct new families of duadic codes and derive from them degenerate quantum codes. This provides a solution to a question raised by Aly, Klappenecker and Sarvepalli on the existence of quantum codes from duadic codes when the multiplicative order of q modulo n is even. We also characterize the affine-invariant maximal extended cyclic codes. Then by the CSS construction, we give a family of quantum code.

The entanglement evolution of a pair of two-level systems is studied in this paper. The two systems without mutual interaction are independently coupled to different two-band non-Markovian environments. By comparing our results to others in the literature, we find that taking one of the Bell states as the initial state, certain non-Markovian effect protects the entanglement in short time scale, while on long time scale it leads to the entanglement sudden death (ESD), which never occurs for this initial state in the Markovian case. Finally, by analyzing the parameters in our model, a relation between disentanglement and decoherence is established and discussed.

In this letter we show that in the relativistic regime, maximally entangled state of two spin-1/2 particles not only gives maximal violation of the Bell-CHSH inequality but also gives the largest violation attainable for any pairs of four spin observables that are noncommuting for both systems. Also, we extend our results to three spin-1/2 particles. We obtain the largest eigenvalue of Bell operator and show that this value is equal to the expectation value of Bell operator on GHZ state.

An experimentally feasible scheme for teleporting an arbitrary and unknown entangled state is proposed. In this paper, we use a cluster state as the quantum channel, and we do not need any joint Bell-state measurement (BSM). Our scheme is based on Josephson charge qubits, and the successful probability and fidelity of teleportation can both reach unity. Moreover, the current scheme can be realized within the current experimental technology.

In a number of Internet applications, we need to search for objects to download them. This includes peer-to-peer (P2P) file sharing, grid computing and content distribution networks. Here the single object will be searched for in multiple servers. There are many searching algorithms in existence today for this purpose based on the concept of classical physics and classical algorithms. The principles of quantum mechanics can be used to build and analyze a quantum computer and its algorithms. Quantum searching is such an algorithm. In this paper we are proposing a search method based on the quantum physics and quantum algorithms. Our method, the targeted quantum search is found to be more cost effective than any other classical searching algorithms like linear and two-way linear, simulated annealing, including broadcast based searching. Our targeted quantum search method is analyzed and simulated to show the best results.

The three-color continuous-variable entanglement directly produced by a cascaded nonlinear process in an optical superlattice was investigated theoretically. This cascaded nonlinear process can be realized by using quasi-phase-matching technique in a quasiperiodic superlattice. In the first step, signal- and idle-beams can be generated by a parametric down-conversion. Then, the third beam can be obtained through sum-frequency of idle and pump. The degree of quadrature phase amplitude correlations between the three modes was discussed by a sufficient inseparability criterion with a single inequality for continuous-variable entanglement.

The physical motivation for the mathematical formalism of quantum mechanics is made clear and compelling by starting from an obvious fact — essentially, the stability of matter — and inquiring into its preconditions: what does it take to make this obvious fact possible?