We analyze the controlled teleportation protocol through three-qubit mixed states. In particular, we investigate the relation between the faithfulness of the controlled teleportation scheme and entanglement. While our knowledge concerning controlled teleportation and entanglement in pure states is well established, for mixed states it is considerably much harder task and very little has been done in this field. Here, we present counterintuitive results that provide a new light on controlled teleportation protocol. It is shown that even mixed biseparable states are useful for this protocol along with genuine entangled three-qubit states.

We discuss a model comprising a chain of three Kerr-like nonlinear oscillators pumped by two modes of external coherent field. We show that the system can be treated as nonlinear quantum scissors and behave as a three-qubit model. For such situation different types of tripartite entangled states can be generated, even when damping effects are present in the system. Some amount of such entanglement can survive even in a long-time limit. The flow of bipartite entanglement between subsystems of the model and relations among first-, second-order correlations and the entanglement are discussed.

We report an experimental implementation of tripartite controlled quantum teleportation on the quantum optical devices. The protocol is performed through bi-and tripartite entangled channels of discrete variables and qubits encoded in polarization of individual photons. The experimental results demonstrate successful controlled quantum teleportation with a fidelity around 83%, well above the classical limit. By realizing the controlled quantum teleportation through biseparable state, we show that tripartite entangled is not a necessary resource for controlled quantum teleportation and the controller's capability to allow or prohibit the teleportation cannot be considered to be a manifestation of tripartite entanglement. These results open new possibilities for further application of controlled quantum teleportation by lowering teleportation channel's requirements.Introduction.-Quantum teleportation is considered as one of the major protocols in quantum information science. By exploiting the physical resource of entanglement, quantum teleportation has played a prominent role in the development of quantum information theory [1][2][3][4][5] and represents a fundamental ingredient to the progress of many quantum technologies such as quantum gate teleportation [6], quantum repeaters [7,8], measurementbased quantum computing [9], port-based teleportation [10] and quantum network teleportation (QTN) [11][12][13]. Teleportation has also been used as a quantum simulator for 'extreme' phenomena, such as closed timelike curves and the grandfather paradox [14].Quantum teleportation, first proposed by Bennett et al.[1], is a scheme of quantum information processing which allows the transfer of a quantum state between remote physical systems without physical transfer of the information carrier. Specifically, an unknown quantum state of a physical system is measured and subsequently reconstructed at a remote location through the use of classical communication and quantum entanglement [15,16]. Without entanglement, such quantum state transfer would not be possible within the laws of quantum mechanics. For that reason, quantum teleportation is thought of as the quantum information protocol which clearly demonstrates the character of quantum entanglement as a resource.To date, quantum teleportation has been achieved and studied in many different systems, including photonic systems, nuclear magnetic resonance, optical modes, trapped atoms and solid-state systems (see [17] and references therein). Naturally, most attention has been focused on teleporting the state on long-distance [18,19] with the recent satellite-based implementations [20]. However, even though quantum teleportation is a typically bipartite process, it can be extended to multipartite quantum protocols which have not been thoroughly studied yet. Such multipartite protocols are expected to form fundamental components for larger-scale quantum

The excess adsorption Γ in two-dimensional Ising strips (∞ × L) subject to identical boundary fields, at both one-dimensional surfaces decaying in the orthogonal direction j as −h1j −p , is studied for various values of p and along various thermodynamic paths below the critical point by means of the density-matrix renormalization-group method. The crossover behavior between the complete wetting and critical adsorption regimes, occurring in semi-infinite systems, are strongly influenced by confinement effects. Along isotherms T = const the asymptotic power law dependences on the external bulk field, which characterize these two regimes, are undercut by capillary condensation. Along the pseudo first-order phase coexistence line of the strips, which varies with temperature, we find a broad crossover regime where both the thickness of the wetting film and Γ increase as function of the reduced temperature τ but do not follow any power law. Above the wetting temperature the order parameter profiles are not slab-like but exhibit wide interfacial variations and pronounced tails. Inter alia, our explicit calculations demonstrate that, contrary to opposite claims by Kroll and Lipowsky [Phys. Rev. B 28, 5273 (1983)], for p = 2 critical wetting transitions do exist and we determine the corresponding wetting phase diagram in the (h1, T ) plane.