Generation of cluster states with Josephson charge qubits

Abstract

A scheme for the generation of the cluster states based on the Josephson charge qubit is proposed. The twoqubit generating case is first introduced, and then generalized to multi-qubit case. The scheme is simple and easily manipulated, because any two charge qubits can be selectively and effectively coupled by a common inductance. More manipulations can be realized before decoherence sets in. All the devices in the scheme are well within the current technology.PACS numbers: 03.65. Ud, 85.25.Cp, 42.50.Dv Quantum entanglement plays one of the most important roles in the quantum information processing. Many ingenious applications of entanglement have been proposed, such as quantum dense coding [1], quantum teleportation [2], quantum cryptography [3], etc. In achieving the task of quantum communication, entangled states are used as a medium for transferring quantum information. Meanwhile, entangled states are also used for speeding up quantum computation. Therefore, the preparation of entangled states becomes a key step towards quantum computation. Though bipartite entanglement is well understood, the extensive researches of multipartite case are still difficultly proceeding. For a tripartiteentangled quantum system, there are two irreducible classes of entangled states [4]. Recently, Briegel and Raussendorf [5] introduced a new class of N-qubit entangled states, i.e., the cluster states, which have some special properties. In addition to the properties of both Greenberger-Horne-Zeilinger (GHZ) and W-class entangled states, they especially hold a large persistence of entanglement, that is, they (in the case of N > 4) are harder to be destroyed by local operations than GHZ-class states. It has been shown that a new Bell inequality is maximally violated by the four-qubit cluster states, and isn't violated by the four-qubit GHZ states [6]. More significantly, the cluster states are regarded as a resource of multiqubit entangled states, thus cluster states become an important resource in the physics, especially in quantum information.Recently, much attention has been attracted to the quantum computer, which works on the fundamental quantum mechanical principle. The quantum computers can solve some problems exponentially faster than the classical computers. For realizing quantum computing, some physical systems, such as nuclear magnetic resonance [7], trapped irons [8], cavity quantum electrodynamics (QED) [9], and optical systems [10] have been proposed. These systems have the advantage of high quantum coherence, but can't be integrated easily to form large-scale circuits. As is well known, the cluster states are mainly applied to quantum computing. In Ref.[11], Raussendorf and Briegel described the so-called oneway quantum computer, in which information is written onto the cluster and read out from the cluster by one-qubit measure- * Electronic address: xhzheng@ahu.edu.cn † Electronic address: zlcao@ahu.edu.cn (Corresponding Author) ments. A number of applications using cluster states in quantum computation have ...

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