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We propose a scheme to implement a two-qubit controlled-phase gate for single atomic qubits, which works in principle with nearly ideal success probability and fidelity. Our scheme is based on the cavity input-output process and the single photon polarization measurement. We show that, even with the practical imperfections such as atomic spontaneous emission, weak atom-cavity coupling, violation of the Lamb-Dicke condition, cavity photon loss, and detection inefficiency, the proposed gate is feasible for generation of a cluster state in that it meets the scalability criterion and it operates in a conclusive manner. We demonstrate a simple and efficient process to generate a cluster state with our high probabilistic entangling gate.The one-way quantum computation [1,2,3,4,5] has opened up a new paradigm for constructing reliable quantum computers. In their pioneering works [1,2], Raussendorf and Briegel showed that preparation of a particular entangled state, called a cluster state, accompanied with local single-qubit measurements is sufficient for simulating any arbitrary quantum logic operations. Therefore, experimental or intrinsic difficulties in performing two-qubit operations can be substituted with (possibly probabilistic) generation of an entangled state. Especially, Nielsen showed that the resource overhead of a conventional linear optics quantum computer [6] is drastically decreased by combining it with the idea of the one-way quantum computation [4].A cluster state can be visualized as a collection of qubits and lines connecting them. In order to generate a cluster state systematically, one first initializes each qubit in state |+ = 1 √ 2 (|0 + |1 ), where |0 and |1 are the computational basis states, and then performs controlledphase operations between every neighboring qubits connected by the lines. In some previous works [7,8,9], it was shown that in principle there is no threshold value of p required for efficient generation of a cluster state, where p is the success probability of each controlled-phase operation. For a reasonable computational overhead, however, a high success probability p should be attained.In the present work, we propose a scheme to implement a two-qubit controlled-phase gate for single atomic qubits, which works in principle with nearly ideal success probability and fidelity. The proposed entangling gate is suitable for the systematic generation of a cluster state described above for two reasons. The first is that it works between two individually trapped atoms, thus it meets the scalability criterion. Since a large number of qubits should be entangled in a cluster state to perform a nontrivial quantum computation, entangling gates which work only inside a single trapping structure [10,11,12,13] can not be used directly for our goal. The second is that, in contrast to other scalable two-qubitThe setup for the basic building block. A qubit is encoded in two ground levels |0 and |1 of a 3-level atom trapped in an one-sided optical cavity. The transition between states |1 and |e ...

A single-particle entangled state can be generated by illuminating a beam splitter with a single photon. Quantum teleportation utilizing such a single-particle entangled state can be successfully achieved with a simple setup consisting only of linear optical devices such as beam splitters and phase shifters. Application of the locality assumption to a single-particle entangled state leads to Bell's inequality, a violation of which signifies the nonlocal nature of a single particle.

A new approach is presented in which classical mechanics is combined with quantum statistics to describe molecular collisions. In this approach, the dynamics of collisions is described by classical trajectories as in the widely used quasiclassical method. However, initial and final internal states are represented in phase space in a quantum statistical way, using the Wigner distribution function. Results of calculations performed on a collinear He–H2 collision indicate that this new method is more accurate than the quasiclassical method, especially when the initial vibrational energy is low. Moreover, the new method is capable of describing classically forbidden processes that cannot be accounted for by the quasiclassical method.