Narrow Results

The cluster structure of ¹²C is explored and recent measurements of proton inelastic scattering, suggesting a 2⁺ state close to 9.6 MeV are presented. Resonant scattering studies of ¹⁰Be + ⁴He used to populate resonances in ¹⁴C are briefly discussed.

We propose a one-step cluster state generation scheme via atomic cavity QED. The current scheme is insensitive to the cavity mode thermal state as well as the atomic spontaneous emission since the gate operations are independent of the cavity mode states and laser power is sufficiently weak. In addition, the needed operation is of geometric nature, so it is robust against random operation errors.

We propose a scheme to create cluster states with superconducting quantum-interference devices (SQUIDs) with the help of an auxiliary qubit, by using a quantized cavity field and classical microwave pulses, via Raman transition. In our scheme, no transfer of quantum information between the SQUIDs and the cavity is required. The cavity is only virtually excited during the whole operation and thus the cavity decay is suppressed during the operations and the requirement on the quality factor of the cavity is much relaxed, which could greatly reduce the experimental challenge.

The differential cross-sections of the elastic and inelastic $\alpha {+}^{13}$C scattering have been measured at E$(\alpha )=29$MeV. The radii of the exited states: 3.09 $(1/2^{+})$ and 8.86 $(1/2^{-})$ MeV were determined using the Modified Diffraction Model. The radii of these excited states are larger than that of the ground state of ${}^{13}$C, confirming the suggestion that the 8.86 $(1/2^{-})$ MeV state could be an analog of the Hoyle state in ${}^{12}$C and the 3.09 $(1/2^{+})$ MeV state has a neutron halo. The possibility of coexistence of various exotic states in the structure of the ${}^{13}$C nucleus is shown.

A method is proposed to characterize and quantify multipartite entanglement in terms of the probability density function of bipartite entanglement over all possible balanced bipartitions of an ensemble of qubits. The method is tested on a class of random pure states.

We design a controlled-phase gate for linear optical quantum computing by using photodetectors that cannot resolve photon number. An intrinsic error-correction circuit corrects errors introduced by the detectors. Our controlled-phase gate has a 1/4 success probability. Recent development in cluster-state quantum computing has shown that a two-qubit gate with non-zero success probability can build an arbitrarily large cluster state only with polynomial overhead. Hence, it is possible to generate optical cluster states without number-resolving detectors and with polynomial overhead.

In this thesis, we describe the one-way quantum computer , a scheme of universal quantum computation that consists entirely of one-qubit measurements on a highly entangled multiparticle state, i.e. the cluster state. We prove the universality of the , describe the underlying computational model and demonstrate that the can be operated fault-tolerantly.

In Sec. 2, we show that the can be regarded as a simulator of quantum logic networks. In this way, we prove the universality and establish the link to the network model — the common model of quantum computation. We also indicate that the description of the as a network simulator is not adequate in every respect.

In Sec. 3, we derive the computational model underlying the , which is very different from the quantum logic network model. The has no quantum input, no quantum output and no quantum register, and the unitary gates from some universal set are not the elementary building blocks of quantum algorithms. Further, all information that is processed with the is the outcomes of one-qubit measurements and thus processing of information exists only at the classical level. The is nevertheless quantum-mechanical, as it uses a highly entangled cluster state as the central physical resource.

In Sec. 4, we show that there exist nonzero error thresholds for fault-tolerant quantum computation with the . Further, we outline the concept of checksums in the context of the , which may become an element in future practical and adequate methods for fault-tolerant computation.

Large-scale quantum information processing and distributed quantum computation require the ability to perform entangling operations on a large number of qubits. We describe a new photonic module which prepares, deterministically, photonic cluster states using an atom in a cavity as an ancilla. Based on this module, we design a network for constructing 2D cluster states and then we extend the architecture to 3D topological cluster states. Advantages of our design include a passive switching mechanism and the possibility of using global control pulses for the atoms in the cavity. The architecture described here is well-suited for integrated photonic circuits on a chip and could be used as a basis of a future quantum optical processor or in a quantum repeater node.

We firstly propose a simultaneous dense coding protocol with two-photon four-qubit cluster states in which two receivers can simultaneously get their respective classical information sent by a sender. Because each photon has two degrees of freedom, the protocol will achieve a high transmittance. The security of the simultaneous dense coding protocol has also been analyzed. Secondly, we investigate how to simultaneously teleport two different quantum states with polarization and path degree of freedom using cluster states to two receivers, respectively, and discuss its security. The preparation and transmission of two-photon four-qubit cluster states is less difficult than that of four-photon entangled states, and it has been experimentally generated with nearly perfect fidelity and high generation rate. Thus, our protocols are feasible with current quantum techniques.