Quantum Entanglement And Quantum Gates Based On Cavity-assisted Interaction

Quantum information science is one of the most exciting scientific progress of physic-s during the past two decades. Its two main branches, i.e., quantum communication andquantum computing, have been made significant research results. Quantum communica-tion is theoretically absolute security determined by the principles of quantum mechanics,and the superposition properties of microscopic physical system enables people to designnovel quantum algorithm with parallel computing ability, which is peculiar to quantumcomputing device. Quantum entanglement is a kind of quantum correlation with non-locality between physical systems. As a central physical resource in a large number ofquantum communication protocols, entanglement has widespread application prospectand scientific research value. In the meanwhile, similar to classical logic operation prin-ciple of the traditional computer, quantum computing process is made up of many kindsof basic quantum logic gates. Therefore, the researches of quantum entanglement andquantum logic gates always occupy important positions in the development of quantuminformation science. Based on cavity quantum electrodynamics(CQED) technique, thisthesis focuses on the implementation of quantum entanglement and quantum logic gates,and then designing feasible schemes of quantum state manipulation.We first review the cavity-assisted single-photon input-output process in an opticalcavities under the CQED regime with atom-cavity mode coupling. Based on the cavity-assisted interaction, we propose a scheme for non-orthogonal quantum state comparison.The quantum state information is encoded on atomic ground levels to restrain the effectof spontaneous emission. Then unambiguous state discrimination is considered usingthe state comparison process as the elementary unit, which has important significancein quantum secret communication protocols. The necessary quantum logic operationsare built with CQED system and some linear optical elements and without any auxiliaryparticle. The theoretical calculation and analysis show that these schemes are feasibleunder the current experimental conditions and have near-optimal successful probabilities.Then two theoretical schemes are proposed to generate the two-atom Knill-Laflamme-Milburn(KLM) states with cavity QED technique. This kind of quantum entangled statecan be used as the effective entanglement channel shared in quantum teleportation andquantum computation fields. The first scheme is devised by the strong coupling atom-cavity-fiber system and realizes the logic operations of entanglement generation withatom-cavity mode interaction and the driving of extra classical field. The second schemeis devised by the single-photon input-output process in low quality cavities and achievesthe e?cient transmission of a bunch of photon pulse between two spatial nodes. Afterthe measurement of photonic states, the two-atom KLM states can be prepared. In bothof these schemes, the KLM states prepared here take the relatively general form owing tothe initial atoms in arbitrary single-atom states, thus one can optionally get the maximal-ly entangled states or non-maximally entangled states by choosing different parameters.The final entanglement is reserved in the atomic ground levels, which is beneficial forentanglement storage and application. And the Analysis and discussion indicate that thesuccessful probabilities of these protocols approach unity in the ideal case.The solid-state spin of diamond nitrogen-vacancy(NV) center is identified as a scal-able experimental platform for room-temperature quantum information and computingin recent years. We implement the spin entangled states of NV centers and universaltwo-qubit quantum gates based on the model of NV centers in diamond confined in sep-arated microtoroidal resonators. The basic approaches are cavity-assisted interaction andmanipulation of photonic qubits. In the scheme of entanglement generation, high fidelitytwo-body or three-body NV center entangled states and photonic entangled states are gen-erated, respectively. Then utilizing the prepared entanglement the quantum state transfer(QST) for distant NV centers or even between NV center and photonic qubit are achieved.In the scheme of universal quantum gates, A hybrid polarization-spin controlled-NOTgate and a two-qubit controlled phase gate between NV centers are demonstrated. And onthis basis, an exchange interface of quantum states between the photonic qubit and the NVcenter qubit is designed. These presented schemes are of both advantages of solid-statequbits and optical micro-cavity, and do not need high quality cavity and strong-couplingregime. The final analyses of the physical devices and theoretical schemes show that ourschemes are deterministic and may have good performance in the current experimentalconditions.The prepared entangled states tend to suffer from the influence of the outside noiseon quality during the storage and transmission process, e.g., the maximally entangled s-tates degrade into the non-maximally entangled states. Entanglement concentration worksto extract the maximally entangled states from the non-maximally entangled states. Us-ing single-particle hybrid entanglement within two different degrees of freedom, we pro-pose two entanglement concentration schemes for two-particle non-maximally entangledstates. The first is entanglement concentration for unknown entangled states based onthe entanglement swapping, and the second is entanglement concentration for entangledstates known in advance by introducing auxiliary particles. The two schemes are applica-ble to a variety of physical systems and qubit carriers. And the second scheme is extendedto multi-particle entanglement concentration process.