Entangled State Fusion And Conversion

Quantum information science is an emerging field,which takes advantage of the unique properties of quantum system to process information and promotes the rapid development of information science,for example,the quantum no-cloning theorem guarantees the security of quantum communication and the principle of superposition of quantum states guarantees the parallelism of quantum computation.All of these progress are inseparable from quantum entanglement.Quantum entanglement is a quantum correlation with non-classical and non-locality between two or more physical systems,it is one of the most important basic resources in quantum information,especially the multi-particle entangled state which plays an important role in quantum information processing.During the past decades,the preparation technology of quantum entangled states has matured.However,as the number of particles increases,the dynamics of the system will become more complex and leading to the process of preparing entangled states becomes more and more difficult,therefore,a method of simple and effective for preparing large scale multi-particles entangled states is very important.The main purpose of this dissertation is to obtain the multi-particles entangled states by means of entanglement fusion and conversion.In the process of entanglement fusion,we adopt two different models.In the first model,the quantum information is encoded into the ground state of?type three-level atom.The two atoms belonging to an-atom state and an-atom state,respectively,are send into a vacuum cavity to complete the entangled state fusion.The two atoms are trapped in the cavity during the process of fusion,and each atom is driven by a classical laser field with the corresponding Rabi frequency.If the Zeno condition is satisfied,we can obtain the effective Hamiltonian from the initial Hamiltonian,also we can obtain the systemic state at a particular time.Using these specific evolutionary states,the two multi-particle states can be fused into a?+-2?-atom state by performing the corresponding single bit operation according the result of measurements.The numerical results show that the scheme is robust against both spontaneous emission of atoms and decay of cavity.This is because that in the Zeno subspace,the state of the cavity is always in the vacuum state,therefore,the cavity decay terms have no influence on the evolution of the encoded qubit states.The further large detuning condition excludes the excited states,so this process is also robust against the decoherence induced by spontaneous emission.It is difficult in to operate the atoms in the experiment when the two atoms are trapped in one cavity,thus we consider the case of the two atoms are trapped in two cavities,respectively,and the two cavities are connected with optical fiber.Similar to the previous,we can obtain the effective Hamiltonian for the system by using the quantum Zeno dynamics and obtain the state of the system at a particular time.This scheme has the advantages which the above scheme has and is easier to implement in experiments.The only difference is the decoherence of optical fiber is introduced.Fortunately,the decay of optical fiber is small enough to ignore.The numerical simulation also shows that the influence of the optical fiber is very small on fidelity.In the second model,the ground states of Rydeberg atom are used as the carrier of quantum information.When Rydberg atoms are in the Rydberg states,the dipole-dipole interaction or van der Waals interaction always make the Rydberg energy level shifted,thus inhibiting the resonant optical excitation of multiple Rydberg atoms,which is so called the Rydberg blockade effect and it is widely used in many quantum information protocols.However,when the detuning between the atom transition frequency and the frequency of a classical laser satisfies some conditions with Rydberg interaction strength,the atoms also can be excited to the collective Rydberg states.Hence,the Rydberg anti-blockade regime can be generated.Then,the effective Rabi oscillation between the two ground states and the two excited Rydberg states are generated.The large-scale entangled GHZ states and states with neutral Rydberg atoms can be fused based on this mechanism.The scheme has some characteristics,i.e.,the quantum information is encoded into the stable hyperfine ground state and distant atoms interact with each other through the Rydberg interaction and not only two multi-particle states can be fused but also two multi-particle GHZ states can be fused in this model,especially the success probability for fusion of GHZ states can reach unit.We can also obtain the multi-particle entangled state by means of conversion between different entangled states.We suggest a high-fidelity CNOT gate between two photons can be constructed resorting to NV center and single-photon input-output process.According to the measurement results of NV center,the corresponding single bit operation should be performed on photon.In this process,the NV center which act as auxiliary system is fixed on the surface of a microtoroidal resonator and can be coupled to the cavity mode.What's more,the schemes of entangled states conversion are presented for converting three-photon,four-photon and five-photon GHZ state to a state or Dicke state by applying the CNOT gate and the phase discrimination detection of coherent light field.The schemes has the following advantages:the minimum success probability of conversion a three-photon GHZ state to a state is 3/4,while by iterating the conversion process,the total success probability can be to unit,but the conversion for four-photon GHZ to a state does not need iteration.For five-photon GHZ state,the total success probability for converting to a W state can be to 1/3 by iterating the conversion process.Meanwhile,we can obtain a Dicke state|D₅²>with the success probability 2/3.The analysis of feasibility shows that our scheme is feasible for current experimental technology.