The Implementation Of Quantum Phase Gate And The Preparation Of Entanglement State Base On Atomic Ensemble Systems

Quantum information science is a new interdiscipline composed of quantum mechanics and information science. Its vitality and vigour carve out a progressional way for information science. Quantum entanglement state is a basic physical resource for realizing quantum information processing. Especially, multiqubit entangled state plays a important role in quantum information science. Quantum logic gate is the elementary unit of quantum logic circuit. The advantages of quantum computation, which is realized by the universal quantum logic gate, are incomparable compared with classic one.With the development of cold atom technology, Rydberg atomic ensemble system is known as one of the very promising physical system to achieve quantum information processing for scalable quantum computing. Rydberg atoms possess wonderful physical properties and atomic ensemble has collective enhancement effect. Thus, it has the important theoretical value and practical significance for researching construction of quantum logic gate and preparation of multiqubit entangled state based on Rydberg atom ensemble. The main works include the three aspects:1、We propose a scheme for realizing an unconventional three-qubit controlled-phase gate via the Rydberg blockade mechanism. The qubit is encoded by atomic ensembles that are trapped in optical traps and fixed on an atom chip. Due to the collective nature of the encoding and the Rydberg blockade mechanism, the scheme do not require separate addressing of individual atoms. The time needed for the gate operation is much shorter than that in a similar scheme. In addition, we show the gate can be used as a basic tool for effective generation of large-scale2D cluster states.2、Based on collective encoding in an ensemble with2N+1stable ground sates and N Rydberg states, we realize successfully the preparation of multiparticle W state via adjusting suitable interaction time between laser and ensemble and making use of the dipole blockade effect. The encoding scheme effectively overcome the limit of qubit number caused by the dipole blockade effect being effective only within a certain range. Finally, we discuss the scheme’s experimental feasibility.3、Based on collective encoding in an ensemble with N+1stable ground sates, a multiqubit controlled-phase gate with any number qubits is implemented by utilizing the dipole blockade effect. Where it is no need for addressing individual atom, and the unique energy level structure effectively overcomes the limitations of qubit number induced by the dipole blockade effect effective distance. Meanwhile, The multiqubit controlled-phase gate can be used to effectively prepare large scale two-dimension and three-dimension graph states.