Quantum Non-demolition Measurement Of Electron Spins State

Quantummeasurementplaysanimportantroleforobtainingtheinformationaboutthe microscopic world. There is a kind of measurements in which the measurementdoesn't perturb the quantum state of the measured system, and continually read someobservables many times. This kind of measurements is called quantum nondemoli-tion measurement. Some physical quantities can be measured with high precision forbeating the standard quantum limit with quantum nondemolition measurements. Quan-tum nondemolition measurement occupies an important place in quantum informationfield,andisindispensableformanyapplications,suchasquantumerrorcorrection,statepreparation by measurements and one-way quantum computation. In the thesis, quan-tum nondemolition measurement of electron spins is implemented by coupling themto a superconducting flux qubit. The interacting Hamiltonian is engineered especial-ly for satisfying the conditions of quantum nondemolition measurement. Due to theweak coupling between a single electron spin and the superconducting qubit, it is hardto achieve a project measurement of it. Then a ultracold ensemble of electron spinsis introduced for enhancing the coupling strength between the electron spins and thesuperconducting flux qubit. Making use of the strong coupling, a CNOT gate is im-plemented between the electron spins and the superconducting flux qubit. Through theCNOT operations, it is achieved for quantum nondemolition measurement of electronspins with the property of project measurement.There are some advantages in the hybrid quantum network constructed by the elec-tron spins and the superconducting flux qubit, such as long coherence time of electronspins and strong coupling of superconducting flux qubit. The more powerful hybridquantum information processing devices may be constructed by utilizing of the excel-lence of the electron spins and superconducting flux qubit and overcoming their short-comings. Two applications are listed in the thesis: one is the quantum informationread-write devices; the other is hybrid entangle network.