Trap And Manipulation Of Single Atom In Far-off-resonance Microscopic Optical Dipole Trap

In recent years, the coherence manipulation of quantum system has been studied experimentally and theoretically. And its application has been expanded from research of physical problems to quantum communication field. Many physical systems can be vectors of quantum system, like individual quantum dots, color centers, single molecules, single atom, cavity-QED based on trapping single atom and ion. Single atom is a perfect quantum system, and its cooling and trapping can be easily achieved by laser cooling technique. Compared to ion and other particle, the single atom is hardly influenced by the electric and magnetic fields. Due to the weakly-coupled property with environment, it is an advantage for keeping the coherence of quantum state. The quantum system based on single atom can demonstrate some non-classical effect and perfect quantum theory. Meanwhile, the quantum system can be widely applied to quantum calculation and quantum communication. Qubits (Quantum bits) are the quantum-mechanical states of |0) and |1> or superposition states of the logical states|0) and |1>. Based on single atom can realize the writing-in and reading-out of information. For example, quantum bit is the of classical logical state "O" and "1", and the quantum bit based on single atom can realize the writing-in and reading-out of information. By processing qubits with quantum gates, a quantum computer is fundamentally superior to classical computers in certain cases. As the CNOT gate of quantum bit of single atom is basic component of quantum logical calculation, the quantum logical gate of macro quantity can realize quantum calculation. Using the entanglement of photon and atom, the interaction between the quantum state of fly qubit for transferring information and the quantum state of carrier qubit, which can be applied for storage and transfer of quantum information and meaningful for quantum network and quantum communication. Moreover, the single atom trapped by cooling can realize the controlled single-photon source under control. And the single-photon source is important resource for quantum code password and quantum lineal calculation, which is quite meaningful for quantum information treatment.In the laser cooling and trapping atoms by MOT, there are always absorption and spontaneous radiation of atoms for existence of near resonance laser, which will destroy the internal free movement of the trapped atoms. The trapped atoms with pre-cooling produced through interaction of atom and dipole force in optical trap, especially far-off-resonance optical dipole trap (FORT) is quite low in scattering rate because the frequency of trapped laser is far from atomic resonance transition. Therefore, this optical trap can be considered as approximate conservative trap. In this conservative trap, the relativity of internal state of atoms can be kept for a long time; Meanwhile, a great variety of trap shapes can be produced, according to the many possible light configurations which can be attained with laser beams and interference patterns, and the size of the traps can be quite small, which is helpful for space localization of the atoms and can realize control of external degrees of freedom.The loading, recognition and manipulation of the single atom are mainly achieved by detecting the interaction of the atoms in the trap and special light field. The signal of the single atom is weak, which is easily influenced by background environment and limited by precision adjustment. For single atoms, the detection signal is so weak that it is strongly restrained by background and the accuracy of manipulation, and the quality andquantum state of the radio fluorescence photon are determined by the external state locality and internal states coherence of trapped single atom; The trapped atoms in the trap can heat the atoms and decrease the lifetime of the trapped atoms because of energy accumulation of the atoms in the trap caused by collision and noise heating. Therefore, we should firstly consider is how to set up an experiment system which can get effective source of the single atom or definite number of atoms source and how to evaluate the temperature of the single atom in the FORT by the effective temperature measurement technique, and how to efficiently cool the single atom in the FORT and extend its lifetime in the trap during trapping period.Based on the above several question, this paper does specific work stated below:1. The dependency of the loading rate of MOT and the parameters of the trap are studied from theoretical aspect. And a MOT system with larger magnetic field gradient ia we also designed and a spontaneous collect system is seted up。2. The cooling and trapping of the single atom is achieved in MOT, and the dependency of average atoms number of MOT and the parameters of the trap.3. The parameters of the dependence of the single atom spontaneous and statistical characteristics of radiation field are studied。4. The heating mechanism of the single atom in the FORT is studied. Numerical simulations show that the heating depends on the FORT beam's waist size and the trap depth. The effective temperature of single atom is evaluated by extending the R&R technique to the FORT. The typical effective temperature of a single atom in the tweezer has been is improved from～105μK to～17μK by applying the optimum PGC phase.The typical trapping lifetime has been improved from～6.9 s to～130 s by decreasing the background pressure from～1×10-10 Torr to～2x10-11 Torr and applying one-shot 10-ms laser cooling phase.