Manipulation Of Single Qubits For Quantum Computation With Neutral Atoms

We investigate the manipulations of single qubits for quantum computation with neutral atoms.Fundamental ingredients of architecture for quantum com-putation include the initialization and the readout of scalable quantum registers, gate operations of single- and multi-qubits. Based on "collisional blockade" mech-anism, single atoms can be confined in a microscopic optical trap. We initialize quantum registers by loading single atoms into the ring optical lattice. For alkali-metal atoms, qubits are encoded in the hyperfine structure levels of the ground states. We perform single qubit operations between the internal states by using two-photon stimulated Raman transitions.In this thesis, the main novel results are listed as follows:1. We experimentally demonstrate the strong suppression of dephasing of single qubits by using Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences. Re-garded as the repetition of the spin echo technique, the CPMG pulse sequence has been applied in a variety of systems to improve phase coherence. We extend this technique to single neutral atoms. By employing CPMG sequence with π-pulse number n= 6, the homogeneous dephasing time is prolonged by a factor of 3 in comparison with the result of the spin echo process.2. We demonstrate the transfer of single qubits in the lattice with the aid of an auxiliary moving tweezer and investigate its influences on qubit fidelity. When the moving tweezer has the deeper trap depth and moves across the static tweezer, it is observed that the trapped atom follows the movement. The transfer efficiency of one atom from the initial position to the destination reaches up to 95%. This scheme is suitable for scalable quantum registers because of no influence on the other sites. During the transfer process, the qubit fidelity of the eigenstate is F= 0.94 and can be well preserved. However, the fidelity of the superposition state is influenced, which decreases from F= 0.74 to F= 0.66 at the measurement time T= 20 ms. We find that the loss of qubit fidelity during the transfer results from heating effects induced by this process and pointing instabilities of the trap laser. In comparison with the transport process of single ions, there exist many common requirements and challenges for single neutral atoms. They may need to combine our transfer scheme with alternative methods.3.We investigate the matter wave interference of a single atom in the free space and the optical dipole trap. Atomic interferometer can be created by using a series of Raman pulses and widely applied in the field of precision measurement. In the free space, we observe a Mach-Zehnder type interferometer of only one atom at a time, and measure the coherence length of single atoms wave packet. However, in the dipole trap, the visibilities of interference fringes are decayed rapidly with the increasement of the measurement time. It is due to the harmonic motion of single atoms in the optical dipole trap. An interference scheme contained two pulses in the trap is thus proposed.4. We optimize the laser cooling sequence and prepare single atoms beyond Lamb-Dicke limit in the overlapped trap. In order to tightly confine single atoms in the red-detuned optical trap, a blue-detuned Laguerre-Gaussian laser beam is overlapped. After performing the polarization gradient cooling, we measure the temperature of single atoms by using the release and recapture method. It indicates that single atoms are located in the Lamb-Dicke regime, which is the fundament condition of the Raman sideband cooling.5.We preliminarily realize the super-resolution fluorescence imaging of sin-gle atoms array with a numerical processing method called compressive sensing (CS). When the signals satisfy certain sparsity constraints, the CS technique pro-vides a solver for sparse reconstruction. In order to real time monitor the whole lattice, we collect laser induced fluorescence and image onto an electron multi-plying charge coupled device camera. The CS technique is preliminarily applied to restore super-resolution imaging of single atoms array in a two-site lattice.In conclusion, we demonstrate the manipulations of single atomic qubits in the scalable quantum registers. All of these results pave the way for quantum computation with neutral atoms.