Coherent Manipulation Of Single Atom Qubit

The basic behaviors of quantum mechanics,such as state superposition and entanglement,are applied to quantum information process in quantum computing,which can overcome the difficulties a classical computer faces,such as quantum simulation and factoring large number.Several physical platforms have been shown as the good candidates for realizing the quantum computing,including trapped ions,linear optics,superconducting circuits,quantum dots,NV center,neutral atoms,and so on.The neutral atom system has some advantages,which makes it very suitable for demonstrating the process of quantum information.The coupling of the neutral atom to the external electromagnetic field is weak,which indicates a long coherence time for the ground state of a neutral atom.Therefore,a qubit can be encoded in the Zeeman states.The efficient initialization of the qubit can be achieved by optical pumping.Quantum gate operations are accomplished by the two-photon process or the microwave pulse.A state-selective detection can measure the qubit-specific state.These distinctive features satisfy the Di Vincenzo requirements of the fault-tolerant quantum computing.In addition,single neutral atoms can be easily scaled to 2D or 3D arrays,which provides a possibility for the large-scale integration of quantum information processing.Furthermore,the strong interaction between optical cavity and atomic array can improve the reading efficiency of atomic states,and can indirectly accomplish atom-atom entanglement,and then realize efficient double-qubit logic gates operation,which provides a good experimental platform for the quantum information processing.In this thesis,we take the scalable quantum compution based on neutral atom qubits as the long term goal.And the coherent manipulation of single neutral cesium atom captured by a micro-sized optical dipole trap,extending the coherence time of a single qubit states by means of blue detuned dipole trap and two-photon transition,and the measurement of Wigner function for discrete atomic systems are studied.The main works of this thesis are as follows:1).The single cesium atom is confined separately either in a 1064 nm red detuned dipole trap or a 780 nm blue dipole trap,which possess almost the same atom temperature and the same trap depth.Due to the less heating effect and fewer scattering photons in the blue detuned trap,the atom lifetime can be extended,which is longer than that in the red trap.Besides,the differential AC Stark shift between atomic?qubit?states is minimized in the blue trap,and the inhomogeneous phase decoherence due to atomic motion can be depressed.The blue trap can thus provide a good environment for preparation and manipulation of single atom qubit.2).We present a method to eliminate the differential AC Stark shift between atomic ground states by using a two-photon process,which can depress the inhomogeneous decoherence of the single atom qubit.A new method to precisely measure the hyperfine spectra of a weak atomic transition line with very low probe power is presented by combining the optical pumping and state-selection detection techniques together.By adopting such a method,we demonstrate an experiment to measure the cesium 6S–7S two photon spectra with optically trapped single atoms.The hyperfine splitting between the 7S states is 2.18361±0.00128 GHz and the corresponding hyperfine coupling constant A_hps of the 7S state is 545.90±0.32 MHz,which is in good agreement with former measurements.The power of the probe beam in our experiment is on the micro-watt level,which is much lower than that of traditional fluorescence detection measurements.Our method can also be used to probe the spectroscopy of other weak transitions.3).Based on the coherent manipulation of single atom,we have measured for the first time the complete and continuous Wigner functions of a two-level cesium atom in both a nearly pure state and highly mixed states.We find that the negativity of the Wigner function completely vanishes when the purity of an arbitrary mixed state is less than 2/3,which undergoes a nearly pure dephasing process.Our method and result can be applied straightforwardly to two-level systems for measuring the Wigner function.The work provides an intuitive approach to studying the decoherent properties of quantum states and quantum phenomenon in a dephasing evironment.