Investigation Of 852nm Single Photon Source Based On Manipulation Of Single Atoms In A Microscopic Optical Dipole Trap

As a sort of non–classieal light source,the single-photon sources are most important for fundamental research in quantum optics as well as for many quantum cryptography protocols and linear optics quantum computing.Single-photon emission has been demonstrated for many different sources.Most of these sources are single quantum emitters quantum systems,such as single semiconductor quantum dots,single molecules,single atoms,single ions,and single N-V color centers.Single-atom-based single-photon source has several special chracteristics,such as narrow bandwidth,wavelength matching with the absorption line of the same atomic emsamble,and not sensitive to the environment disturbing.It is much more suitable for achieving nondistinguishable single photons compared with other single-photon scheme.Thus it is very important not only for basic researches in quantum optics field but also for applications in quantum information processing.This thesis mainly focuses on the generation of 852 nm single-photon source and improvement of the indistinguishability of single photons,experimental realization of high-probability single atoms loading,production of a high ON/OFF ratio and high speed wavelength switching of amplified nanosecond(ns)laser pulses system,extending the trapping lifetime of single atoms,construction and measurement of the magic wavelength.The main work shown in the thesis are follows:I).High-probability loading of single atoms.In order to demonstrate 852 nm singlephoton source,we need a stable source of single atoms.We explore a new real-time feedback control technique to obtain a high-probability single atoms loading.Firstly,we rapidly load a preset mean number of cesium atoms into a high-gradient vapor-cell MOT by temporarily lowering its quadrupole magnetic field(QMF)and switching on the ultra-violet(UV)light emitting diode(LED)to fire the light-induced atomic desorption(LIAD)for increasing the density of background cesium atoms.After this forced loading,the QMF is increased for suppressing the loading rate and the UV LED is rapidly switched off.The desired number of trapped atoms is determined by analyzing the laser induced fluorescence photon counting signals.However,the atom trapped in the MOT will be accompanied by the process of absorption and spontaneous emission,leading to the destruction of the atomic internal freedom.So,the atom was transferred from the MOT into an optical dipole trap(ODT).To generate an ODT,a far-red-detuned 1064 nm laser beam is tightly focused by using a high-numerical aperture lens assembly,a typical trap depth is 2 mK and trap waist is 2.3 ?m.We design an automatic compensation identification system to control the loading states by applying LIAD,and to control the collision processes by blue-detuned light-induced atom collisions(LIAC).The probability of trapped single atoms in an ODT is improved to 99.9%.II).Demonstration of 852 nm single-photon source.In order to extend the trapping lifetime under periodical pulse exciting,we investigate single-cesium-atom heating owing to the momentum accumulation process induced by the resonant pulsed excitation in a microscopic ODT formed by a strongly focused 1064-nm laser beam.The heating depends on the trap frequency,which restricts the maximum repetition rate of the pulsed excitation.We experimentally verify the heating of a single atom and then demonstrate how to suppress it with an optimized pulsed excitation and cooling method.The typical trap lifetime of a single cesium atom is extended from ~108 ± 6?s to ~2536 ± 31 ms.In applying this faster cooling method,we use the trapped single cesium atom as a triggered single-photon source at an excitation repetition rate of 10 MHz.The second-order intensity correlations of the emitted single-photon are characterized by implementing Hanbury Brown-Twiss setup.The statistics shows a strong anti-bunching effect with a value of g(2)(?=0)=0.09.Taking the experiment into account,a 852 nm nanosecond laser pulse chain with a high on/off ratio is generated by chopping a continuous-wave laser beam using a Mach–Zehndertype electro-optic intensity modulator(MZ-EOIM).Choosing a matching temperature value,the MZ-EOIM can be optimized near perfect state,and the static ON/OFF ratio can be increased from a typical default value of 20 dB to 41 dB.To further improve the output ON/OFF ratio and increase the mean pulse power,we report a novel optical pulse generation method for high-speed wavelength switching of amplified nanosecond(ns)laser pulses resonant to atomic transitions by using of dynamical injection locking a slave diode laser with ns master laser injection.III.Improving the indistinguishability of the single photons.The trapping laser light induces a light-shift on the atom that shifts the transition frequencies between the ground and the excited states.This shift is position-dependent and time-dependent.The thermal motion of the atom in the ODT will explore different light shifts of the transition,which will lead to a broadening of the emission spectrum and can thereby reduce the achievable two-photon interference contrast.In order to eliminate the light-shifts,a state-insensitive ODT with 937.7nm laser beam is constructed,where the ground state and the excited state have the same light shift.With this method,the transition frequency and the emission of the photons are therefore the same as in free space.We measure the differential light shift of the relevant optical transition as a function of the trapping light wavelength and analyze the various influence mechanisms of the result of the measurement.The light shift is measured by measuring the fluorescent spectrum of single cesium trapped in an ODT with ?+ polarization probe beams.We also perform a Hong-Ou-Mandel two-photon interference experiment to analyze the indistinguishability of the single photons.Compared with the 1064 nm ODT,the indistinguishability of single photon is apparently improved by using a magic wavelength ODT.