Efficient Preparation And Coherent Manipulation Of Single Atom In A Magneto-optical Trap And In A Far-off-resonance Optical Dipole Trap

Based on the requirement of quantum computation and quantum communication, quantum systems become research focus in the world. Until now, the carriers of the quantum system mainly include:photons, quantum dots, superconducting ring, the N-V center of diamond, single ions, and neutral atoms. Compared with ions or other systems, signal atoms are less susceptible to the interference of external electric field and magnetic field, therefore has many advantages to maintain its coherence of the states, which provides a sufficiently long decoherence time for coherence manipulation of atoms in the ground state; Meanwhile, single atoms can interact with specific laser, thus the single atom become one of the research focuses. As a quantum system, single atoms can demonstrate some non-classical effects, can verify and improve some quantum theory; and the expanded array of single atoms can be applied to quantum computing and quantum communications. According to the DiVincenzo criterion, building a quantum computer need five basic conditions: ① Scalable system with well characterized qubits, ② Ability to initialize the qubits to some simple state, ③ Decoherence times longer than any relevant gate times, ④ Universal set of quantum gates, ⑤ Qubit specific state measurement capability. Obtained from DiVincenzo criteria, to build quantum computers must have the ability which can be initialized to arbitrary coherent superposition and scalable quantum bits, and can be reliably read each quantum state. To achieve coherent manipulation of the ground state of the cesium atom, we carry out a series of work.This paper focuses on the ground state coherent manipulation of single cesium atom and includes the following aspects:1. In order to complete the coherent manipulation of the ground state of the cesium atom, realize the preparation of quantum bit, we need a stable source of single atoms. Based on the technology of laser cooling and trapping, trapping atoms in the magneto-optical trap (MOT) are easily. By optimizing the magnetic field, light field, and the background vacuum degree, we can trap single atoms in the MOT. The atoms in the MOT will be accompanied by the process of absorption and spontaneous emission, leading to the destruction of the atomic internal freedom. In the optical dipole trap (ODT), especially, far off resonant trap (FORT), the frequency of the light is far from the atomic resonance frequency, the atomic scattering rate is very low, so the FORT can be regarded as a kind of nearly conservative potential trap in which the coherence of atomic state can be maintained for longer time. But due to the influence of the collision and the noise to the atoms, atoms will been soon heated out of the trap. By reducing the vacuum of the background, using the polarization gradient cooling, reducing light intensity fluctuations of the dipole trap and other methods, we get stable single atom of long life (130 seconds). In order to accelerate the single atom trapping, we increases the rate of a single cesium atom by light assisted collision effects, feedback control magnetic field gradient of the magneto-optical trap and using purple LED’s light cesium atomic chamber to improve the single cesium atom trapping rate, so that we can quickly trap a single atom for a long time which provides a stable source of signal cesium atom for coherent manipulation of single atoms in the ground state. We have measured the statistics of photon generated by single atom in the FORT using Hanbury Brown-Twiss (HBT) experiment.2. Coherent control of the ground state of the cesium atom also requires two phase coherent laser beams with a constant phase difference and frequency difference being equal to clock transition frequency (9.192631770 GHz), which referred to as the Raman laser. By directly RF modulating slave laser or electro-optic modulator (EOM), then optical injection locking method, we obtain two sets of Raman optical systems. We verify the phase coherence of the beams through beat note experiment. With one beam locked to cesium atomic transition line, the other beam scanning around the cesium atomic resonance transition line, atoms are prepared to the coherent superposition dark state of the two ground states by two-photon STIRAP process, namely, coherent population trapping (CPT). The occurrence of coherent population trapping (CPT) phenomenon, which can further confirm the phase coherence of the two sets laser systems.3. With the help of the temperature controlled Etalon, we can realize the locking of two Raman lasers which is two-photon resonance and single photon large detuning. Finally, the Raman light is locked to the cesium atomic negative detuning D2 transition line at 10GHz-40GHz, which can drive the coherently manipulation of ground state of cesium atomic.4. We prepare the cesium atoms to the corresponding hyperfine state and Zeeman state, and complete the preparation of the initial state of the cesium atom, combined with large detuning Raman laser lock; we initially realized the coherent manipulation of the ground state of the cesium atom.