Loading And Manipulation Of A Single Atom In An Optical Dipole Trap

As one of the most popular frontier research fields in the past two decades,quantum computation plays an irreplaceable role in simulating the evolution of physical systems and dealing with certain mathematical problems.Researchers are trying to realize quantum computers in different physical systems,the most promising of which include superconducting circuits and ion trap platforms.The neutral atomic array platform rose as a dark horse qubit candidate in the past ten years,and more and more experimental research groups and commercial companies have chosen atomic arrays as their platforms to develop quantum computers.This is because the atomic arrays have the following advantages:(1)Naturally identical atomic qubits have inherent scalability,which can be trapped in dense one-,two-or three-dimensional arrays;(2)High fidelity entanglement between atoms can be achieved through the strong Rydberg interaction;(3)Atoms can be transferred without disturbing the entanglement,so the long-distance entanglement between two atoms can be achieved via moving them together,which provides more opportunities for quantum algorithms and error correction codings.During my Ph.D.,I built a single-atom array experimental platform,discovered several physical phenomena of the light-atom interaction and realized precise manipulation of multiple degrees of freedom of atoms.These works constitute the main content of this paper.The details are as follows:1.The manipulation of the internal state of the atom was realized in the thermal atom ensemble,and a magneto-free isolator with the highest known isolation ratio was realized.The manipulation of the internal state of neutral atoms is an indispensable technology for quantum state initialization and state detection.We explored the manipulation method of internal state degrees of freedom in hot atom ensembles,and realized optical non-reciprocity generated by atomic spin polarization in this process.Utilizing the polarization effect of the circularly polarized light field on the atomic ground state,as well as the effect that the polarized atom will produce different refractive indexes for two circularly polarized light fields,we proposed a new physical concept as nonlinear non-reciprocity.Based on this concept,we experimentally demonstrated a non-reciprocal isolator with a maximum isolation ratio of 63.4 dB and a 20 dB isolation bandwidth of 12.5 GHz.The 63.4 dB isolation ratio reaches the highest record among the non-magnetic isolators so far,and the isolation bandwidth is more than one order of magnitude higher than the previous results performed on atomic ensembles.Furthermore,we put the nonlinear non-reciprocal medium into an asymmetric cavity,realizing a completely passive non-magnetic isolator.Besides,the nonlinear non-reciprocal principle can also be utilized to make a circularly polarized purifier,which has very high robustness for the polarization of the input light.It can obtain circularly polarized light with a purity higher than 60 dB.The high-purity circular polarized light realized in this work is conducive to the realization of high-fidelity state preparation and gate operation in quantum computing.2.The construction of special-shaped magneto-optical traps and single-atom dipole traps was completed,and the motion state of single atoms in the dipole trap was characterized.This work provided a basic platform for subsequent experiments.Single atoms loaded in dipole traps are the foundation of the atomic array quantum computer,while the cooling of atoms from room temperature through a magneto-optical trap(MOT)is a precondition of the single atom loading process.We analyzed and calculated the requirement of the cooling laser intensity for our special-shaped MOT.With the assistance of an ordinary MOT,we successfully realized the special-shaped MOT through careful adjustment.We used a red-detuned far off-resonant dipole trap to capture a single atom from the cold atomic ensemble with a loading rate of 55%and an averaged lifetime longer than 10 s.After that,we measured the atomic temperature and the resonant frequency of the atoms in the trap.3.A model for simulating the kinetic process of atomic loading in a dipole trap was developed,and the experimental methods to adjust the main parameters in this model were given.This work provides a new way to control both the loading process of the atom array and the collision rate between atoms in one dipole trap,realizing the manipulation of the atom loading process.To characterize the atomic loading process more distinctly,we built a mathematical model to simulate this process.There are three parameters in this model,the singleatom loading rate R,the single-atom loss rate γ,and the two-atom collision loss rate β.We first proposed how to extract model parameters from experimental data.Then we verified the accuracy of our model by comparing the original experimental data with the simulated data from Monte Carlo simulations.We summarized how to continually vary the R and γ in the experiment,and proposed a new method to control the two-atom collision rate β.Compared to the previous method to change β,our method requires less laser power to achieve the same result.Finally,we theoretically studied the influences of R,γ and β on the loading probability of different atom number,which provides a reference for multi-atomic loading experiments.4.The coupling between the vibrating phonons of the atoms in a dipole trap and the atomic internal spins was studied from both theoretical and experimental aspects.This coupling can be used to cool atoms under the assistance of circularly polarized pumped light.The temperature of the single atom in the dipole trap represents the kinetic energy of the atom.Atoms with lower temperatures will endure less decoherence from the inhomogeneous environment.In this work,we proposed a novel cooling mechanism that only requires one single-frequency circularly polarized laser beam.The cooling effect comes from the coupling between the Zeeman sub-level transition and the phonon of the atomic oscillation in the dipole trap.We experimentally measured the relationship between atomic temperature and magnetic field and verified that this mechanism could reduce the atomic temperature.This cooling mechanism provides a new scheme to achieve non-loss measurement of the quantum state of the atoms in the array,allowing the reuse of the atomic array for quantum computing operations and increase the calculation speed of the atomic array quantum computer.