Quantum Coherence Effect And Quantum Coherence Manipulation In Ultra-cold Rydberg Atomic Ensemble

The quantum coherence effect in the light-matter system is the focus in the development of quantum optics,and it has brought new vitality in the quantum information area,so we review several typical atomic coherence effects including the stimulated Raman adiabatic passage(STIRAP)and electromagnetically induced transparency(EIT)and the relevant applications in quantum information area at the beginning of the thesis.It is possible to implement the efficient population transfer,quantum entanglement,quantum gate,photonic crystal and light storage utilizing the atomic coherence effects.Rydberg atoms have exaggerated properties that is different from ordinary neutral atoms as they have high principal quantum numbers.For instance,Rydberg atoms have large orbit radiuses,long lifetimes and huge dipole moments.The dipole-dipole interaction is typically enhanced by 20 orders of magnitude compared to two ground state atoms as the dipole moments of Rydberg atoms are very large.Based on these novel properties,the quantum coherence effects in the Rydberg atomic ensemble will be different from the common cold atomic ensemble,so the study on Rydberg atomic ensemble has aroused intensive attention.With the development of technology and the improvement of the theoretical method,the study of Rydberg atomic ensemble is more and more in-depth,so the thesis mainly focuses on the quantum coherence effects and quantum coherence manipulation in ultra-cold Rydberg atomic ensemble.Our research includes the STIRAP and EIT in Rydberg atomic ensemble.In addition,we propose a viable scheme to implement the single photon source by combining the dipole blockade effect and the light storage process in the Rydberg atomic ensemble.In Chapter three,we consider the adiabatic passage for pairs of interacting cold atoms driven into three-level ladder configuration and four-level Y configuration which include two Rydberg states,respectively.The long range interaction in Rydberg medium will lead to the dipole blockade effect which can block the excitation of two or more atoms to Rydberg states.However,we find,with proper single-photon and twophoton detunings,that is viable to(i)achieve efficient population transfer from the ground state to either Rydberg state by fully overcoming the dipole blockade effect and(ii)implement maximal entangled states by partially overcoming the diploe blockade effect.After careful optimization,we obtain the parameters for the implement of complete population transfer and maximal entangled states with high fidelities.We expect that similar multipartite entanglements could be further attained if our scheme is extended to study three or more atoms,which is significant for quantum communication.Additionally,by the study of four-level Y configuration,we can figure out the different blockade effect to different states arising from different kinds of interactions,which enables us to get a deep understanding of the blockade and antiblockade effects.In Chapter four,we study the electromagnetically induced transparency in a Y system with single Rydberg state via the accurate superatom(SA)model.In the numerical calculation,we break the limitation of the weak field approximation and consider all the relevant high-order collective states,which makes the results more accurate and reliable.It is more convenient to compare the differences between the EIT windows arising from the ordinary excited state and the Rydberg state.Two EIT windows generate in the transmission spectrum.One window is low and immune to the probe field intensity since it is caused by the two-photon resonance from the ground state to the short-lived ordinary excited state.The other window caused by the resonance from the ground state to the long-lived Rydberg state exhibits cooperative optical nonlinearity with the probe field.When two EIT windows overlap,the degenerate window becomes a sharp non-Lorenz curve and the cooperative optical nonlinearity plays a leading role.In addition,the statistical property of the probe field changes in the propagation process.It is viable to change the degree of photon correlation by controlling the probe and coupling fields.In Chapter five,we propose a scheme to implement a controllable single-photon source utilizing the storage process in Rydberg atomic ensemble adopting the superatom method.We store probe photons in highly excited collective Rydberg states as Rydberg dark state polaritons of cold atomic ensemble using EIT and subsequently convert them into light.Only one atom can be excited to the Rydberg state in the blockade sphere owing to the blockade effect,which means that we can only store one photon in each SA,so at the sample exit,it is viable to attain the single-photon sequence which can be certified by the results that two-photon correlation of the probe field is less than 1.0 and the output photon energy shows nonlinear dependence of the input photon energy.If we use the stable probe field instead of the Gaussian pulse,it is feasible to attain the homogenous single photon sequence.Compared with the singlephoton device implemented by the propagation process,our scheme is controllable and the benefit is obvious.