Research On Correlation In A System Of Interacting Atoms

The interaction between photons propagating in vacuum is extremely difficult.This characteristic of light propagation,coupled with enough high frequency and large bandwidth,makes the optical signal become the preferred medium for long-distance communication.However,sometimes the processing of information requires some form of interaction between signals,which can often be achieved by nonlinear optical processes.In the 1970 s,it was found that light fields can interact with each other in nonlinear optical media.However,at the same optical power,the nonlinearity of traditional materials is negligible compared with that of a single photon.Therefore,it is a long-term goal to achieve nonlinear effects while reducing optical power or pulse energy,the research under this limit is called quantum nonlinear optics.The realization of quantum nonlinear optics can improve the performance of classical nonlinear devices,such as fast energy saving optical transistors to avoid heat generation.In addition,nonlinear switches controlled by single photons can realize optical quantum information processing,quantum communication,and other applications that depend on the generation and manipulation of non-classical light fields.There are many ways to generate optical nonlinearity at the single photon level,such as coupling with a single atom through a cavity,mapping photons to atom ensembles,atom-atom interaction and so on.These systems show strong photon-photon interactions,which make many unique applications.Here,we choose an atom-atom interaction system without a cavity to realize the strong nonlinear interaction between a single photon by making use of the strong long-range interaction between Rydberg atoms.The specific research focuses on the third and fourth chapters:In Chapter 3,the transmitted properties of a quantum probe field traveling through a one-dimensional sample of ultracold Rydberg atoms has been studied.Based on the dipole blockade effect,the steady-state spectrum of the electromagnetically induced transparency(EIT)in a four-level configuration exhibits the typical nonlinearity at single-photon level,namely,the transmitted intensity and photon correlation are all sensitive to the input probe intensity before saturation.On one hand,this allows us to flexibly manipulate nonlinear EIT by changing the single-photon detuning of the two classical fields.On the other hand,transformation of EIT nonlinearities from the four-level atomic configuration to the three-level ladder type one is also observed via the transmitted spectrum by varying the ratio of the Rabi frequencies of two classical fields.In Chapter 4,we study the transmitted properties of weak probe fields in interacting atom systems in a typical Rydberg electromagnetically induced transparency system,focusing on the cooperative optical nonlinear behavior of the probe field phase based on the dipole blockade effect.By comparing with the transmitted intensity and photon correlation,it is found that the optical response of the phase has new characteristics: the phase is insensitive to the transmitted intensity and the initial photon correlation under resonance and Autler-Townes splitting conditions,but the frequency between the phase response within the range is nonlinear,especially at the classical optical frequencies.In addition,increasing the main quantum number and atom density will promote the nonlinear effect of the phase.In summary,like the transmitted intensity and photon correlation,phase can be used as another indicator of cooperative optical nonlinear to characterize nonlinear phenomena,which is a powerful complement to Rydberg electromagnetically induced transparency research.