Investigation On Coherent Feedback And Quantum Entanglement Based On Four-Wave Mixing In Atomic Ensemble

Quantum information is a new interdisciplinary subject,which integrates quantum mechanics,quantum optics,information science,etc.Among them,nonclassical states,such as squeezed state,entangled state,single-photon state and Schrodinger cat state,are considered as important quantum resources for quantum information transmission,quantum metrology,quantum computation due to their quantum correlation.A quantum state with high quantum correlation can effectively improve the communication fidelity of quantum information and the measurement precision of quantum metrology,so it is very important to improve the quantum correlation of the quantum state.We apply the feedback control strategy to the quantum system,and propose two coherent feedback control schemes to enhance the quantum correlation of quantum system.In addition,multiplexing is a very important concept in information communication.By combining multiple channels into a single channel,multiplexing can largely enhance the data-carrying capacity of the optical carrier.Orbital angular momentum?OAM?of light,one of the most fundamental physical quantities,has been used to realize information communication due to the infinite range of possibly achievable OAM modes.We combine the multiplexing with OAM and apply them to continuous variable?CV?quantum system.We demonstrate the OAM multiplexed CV entanglement,which can greatly improve the communication capacity of quantum system.The main contents of this paper are as follows:1.We implement a linear coherent feedback control system based on four-wave mixing?FWM?process in hot rubidium atomic vapor.By adjusting the feedback ratio of the linear feedback controller,we can improve the quantum correlation of the output fields.In addition,we find that there exists an optimal feedback ratio of the linear feedback controller to maximize the quantum correlation of the output fields.2.We implement a nonlinear coherent feedback control system based on FWM process in hot rubidium atomic vapor.By adjusting the intensity gain of the nonlinear feedback controller,we can improve the quantum correlation of the output fields.In addition,we find that there exists an optimal intensity gain of the nonlinear feedback controller to maximize the quantum correlation of the output fields.3.We implement OAM multiplexed CV entanglement based on FWM process in hot rubidium atomic vapor,in which 13 pairs of entangled Laguerre-Gauss?LG?modes:LG_?,prand LG_-?,conj,are deterministically generated,where?is the topological charge corresponding to the OAM mode and Pr?Conj?indicates probe?conjugate?beam.In the meanwhile,we experimentally demonstrate that there is no entanglement between the LG_?,prand LG_?,conj?l?0?modes.These results clearly confirm the conservation of OAM in FWM process from the viewpoint of CV system.In addition,we investigate the entanglement properties of three types of coherent OAM superposition modes.In the end,we also study the effect of pump beam radius on the number of OAM multiplexing.