Experimental Study On Performance Optimization Of Quantum Memory Based On Cold Atom Ensemble

Quantum communication makes use of the uncertainty and nonclonability of quantum superposition state and the collapse principle of entanglement measurement to become an absolutely secure communication mode of quantum information transmission which cannot be eavesdropped and cracked.In the process of quantum communication,long-distance transmission of quantum information will have a great loss.Quantum repeater scheme uses quantum memory as a node to realize quantum states information transmission,requiring that the memory as a quantum repeater can maintain the single quantum state information for a long time,and can also recover with high fidelity and high efficiency after a long-distance transmission.Quantum repeater is conducive to the construction of large scale and long distance quantum networks.DLCZ scheme uses atomic ensemble and linear optics to construct quantum repeater nodes.Cold atom ensembles are a suitable storage medium based on the collective enhancement effect.By the interaction between the write light and the atomic ensemble,the entangled photon pairs are prepared,and the coincidence counting measurement is carried out for the quantum memory.The relevant parameters characterizing the entanglement characteristics are obtained as the indicators to measure the performance of the quantum memory,and as the measurement standard for further testing and optimization.On the basis of generating and storing entangled photons in cold atom ensembles,we have carried out the following work to optimize memory performance:(1)Noise suppression in a temporal-multimode quantum memory entangled with a photon via an asymmetrical photon-collection channel.In this chapter,we propose an experimental scheme using asymmetric quantum channels to suppress the additional noise of time multipath quantum interface(QI).The scheme uses an asymmetric channel to collect entangled photons generated from time-multiplexed QI,compared with QI for photon collection using a symmetrical channel.When 14 modes are stored,the Bell measurement parameters S measured using asymmetric and symmetric photon acquisition channels are 2.36±0.03 and 2.24±0.04.The results show that the use of asymmetric channels increases the entanglement fidelity by 3%,which means the extra noise is reduced by 1.7 times.Using asymmetric and symmetric acquisition channels to store 14 entangled modes of quantum memory,the memory time can reach 25μs and 20μs,respectively,but still violate the Bell inequality,in addition,according to the decline trend of multi-mode memory of multi-mode memory,it is predicted that the multi-mode memory capacity of the asymmetric channel acquisition mode increases to 42 modes from 26 modes of symmetric channel acquisition.The results show that the asymmetric quantum channel has a longer entanglement lifetime and its multi-mode memory performance is optimized.(2)The generation of entanglement between photon and atomic memory with tunable pulse width.We demonstrate that the Stokes photon wave packet with pulse width of 40ns～50μs is generated by changing the interaction time between light and atom.The relationship between the recovery efficiency and the Bell parameter S and the Stokes photon pulse width was measured.The results show that the S parameter decreases with the pulse duration because the background noise increases with the pulse duration.(3)The preparation of a long lifetime entanglement source for photonatom.By applying an axial magnetic field to screen out magnetically insensitive and magnetically sensitive spin waves,and by using a phase-stable polarization interferometer and coupling mirror to configure the write and probe paths in a small Angle near collinear configuration,the established quantum entanglement generation system can be used to generate three kinds of spin wave superposition qubits storage and measurement.This work lays a foundation for making full use of quantum resources to produce multi-mode entanglement sources.Based on the above experimental studies on the preparation of entanglement source and the optimization of memory performance of cold atom ensemble,we effectively improve the recovery efficiency,memory lifetime and multi-mode memory capacity of quantum memory,and effectively solve the pulse-width matching problem of hybrid quantum network nodes through the preparation of tunable pulse-width photon wave packet.This work lays a foundation for developing high-performance quantum memory,constructing efficient quantum relay and large-scale quantum network.