The Memory And Frequency Conversion Of High-dimensional State Based On Rubidium Atoms

As one of the important research directions of quantum information,quantum communication is a communication process by transferring quantum states between two nodes with long distances.Currently,in quantum communication networks,the main concern is the distance and rate of communication.In terms of increasing the communication rate,there are typically two solutions:the first is to increase the sending rate of the signal from the physical source,and the second is by increasing the capacity of the carrier.For the former,the sending rate of the carrier is limited by the performance of the actual system,while the latter can greatly increase the channel capacity by encoding the information in the high-dimensional Hilbert space of the photon.In this thesis,we use the second scheme to increase the capacity of the carrier by encoding the information in the orbital angular momentum(OAM)degree of freedom of light.In terms of communication distance,quantum repeaters are the essential to achieve long-distance quantum communication,and it plays a role to the decay of light from an exponential correlation to a polynomial correlation in long-distance communication.In the scheme of quantum repeater,quantum memory and frequency convertor are the important devices for long-distance quantum communication.The quantum memory plays the role of synchronizing the quantum states between different nodes,and the frequency convertor serves to convert the wavelength of light to the communication band so that the distance of entanglement distribution can be extended.Meanwhile,the efficiency and the fidelity of the quantum memory and frequency convertor become important performance criteria to achieve long-distance quantum communication,where the storage(frequency conversion)efficiency represents to the ratio of the energy of light after and before it passes through the node,and the fidelity represents to the integrity of the information after passing through the node.Also,since atomic systems have a spectrally consistent property in the study of quantum storage and frequency conversion.In this thesis,We focus on the combination of 85Rb atomic systems with the OAM degrees of freedom of light,and investigate the storage process and frequency conversion process of high-dimensional state which is composed of the OAM light.Here,we improve the efficiency of storage and frequency conversion by increasing the optical depth(OD)of the system,and explore the high-fidelity storage and frequency conversion processes of high-dimensional state by combining the perfect optical vortex(POV)beam.Such high-efficiency quantum memory and frequency conversion of high-dimensional states have a promising application in high-capacity and long-distance quantum communication networks based on the scheme of quantum repeater.The main contents of this thesis includes:1.We experimentally prepared a cold 85Rb atomic cloud with high OD through the technique of dark spontaneous-force optical trap,and achieved frequency down conversion of light by a Four-wave mixing process within this cold atomic ensemble.During this frequency conversion process,the signal light at 795.0 nm was attenuated to the single photon level and converted to the telecom light at 1529.3 nm.Then we experimentally investigated the effect of different OD on the frequency conversion effciency and theoretically fitted and analyzed the results,where the frequency conversion efficiency reaches up to 32%at an OD of 190.2.The conventional optical vortex(COV)beam has the characteristic that the ring radius varies significantly with the topological charge number l,which makes it difficult to realize a high-fidelity frequency conversion process of high-dimensional state by using the COV light.Therefore,we experimentally use a POV light that carries OAM phase information,and its ring radius is independent of l.Then,by combining the POV light and the hot 85Rb atomic system,we realize the frequency conversion process with l-independent efficiency at least in a subspace of l ∈[-6,6].Finally,based on the uniform frequency conversion efficiency of POV light with different l,we achieve a high-fidelity frequency conversion process for the 7-dimensional superposition state,and the fidelity reaches up to 88.67±1.23%after frequency conversion.We also experimentally tested the frequency conversion process of the 3-dimensional OAM superposition state based on the COV light,and its fidelity after frequency conversion was only 50.97±0.19%,which is much smaller than the frequency-conversion fidelity of the 7-dimensional superposition state by using the POV optical field.In terms of achieving frequency conversion of high-dimensional state,the POV light has an advantage over the COV light.3.The storage efficiency,dimensionality and fidelity are important parameters in the quantum memory of high-dimensional state.In this thesis,by combining a POV light and a 4-f imaging system(used to reduce beams),we achieve a storage process of high-dimensional state with an efficiency close to 60%in a high coId 85Rb atomic system,where the intensity of the input light is attenuated to the single photon level.In addition,the storage efficiency with l-independent was experimentally found in the subspace of l ∈[-12,12],and high-efficiency and high-fidelity(up to 90.3±0.6%after subtracting the background noise)storage of a 25-dimensional qudit was achieved on this basis.There are no reports about a storage process of high-dimensional state with a dimension up to 25 and an efficiency of over 50%by using OAM light.Remarkably,the fidelity of the 25-dimensional qudit state after storage far exceeds the classical fidelity threshold of 9.2%,which proves that our memory can work well in the quantum domain.We also theoretically analyze the capacity and efficiency of the memory and find that the dimensionality of the memory can be extended to 100 by changing the parameters of POV light and the scaling ratio of the imaging system,which provides a reference for further improvement of the storage capacity and efficiency in the future.The main features and innovations of this thesis are as follows:1.The frequency conversion process from the operating wavelength of the memory to the wavelength in the near C-band that has low propagation loss in the fiber is realized in the 85Rb atomic system,and the relationship between OD and frequency conversion efficiency is investigated both experimentally and theoretically,which provides a reference for the realization of efficient frequency conversion of high-dimensional state in the future.2.We propose a scheme that combines a POV light and a hot 85Rb atomic system to realize a frequency conversion process of high-dimensional state.This POV light solves the problem that the efficiency of the OAM light varies significantly with l during the storage process.Meanwhile,we achieve the frequency conversion process of 7-dimensional state with a fidelity up to 88.67±1.23%in the 85Rb atomic system.In contrast,the fidelity of the 3-dimensional OAM superposition state based on the COV light can only achieve 50.97±0.19%in the same frequency conversion process.This frequency conversion scheme can be combined with a high OD cold atomic system mentioned above,and provide a reference for the realization of efficient frequency conversion process of high-dimensional state.3.By combining POV light and cold 85Rb atomic ensemble,we achieve the storage process of high-dimensional state with a dimensionality up to 25,a storage efficiency close to 60%and a storage fidelity up to 90.3±0.6%(after subtracting background noise).Currently there is no report on achieving these indicators simultaneously by using the OAM degree of freedom of light.More importantly,the capacity and efficiency of the memory can theoretically be further improved by tuning the parameters of the POV beam and the experimental optical path.This indicates that our memory has a large potential for achieving a quantum memory with a higher dimensionality.This efficient storage process of high-dimensional state can be combined with an efficient frequency conversion process of of high-dimensional state that serves in long-distance and high-capacity quantum communication networks.