Generation Of Non-classical Source And Quantum Storage Based On Cold Atomic Ensemble

The nature of photons has abstracted many interests of scientists for centuries.The famous Young-type double-slit experiment revealed the wave-like behavior of light,however the explanation of photoelectric effect proposed by Einstein implied the particle-like behavior of photons.Therefore,these contribute to the current explanation of photon that is wave-particle duality.The quantum light theory proposed by Einstein has not only improved the people’s understanding of photons,but also pushed the development of quantum mechanics.As the fundamental particle of quantum mechanics,photons have been widely applied in many quantum information processes,such as quantum computing,quantum communication,quantum image and quantum cryptography etc.In the study area of quantum information,the photons can be encoded in different degree of freedoms such as polarization,frequency and orbital angular momentum,and the generation and storage of quantum entangled states are the basic requirements for quantum information processing.Meanwhile,many interests have been attracted to enlarge the capacity of information encoded in entangled photons,in which multi-degree-of-freedom entanglement has been proposed for high-capacity of quantum networks.In addition,nowadays multi-particle entanglement generation is emergent for fault-tolerant quantum computing and high-accuracy quantum metrology.This thesis is mainly focus on the realization of quantum entangled state carried with high capacity information and multi-particle entanglement.We have experimentally achieved entangled states in multi-degree-of-freedom as well as the generation and storage of multi-photons entanglement based on atomic ensembles.This works in the thesis are promising for quantum computing and high-capacity quantum communication.The main content of this thesis includes:1.The generation of two-color hyper-entangled photon pairs.We firstly derive a cold 85Rb atomic cloud through laser cooling and trapping.Then we directly generate the hyper-entangled photon pairs with wavelength of 795 nm and 1475 nm through spontaneous four-wave mixing(SFWM)process in a diamond atomic configuration,which is entangled in both time-frequency and polarization degree of freedom.To characterize the hyper-entanglement,we use the quantum beat phenomenon to demonstrate the time-frequency entanglement of photon pairs,and we measure the two-photon interference and check the violation of Clauser-Horne-Shimony-Holt(CHSH)inequality to verify the polarization entanglement of photon pairs.The experimental results show that we have successfully generated a hyper-entangled state.2.The generation of narrow-band four-photon Greenberger-Horne-Zeilinger(GHZ)state based on an atomic ensemble.We firstly generate two pairs of entangled states via multiplexing two spontaneous four-wave mixings in a single cold 85Rb atomic ensemble.Then,a parity check gate is utilized to postselect the desired GHZ state.Ultimately,we adopt the quantum state tomography to investigate the four-photon entangled state,which confirms that we have prepared the genuine narrow-band GHZ state well.3.The storage of a two-photon NOON state based on atomic quantum memory.Firstly,we prepare two cold atomic ensembles,one of which is utilized to generate two photons with identical frequency by using SFWM process in a double-Λ energy level configuration.The other one acting as a quantum memory is used to store the NOON state with an aid of active-locked Mach-Zehnder interferometer since the NOON state has been prepared by combining two photons emitted from the first atomic ensemble in a HOM interferometer.The experimental results confirm that NOON state has been stored well in the atomic quantum memory.4.The study of Wheeler’s delayed-choice experiment based on atomic quantum memory.We use three cold atomic ensembles to demonstrate Wheeler’s idea in the temporal domain.A heralded single photon is generated from the first atomic ensemble,and the other two atomic memories based on Raman storage protocol act as memory-based beam splitters,which construct a temporal Mach-Zender interferometer.This delayed-choice experiment using quantum random number and temporal interferometer demonstrates that it makes no sense to illustrate the wave-like or particle-like behavior of light before the measurement happens.The main features and innovations of this thesis are:1.We for the first time realize the hyper-entanglement between visible and infrared photons.Because the generated visible photon can couple well with atomic energy levels,it is suitable for interfacing with quantum repeater based on atomic quantum memory.In addition,the prepared infrared photon is at telecom-wavelength of fiber-optic communication,therefore it is feasible for long-distance quantum communication.Compared to the single degree-of-freedom entangled photons,the generated photons in our work are entangled both in time-frequency and polarization degree-of-freedom,greatly enlarging the capacity of information encoded in photons.Thereby,this two-color hyper-entangled photon pair is promising for high-capacity and long-distance quantum communication.2.For the first time,we experimentally achieve the narrow-band multi-photon GHZ state in an atomic ensemble,where the number of involved photons is larger than three.In our experiment,we multiplex two SFWM processes in a single atomic ensemble,which saves the number of atomic ensembles and simplifies the experimental configuration.In addition,the generated four-photon GHZ state with narrow bandwidth can interact with atoms effectively,which has many potential applications in quantum information processing based on light-atom interaction.3.For the first time,we realize the storage of multi-photon NOON state.We successfully store the two-photon NOON state via the Raman storage protocol based on a cold atomic ensemble,and thus interfacing the two-photon NOON state with atomic quantum memory,which paves the way for the storage of multi-photon entangled state.4.We for the first time demonstrate the Wheeler’s delayed-choice experiment in a hybrid system consisting of photons and atoms simultaneously.We explore the atomic quantum memories to construct a temporal Mach-Zehnder interferometer to demonstrate the idea of Wheeler,and this work is benefit for improving the comprehension of complementarity under the picture of light and matter interaction.