Hybrid Integrated Photon-atom Circuits For Quantum Internet Applications

The development of quantum mechanics has impacted the process of nature sci-ences deeply and permanently since 20th century.Combining both conventional com-putation and information science with quantum mechanics,the two derivative disci-plines quantum computation and quantum information show some incomparable advan-tages.For instance,the quantum algorithms aimed at specific proposals could solve the problems which are unrealizable in classical computation.Furthermore,the features of the quantum state including forbidden cloning and quantum entanglement can confirm the security of quantum communication based on single photons.For these advantages quantum computing and information are thought to be promising physical frontiers,and have extremely attractive prospects with both opportunities and challenges.One of the most valuable applications of quantum computation and quantum in-formation is quantum internet.Quantum internet is consisted of three basic elements,which are quantum nodes,quantum channels and quantum repeaters.Quantum nodes,which generate,process and store quantum information,are connected by long-distance quantum channels,while quantum repeaters establish and distribute entanglement.At present,investigators are attracted to seek reliable physical platforms to construct the three elements.Here we focus on building quantum nodes we mentioned above and quantum interfaces which link nodes and quantum channels.For quantum nodes,we devote to combining integrated photonic circuits with single cold atoms under the princi-ple of cavity quantum electrodynamics(C-QED).For quantum interfaces,we study the on-chip frequency converter based on AlN microcavities and discuss the tuning effect of telecom signal to the atomic fluorescence.Our demonstration has great advantages of scalability and security,and all our contribution can be summarized as:1.Fundamental studies about optical C-QED systemWe started to build a Fabry-Perot cavity at very beginning to investigate the op-tical C-QED system theoretically and experimentally.A glass vapor cell which is full of hot Rb ensemble is placed inside the cavity to investigate the light-atom interaction.We finished two work based on this setup:(1)the cavity-enhanced electromagnetical-ly induced transparency(EIT)in hot-atom ensemble;(2)An all-optical isolator based on the incoherent atomic population transfer.In the first section we observed the EIT phenomenon where both probe and control lasers are on-resonant with the two-mirror cavity.A new theoretical model was built to describe such a doubly-resonant effect,and the results indicated that the doubly-resonant EIT is equivalent to a non-hermitian three-wave mixing process.In the second section we transformed the two-mirror standing-wave cavity to a four-mirror traveling-wave cavity.Based on the circular dichronism and incoherent population transfer of hot atom ensemble,the isolation ratio of the de-vice exceeded 51 dB,which could be one of the highest parameter people have ever reported previously.Besides,the device has great advantages on parametric stability,bandwidth,working range and noise.Our demonstration paves the way for future stud-ies of magnetic-free photonic non-reciprocal devices,unidirectional quantum storage and quantum state transfer,and also studies of topological photonics on a chip.2.Magneto-optical Trap(MOT)and single Rb atom trapIn this part we focus on trapping the cold atom ensemble and single atom of Rb,which are the crucial steps to establish the quantum nodes by C-QED system.Based on the principle of laser cooling,we built a MOT to trap the cold atom ensemble and estimated the temperature at about 42.3K by absorption imaging.Besides,a far-off-resonant trap(FORT)with high intensity and a tight focusing waist is built by an in-frared laser at 850nm and a home-made objective,where a single Rb atom is success-fully trapped.We observed and analyzed the telegraph and histogram of single atom fluorescence,as well as the second order correlation function.The results prove the successful capture of a single atom.3.Resonance tuning of the AlN microcavity frequency converterThe 780nm photons transmitted from quantum nodes should be converted to around 1550nm by different frequency generation to reduce the loss of fiber-based quantum channels.In this part we investigate the resonance tuning of AlN microcavity frequen-cy converter based on second-harmonic generation,which is the inverse process of dif-ferent frequency generation.Two approaches,including auxiliary laser and controlled heater have been studied.We investigate the tuning effect of the two approaches,and build theoretical models to describe the dynamics process intracavity.4.Packaged photon-atom deviceThe device,which is filled with hot Rb ensemble and bonded to integrated SiN waveguide chip,is designed and fabricated for basic integrated photon-atom interac-tion due to the experimental obstacle of trapping a single cold atom on the surface of integrated photonic circuits.We study the light-atom interaction between the evanes-cent field and hot atom ensemble,and two specific phenomena are discussed:(1)the saturated absorption of atoms flying across the waveguide evanescent field;(2)the chi-ral effect based on Zeeman effect and polarization of evanescent field under a strong magnetic field.The results pave the way of building the quantum nodes based on inte-grated photonic circuits and single cold atoms.