| In recent years,with the development of quantum information technology,researchers have realized that different quantum systems have their own advantages in dealing with different problems.Because a single quantum system can not solve all the needs of practical problems,the hybrid quantum network composed by different quantum systems has become the solution to this problem.In order to make full use of the advantages of different quantum systems in hybrid quantum networks,it is necessary to realize the quantum connection between them.As a natural flying bit,photon is very suitable for connecting different quantum systems.As the core unit of quantum network,quantum node needs to be able to realize the reliable logical operation of quantum bits.At present,the systems that can be used as quantum nodes include trapped atoms,trapped ions,NV color centers and quantum dots.Because the trapped ion system has the characteristics of high operation fidelity,long coherence time and identical qubits,it is favored by researchers.In addition,some nodes in quantum networks are expected to have the ability of quantum state storage.In this regard,solid-state quantum memory system has unique advantages.At present,one hour coherent optical storage has been realized.In addition,the system also has the advantages of multi-degrees of freedom,multi-mode storage and high storage fidelity.It is very suitable to be used as a storage node in quantum networks.Therefore,the hybrid quantum network structure composed of ion trap system as operation node and solid-state quantum memory as storage node has great research prospects.In this paper,we choose to study the hybrid quantum network with this structure,focusing on solving the photon interface problem when two kinds of quantum nodes are connected,mainly including the generation of connected photons(the preparation of ion-photon entangled states)and wavelength conversion.In this thesis,the preparation of entangled states is completed in the ion trap and considering the use of continuous light excitation preparation of ion-photon entangled states will lead to the decrease of ion-photon entanglement fidelity and entanglement generation rate,we use picosecond pulse to complete the excitation of ions.In addition,the fluorescence wavelength in the ion trap is 369.5 nm,while the working wavelength of solid-state quantum memory is 580 nm.There is a great difference between them,so it is difficult to realize direct conversion.This is also a common problem in the research of hybrid quantum networks.A common way to solve this problem is quantum frequency conversion,which can effectively maintain the coherence of quantum states before and after conversion and great progress has been made in visible and nearinfrared bands.However,there are few experiments to study the quantum frequency conversion in ultraviolet(UV)band,and the conversion efficiency is far lower than that in visible and near-infrared bands.Frequency conversion of UV wavelength is subject to two aspects:Firstly,commonly used UV nonlinear crystals,such as LBO and BBO cannot be used for quantum frequency conversion of ultraviolet wavelength because it cannot realize quasi-phase-matching and the choice of crystal types is limited.Secondly,for the commonly used quasi-phase-matching crystals,such as PPKTP(periodic polarized potassium titanyl phosphate)and PPLN(periodically polarized lithium lithium oxide),the former has strong absorption in UV band,while the latter is vulnerable to photorefractive damage,and their first-order quasi-phase-matching polarization period is very short(～2μm),the current processing technology is difficult to make such a short and uniform polarization electrode on the crystal.Through investigation and comparison,we found that PPSLT(periodically polarized lithium tantalate)crystal has high UV transmittance and damage threshold,which is more suitable for quantum frequency conversion in UV band.The main work and innovative achievements of this paper are as follows:1.Completed the ion-photon entangled state preparation by continuous light excitedis ion.The scheme of polarization entanglement is adopted in the ion trap,and the ion photon polarization entangled state with entanglement fidelity of 84.7%is prepared by continuous light weak excitation,which completes the relevant experimental preparation for the establishment of hybrid quantum network in the future.2.Developed the picosecond single pulse selection device independently,and the picosecond single pulse excitation of 171 Yb+ions was completed.Based on the twostage cascade AOM,the selective output of single pulse is realized,and the single pulse isolation ratio reaches the order of 106.The device will effectively improve the generation rate and fidelity of ion-photon entanglement.3.Solved the frequency locking problem of UV diode laser with large tuning range by using the self-developed symmetrical piezoelectric frequency reference cavity.The frequency reference has the characteristics of vacuum-free,simple structure and stable performance.The tuning range is>14.7 GHz and the short-term fluctuation of frequency is less than 0.6 MHz,which is base for ion Doppler cooling and subsequent waveguide performance test.4.Completed the design and fabrication of broadband waveguide,and the quantum frequency conversion from ultraviolet(369 nm)to visible(580 nm)band is realized for the first time.Using femtosecond laser direct writing technology,the broadband waveguide is fabricated in lithium tantalate crystal,and a symmetrical polarization quantum state conversion device is designed and realized the quantum frequency conversion channel with fidelity of 96.13%,the conversion efficiency of single photon is 0.26%W-1cm-1,and the signal-to-noise ratio in the conversion process is>500.This result lays a foundation for the construction of hybrid quantum networks of 171 Yb+ ion nodes and solid-state quantum storage nodes. |
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