Integrated Solid-state Quantum Memory Based On Femtosecond Laser Direct Writing

Optical quantum memories are crucial devices in quantum information science.They have various applications,for instance,converting the heralded photons to on-demand photons,enhancing the precision of quantum metrology,synchronizing the op-erations in quantum computation,more importantly,constructing quantum repeaters to overcome the exponential attenuation of photons in the transmission channels,thus making it possible for long-distance quantum communication.Great progress has been made on the researches of quantum memories,however,for the purpose of promoting these devices to practical applications,developing quantum memories that can be minia-turized and integrated with other devices is an urgent requirement.Quantum memories based on solid-state systems have stable physical and chemical properties,and are easy to be manufactured for the development of integrated quantum memories.Rare-earth ions doped in solids possess ultra-long coherence time at liquid helium temperature,which are promising candidates for quantum memories.During the Ph.D.,my major research topic is developing an integrated quantum menory based on the rare-earth-ion-doped crystals by utilizing the femtosecond laser direct writing technique.The main results of this thesis are:1.determining the spin Hamiltonian of the optical ground state of the 143Nd3+:Y2SiO5 crystal.Quantum memory relies on specific energy levels in the storage medium.We have to choose a suitable energy level system,for instance,a A system,before we can apply the quantum memory schemes.Therefore,we need to have full knowledge of the energy level structure of the storage medium,i.e.,we have to determine its Hamiltonian.As a typical Kramers ion,143Nd3+occupies electron spin S=1/2 and nuclear spin I=7/2.It's hard to determine the spin Hamiltonian of its optical ground state,which contains 16 energy levels,with the traditional optical spectroscopic methods.By utilizing the pulsed ultra-low-temperature electron paramagnetic resonance spectrometer that I participated in the development of,in combination with a home-made program based on M AT LAB and Easy Spin,I determined the spin Hamiltonian of the optical ground state of 143Nd3+with an average deviation compatible with the experimental error.In principle,this method can be directly employed to determine the spin Hamiltonian of other Kramers ions.2.participating in building up the integrated quantum memory platform based on the rare-earth-ion-doped crystals.The femtosecond laser direct writing technique has the property of high fabrication accuracy,repeatability,and stability.I utilized this technique to fabricate type ?,? and? optical waveguides in the rare-earth-ion-doped crystals,and each waveguide can be utilized as a quantum memory.These different kinds of optical waveguides have differ-ent applications in quantum Information science,for instance,type ? waveguides can greatly enhance the power intensity of the light field,type ? waveguides can support the single-mode transmission of light with different polarization,and type ? waveg-uides lie 20 ?m below the surface of the crystal,thus can be easily integrated with other on-chip devices.3.demonstrating high-fidelity coherent optical memory based on the 151 Eu3+:Y2SiO5 crystal.151 Eu3+:Y2SiO5 crystals can possess coherence time as long as 6 hours,which is the longest coherence time for matter systems,making it have promising applications in quantum information science.? fabricated type ? waveguides in a 151Eu3+:Y2SiO5 crystal.The waveguides are compatible with single-mode fibers,and have an insertion loss of 4.95 d B.The reliability of this device is confirmed,by conducting coherent optical memory based on two different memory schemes in a waveguide,both with storage fidelity of approximately 99%.4.developing the integrated quantum memory with on-demand read-out.I fabricated on-chip waveguides on the surface of a 151Eu3+:Y2SiO5 crystal,and introduced two electrodes beside the waveguides to perform the Stark-modulated atomic frequency comb memory scheme,achieving on-demand storage of photonic qubits with a fidelity of 99.3%±0.2%,which is close to the highest fidelity record(99.9%)ob-tained in the bulk crystal.The storage time can be actively controlled by applying two electric pulses on the electrodes with a peak voltage of 5 V.This transistor-transistor-logic-compatiable setup paves the way for the further extension and integration of our waveguide-based quantum memories.