Investigations On The Optical Spectroscopy And The Magnetic Resonance Spectroscopy Of Er³⁺ Ions

Quantum repeater is one of the key technologies for implementing quantum networks.However,the physical systems currently used to demonstrate quantum repeater are not in the communication band,therefore requiring frequency conversion or nondegenerate photon pair sources to connect with the communication band,which can lead to additional efficiency loss.The rare-earth ion Er3+ is one of the popular candidate systems for quantum repeater because its optical transition is in the communication band and it has an excellent optical coherence time.However,the current application of Er3+ ions in quantum repeater still faces problems such as too short electron spin coherence time and too long optical relaxation time.This article focuses on the spectroscopic and magnetic resonance studies of erbium ions regarding the above-mentioned issues.The specific details are as follows:1.Coherent spin dynamics of Er3+ in Y2SiO5 explored with electron-nuclear double resonance at subkelvin temperatures.The electron spin of Er3+ ions in solid state is subject to magnetic disturbance caused by spin flips of surrounding ions,resulting in a short spin coherence time.Sufficient polarization of the spins can reduce the spin flip of Er3+ ions.According to the Boltzmann distribution,lowering the temperature can achieve the full polarization of spins of Er3+ ions.Therefore,we used our self-built cryogenic electron-nuclear double resonance spectrometer to study the spin dynamics of Er3+:Y2SiO5 crystal.Experimental results show that with decreasing temperature,the relaxation lifetime of electron spin and the coherence time of both electron and nuclear spins are significantly improved.Under the conditions of the lowest temperature achievable in the cryostat and a specific magnetic field angle,the electron spin relaxation lifetime reached 0.5 s,and the coherence time of the electron spin reached 290μs,nearly 40 times longer than that reported in previous works.2.Control of the relaxation lifetime of Er3+ optical transitions based on a photonic crystal cavity.Due to the parity forbidden transitions between the 4f levels of rare-earth ions,the optical excited state lifetimes of rare-earth ions in solids are relatively long.Among them,the optical relaxation lifetime of Er3+ ions reaches 11.4 ms,seriously affecting the luminescence efficiency and optical pumping efficiency of Er3+ ions,hindering their use in quantum storage and quantum light sources.The use of optical cavities can enhance the coupling between light and Er3+ ions,thereby reducing their optical relaxation time.Therefore,we used photonic crystal cavities to achieve Purcell enhancement of Er3+ ions in Y2SiO5 crystals.We measured the minimum fluorescence lifetime of Er3+ions to be 12.5μs,which is nearly 900 times lower than 11.4 ms.In addition,we tested the excitation fluorescence spectrum within the resonance frequency range of the photonic crystal cavity and obtained a fluorescence peak with g(2)[0]=0.10,achieving frequency-domain addressing of individual Er3+ ions.3.Frequency tuning of a single Er3+ ionIt is necessary to ensure that photons emitted from independent Er3+ ions are indistinguishable,which is a prerequisite for achieving entanglement swapping.However,due to the complex local environment within the solid,different Er3+ ions exhibit different optical transition frequencies.Therefore,we utilized the Stark effect to change the transition frequency of a single Er3+ ion by applying an electric field.In the experiment,we achieved a maximum frequency shift of 182.9±0.9 MHz,which is 27 times the linewidth of its emission spectrum.This experiment provides a useful solution for frequency alignment for entangling different Er3+ ions,laying the foundation for constructing quantum repeater with Er3+ ions.The innovations of this paper are as follows:1.This is the first experimental study of electron-nuclear spin double resonance on Er3+ ions at subkelvin temperatures.The electron spin coherence time of Er3+ ions was measured to be 290 μs,which is 40 times longer than previous research results.2.The experimental study investigated the effect of photonic crystal cavity on the radiative transition rate of Er3+ ions,achieving Purcell enhancement of Er3+ ions.The lowest measured relaxation lifetime was 12.5 μs,and the Purcell factor reached as high as 900.Additionally,single Er3+ ions were successfully addressed.3.The paper demonstrates for the first time the Stark frequency adjustment of a single Er3+ ion,achieving a maximum tuning range of 182.9±0.9 MHz,which is 27 times wider than the emission linewidth.This approach provides a solution for frequency alignment between different Er3+ ions for entanglement generation.Moreover,it is the first study to use a single-ion method to investigate the Stark effect of Er3+ ions,which is more detailed and intuitive than the ensemble method.