Coherent Dynamics Of Rare-Earth Ions At Ultra-Low Temperatures

Quantum memory is a key component for the construction of a long-range quan-tum network.Rare-earth ions(REIs)in solids have been widely recognized as one of the most promising candidate material for the realization of quantum memories due to their abundant working wavelengths,long coherence lifetimes,large storage bandwidths,ex-cellent multimode capacities,and the capacity to be easily processed and integrated.For quantum memory applications based on REIs in solids,to transfer the optical excitation to the ground-state nuclear-spin energy levels can largely extend the storage lifetime,and can acquire the capacity of on-demand retrieval.For the electronic spins of the REIs,both the Zeeman interaction experienced inside the external magnetic field,and the hy-perfine interaction with their coupled nuclear spins can reach the magnitude of?GHz.and the quantum memory with large storage bandwidth and multimode capacity can be efficiently supported.The existence of the large electronic magnetic moment can to a large extent prohibit the decoherence effect induced by the flip-flops of the host nuclear spins.Moreover,strong coupling have been achieved between the rare-earth electronic spins and the microwave photons in the superconducting circuits,which makes the REIs in solids the appealing candidate for the realization of an interface between quantum computation and quantum communication.Therefore,the coherent dynamics of the electronic and nuclear spin of REIs in solids have attracted a lot of interests.However,to address the energy levels of the electron-nuclear coupled spin system is very hard using the traditional optical tech-niques because of the large optical inhomogeneous broadening of the REIs in solids.Pulsed electron paramagnetic resonance(EPR)and electron-nuclear double resonance(ENDOR)are the classical methodologies for the investigation of the coherent proper-ties of an electron-nuclear coupled spin system.Previous studies have shown that,to bring the REI-doped solids into the working environment with ultra-low temperature will largely enhance the population and coherence lifetimes for both the electronic and the nuclear spins.However due to the inevitable heating effect caused by the high-power microwave and RF pulses,it is widely believed that the pulsed EPR/ENDOR spectrome-ter which is compatible for a general bulk sample and the ultralow-temperature working environment cannot be constructed.Moreover,our long-term goal is to build an exper-imental platform integrated with the operating capabilities for both of the optical and spin transitions.In consideration of these circumstances,the work completed in the thesis is concluded as follows:(1)we have successfully combined a classical pulsed EPR spectrometer within a cryogen-free dilution refrigerator.The lowest working temperature is evaluated to be no more than 100 mK.The temperature evaluation is rigid and is performed under different criteria.With the aid of the electron spin polarization under 100 mK,the ground-state hyperfine level structure(with 16 energy levels in total)of 143Nd3+:Y2Si05 is directly measured with a series of novel ENDOR sequences.The problem of the uncertainty of the effective spin Hamiltonian is resolved.We have further performed the studies on the coherent dynamics of the coupled electronic and nuclear spins.The experimental results show that,along with the decreasing of the sample temperature,the population and coherence lifetimes of both the electronic and nuclear spins have experienced a comprehensive enhancement.Acceleration of the enhancement of these lifetimes is observed under 1 K.At the lowest working temperatures,the two-pulse-echo coherence lifetime of the electronic and nuclear spin exceeds 2 ms and 40 ms,respectively.The population lifetime exceeds 15 s and 10 min,respectively.The coherence time of the electronic spins of REIs have therefore been promoted for an order of magnitude and reach the ms time scale for the first time.(2)We have further constructed a preliminary optical working platform,based on which the basic coherent optical spectroscopy of 167Er3+:YVO4 is investigated.At zero magnetic field the 1.5-?m optical transition is divided into three absorption bands;each two of them has a separation of approximately 2 GHz.As the magnetic field is enhanced up to 1 T,the optical coherence lifetime reaches its superhyperfine limit,corresponding to the effective homogeneous linewidth less than 1kHz.The ground-state relaxation time is measured to be greater than 15s,with is more than 3 orders of magnitude com-pared with the relaxation time of the optical transition.These results have solidly convinced the advantages of the deeply-cooled REI-doped solids to be utilized in the applications of quantum memory and microwave-optical quantum transduction.Besides,the scope of applications for an EPR spectrom-eter with ultralow working temperature based on a conventional 3D resonator can be considerably large.For example,the coherence lifetime of molecular nanomagnets is expected to be greatly enhanced.Making use of the slow decoherence and relaxation rates under ultralow temperatures,some previous inaccessible resonance signals can be expected to be observed,such as the cases for the[NiFe]-hydrogenase,and Mn clus-ters in Photosystem ?.The large degree of electron spin polarization is beneficial for the investigation of the electronic ground states of the moleculer nanomagnets,and the complete characterization for the hyperfine constants inside a spin Hamiltonian.