Integrated Spin-wave Solid-state Quantum Storage

Optical quantum memories,as an interface between light and matter,can store optical quantum qubits.There are core devices to overcome the channel loss of longdistance quantum networks and play an important role in the construction of largescale quantum networks.For scalable and convenient practical applications,integrative,especially on-chip,quantum memories are crucial.The on-chip feature facilitates the combination of memories,surface electrodes,and integrated photonic circuits to achieve complete integration.There are many physical systems to realize quantum memory,such as single atoms,atomic gases,rare-earth-ion-doped crystals,etc.,and remarkable achievements have been reported.Rare-earth-ion-doped crystals have many advantages,such as large bandwidth,long lifetime,high fidelity,and multi-mode potential.And as a solid-state system,it has a natural advantage in realizing integrated quantum memories.The integration of quantum memories based on various fabrication techniques has been demonstrated in rare-earth-ion-doped crystals,such as LiNbQ3 waveguides,erbium-doped fibers,focused-ion-beam lithography,silicon-based photonic cavities,and femtosecond laser micromachining.Femtosecond laser micromachining is very suitable for preparing optically integrated structures in rare-earth-ion-doped crystal because of its μm-level three-dimensional processing accuracy and less damage to materials.For quantum memories,spin-wave quantum memories,which can support long-lived on-demand retrieval,are indispensable for practical applications but have never been demonstrated in an integrated solid-state device.The research goal of this dissertation is towards the integration of quantum memories,and finally realizing the demonstration of an integrated spin-wave quantum memory.Based on femtosecond-laser-micromachining,the author fabricated optical waveguide structures on rare-earth-ion-doped crystals close to the crystal surface,realizing the development of a series of on-chip quantum memories for different applications.The main research results obtained in this paper are as follows:1.Coherent optical memory based on type-Ⅳ optical waveguideCompared with other types of optical waveguides,type-Ⅳ optical waveguides are located on the upper surface of the crystal,which have the advantage of being conveniently combined with other on-chip integrated structures.The innovations of this work are:based on the Femtosecond-laser-micromachining system,we fabricated type-Ⅳoptical waveguides on the surface of rare-earth-ion-doped crystals(151Eu3+:Y2SiO5)and verified the optical properties of the Eu3+ ions in the waveguide are similar to those in the bulk crystal.Based on this waveguide,we demonstrate coherent optical storage using a spin-wave atomic-frequency-comb scheme.The high interference visibility 97±1%between the read pulse and the reference pulse confirms the reliability of the device.2.On-demand integrated quantum memory based on type-Ⅳ optical waveguideOn-demand storage means that the storage time can be read on-demand,which is an important indicator for quantum memories.The innovations of this work are:on the basis of the first work,we optimized the femtosecond processing parameters and process of the type-Ⅳ waveguide by using the method of multiple tracks whose per-pulseenergy near the critical power,so that the insertion loss of the waveguide was greatly reduced.At the same time,femtosecond processing is used to prepare groove structures on both sides of the optical waveguide to form buried electrodes to realize Stark modulation of ions in the optical waveguide region.Based on this on-chip structure and a Stark-modulated atomic-frequency-comb quantum storage scheme,we demonstrate ondemand quantum storage based on time-bin optical qubits.Using single-photon-level weak coherent pulses,the fidelity of qubit storage is verified to be 99.3±0.2%,which far exceeds the classical bound(81.4%).3.On-demand integrated quantum memory for polarization qubits based on type-Ⅲ optical waveguidePhotonic polarization qubits are widely used in quantum computation and quantum communication due to their robustness in transmission and easy manipulation.However,due to the anisotropic optical absorption depth of crystals,and the integrated structures only supporting a single polarization at that time in quantum memory,it is a long-term challenge to on-demand storage polarization qubits in an integrated quantum memory.The innovations of this work are:For the first time,a type-Ⅲ optical waveguide,known as a depressed cladding waveguide,is fabricated in 151Eu3+:Y2SiO5 crystal.And its cross-sectional structure was designed as a circle symmetry so that any polarization mode can be conducted.The site-2 151Eu3+ions doping in the Y2SiO5 crystal provides a nearly uniform optical absorption depth.In order to increase the optical absorption depth and improve the storage efficiency,we designed a new optical pumping scheme,and a flattened enhanced absorption band is prepared.Finally,based on the Stark-modulated atomic-frequency-comb scheme,we demonstrate on-demand quantum storage of polarization qubits with a 10 MHz bandwidth.When the average number of photons in the incident qubits is 0.32,the fidelity of the stored process is verified to be 99.4 ± 0.6%.4.An Integrated Spin-wave Quantum MemorySpin-wave quantum storage can realize long-lifetime on-demand storage,which is indispensable in the practical application of large-scale quantum networks.However,all previous demonstrations of integrated optical quantum memories have been limited to optically excited-state storage with short storage lifetimes and little scalability.The spin-wave quantum storage,which can enable on-demand retrieval with a long lifetime,is indispensable for practical applications but has never been demonstrated in an integrated solid-state device.However,it has not yet been realized in an integrable solid-state system.The main reason is that the photon noise brought by the ultra-strong optical control pulse is difficult to be filtered to the single-photon level in an integrated system.Based on the technical accumulation of the first three research achievements,we have successfully overcome this problem.The innovations of this work are:A typeⅢ optical waveguide inclined to the incident plane is designed to realize polarization filtering and avoid end-face reflected light.Combining reverse pumping,filter crystal,and optical gating-based acousto-optic modulation,filtering of the control pulses is achieved.We demonstrate spin-wave quantum storage with signal-to-noise ratio levels comparable to bulk crystals using atomic-frequency-comb and noise-less photon echo schemes,respectively.Based on the noise-free photon echo scheme,the time-bin qubit at the single-photon level is input,and the fidelity of storage and reading is as high as 98.1 ± 0.7%.Such reliable integrated quantum memory can play an important role in integrated quantum networks.Integrability is of great value to the establishment of large-scale quantum networks in the future,and it is also the inevitable direction of development.These results show that integrated waveguides and massive crystals have basically equalized in solid-state quantum memory performance,which provides support and help for the future development of integrated quantum memories.