1.CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei 230026, China 2.CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2017YFA0304100)
Received Date:07 April 2021
Accepted Date:11 April 2021
Available Online:07 June 2021
Published Online:20 August 2021
Abstract:Quantum memory is a crucial component for the large-scale quantum networks. Rare-earth-ion doped crystals have been a promising candidate for the practical quantum memory because of its very long coherence time. However, doped ions cause unwanted lattice distortion, and consequently reduce the optical depth and the storage efficiency. The stoichiometric rare-earth crystals have low lattice distortion and high rare earth ion density, and thus are expected to enable high-efficiency storage. EuCl3·6H2O is a promising material for quantum memory applications because its optical inhomogeneous broadening can be smaller than its hyperfine splitting and the theoretically predicted spin coherence time is up to 1000 seconds. Despite the numerous efforts in solid-state quantum memory based on rare-earth ion doped crystals, optical memory and quantum memory have not been implemented with stoichiometric rare-earth crystals yet. Here, we report the atom frequency comb optical storage using a EuCl3·6H2O crystal. A coherence time of 55.7 μs is obtained by photon echo measurements on $^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$ transition. The two-level atomic frequency comb storage is demonstrated with a storage efficiency of 1.71% at a storage time of 1 μs, showing the potential capability of optical quantum storage of this material. Based on the analysis of the line shift of $^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$ depending on the sample temperature, we highlight the challenge of achieving high-efficiency optical pumping in this material, which imposes a limit on the achievable efficiency. Keywords:quantum memory/ atom frequency comb/ europium chloride hexahydrate
实验装置如图2所示. 激光源为一台倍频的半导体激光器, 通过一个外部超稳参考腔实现频率稳定, 使其激光线宽小于10 kHz. 晶体被置于温度为3.5 K的低温腔中, 后续实验均在低温腔的低震动时间周期内完成. 一个双次通过的声光调制器(AOM 1)被置于样品前的光路中, 以实现光开关和频率移动效果. 另一个单次通过的声光调制器(AOM 2)置于样品之后, 用作光开关以阻断强光进入后面的光电探测器(Thorlabs PDA10A). 两个AOM由一个八通道任意波形发生器(HDAWG, Zuich Instruments)驱动. 图 2 实验装置示意图. 激光器输出的580 nm激光被一个声光调制器(AOM 1)所调制, 并入射到低温系统内的晶体样品(Crystal)上. 输出光场被另一个声光调制器(AOM 2)所调制, 最终由探测器(PD)探测 Figure2. Illustration of the experimental setup. The 580 nm laser is modulated by an acousto-optic modulator (AOM 1) and injected into the crystal sample which is cooled down to 3.5 K by a cryostat. The output laser beam is controlled by another AOM (AOM 2) and finally detected by a photodiode (PD).
该实验使用的EuCl3·6H2O晶体的吸收深度高达1000 dB/cm量级, 当光沿C2轴方向传播时吸收深度最小[34]. 为了获得相对适中的吸收深度, 实验中光场在晶体表面的入射角大约15° (也就是光场传播方向与C2轴夹角). 激光聚焦在晶体上的光斑直径约为35 μm. 在3.5 K温度下晶体中Eu3+离子的$^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$跃迁频率为517.148 THz. 光学跃迁的相干时间通过光子回波方法测得. 两个持续时间为2 μs, 功率密度为730 W/cm–2的光脉冲相继进入晶体. 记第一脉冲进入晶体的时间为0时刻, 第二脉冲进入晶体的时刻为$ \tau/2 $, 则在$ \tau $时刻晶体将自发产生一同向光脉冲. 实验中使用外差方法探测此回波脉冲. 在$ \tau $时刻前后开启AOM 1, 输入一束频率相差46.2 MHz的光脉冲, 则该脉冲与晶体自发产生的回波脉冲将形成拍频. 将光电探测器的信号通过$(46.2 \pm 3)\;{\rm{MHz}}$的带通滤波器后, 送入锁相放大器, 从而解调出回波脉冲信号. 实验测得回波脉冲幅度衰减如图3所示. 对图中数据执行单指数拟合, 可以得到晶体中Eu3+离子的光学相干时间(T2)为$ (55.7\pm $2.3) μs. 此处测得相干时间略低于Ahlefeldt等[34]测得的77 μs. 此处相干时间的差异可能是由于实际测量所处的频率位置不同(相差约1.05 GHz). 图 3 EuCl3·6H2O晶体的光子回波幅度随时间的衰减曲线. 拟合得到相干时间(T2)为(55.7 ± 2.3) μs Figure3. The decay curve of two-pulse photon echo amplitude with time in EuCl3·6H2O. The optical coherence time (T2) is (55.7 ± 2.3) μs by fitting the data to a single exponential decay.
各离子的失谐量为$\delta_j = m_j\varDelta$ ($ m_j $为整数). 于是当时间达到$ 2\pi/\varDelta $时, 各离子随时间演化的相位项$ {\rm e}^{-{\rm i}\delta^j t} $将重新统一, 此时系综将自发形成回波脉冲. 图4(a)所示为153Eu3+离子在EuCl3·6H2O晶体中的$ ^7{\rm{F}}_0 \rightarrow ^5{\rm{D}}_0$跃迁能级结构[33]. 中心频率为$ f_1 $, $ f_2 $和$ f_3 $的三束激光分别与$ {\rm g}_3\rightarrow {\rm e}_3 $, $ {\rm g}_1\rightarrow {\rm e}_2 $, $ {\rm g}_2\rightarrow {\rm e}_3 $跃迁共振, 用来选择非均匀展宽中的一类离子, 称为类清空操作. 三束激光的平均功率约8 mW, 扫频带宽为5 MHz, 持续300 ms. 完成类清空后, 通过扫描$ f_2 $和$ f_3 $跃迁(带宽5 MHz, 持续时间为220 ms), 使得该类离子的电子都处在$ {\rm g}_3 $能级上, 该步骤被称为自旋极化. 类清空和自旋极化完成后, 使用并行制梳脉冲[19]在$ {\rm g}_3 \rightarrow {\rm e}_3 $跃迁上制备带宽为5 MHz的频率梳结构. 制梳过程使用的激光功率约5 mW, 持续时间共226 ms. 图 4 (a) 153Eu3+离子能级结构; (b)在偏离中心频率位置上制备的AFC结构, 其频率中心为517.14873 THz, 插图为放大的AFC光谱结构 Figure4. (a) The energy diagram of 153Eu3+. Three beams with center frequency of $f_1$, $f_2$ and $f_3$ are applied in a certain sequence to accomplish the spectral-hole burning for AFC preparation. (b) The prepared AFC structure with the center frequency is 517.14873 THz. Inset: enlarged view of the AFC structure.
图4(b)为目标频率附近的光谱结构. 跃迁$ f_2 $和$ f_3 $频率处的烧孔效果为类清空和自旋极化的结果, $ f_1 $处为制得的频率梳结构. 由于AFC存储效率受吸收深度影响, 因此选择偏离吸收中心以获得合适的吸收深度. 光存储的效果如图5(a)所示, 黑色曲线为入射光脉冲, 红色曲线表示透射光脉冲和存储1 μs后的再发射脉冲. 存储时间为1 μs时的效率为$(1.71\;\pm$0.04)%. AFC理论存储效率可由AFC结构参数计算得到[9]: 图 5 (a) AFC光存储1 μs结果, 存储1 μs时的效率为$(1.71\pm 0.04)\%$, 与根据图4(b)得到的AFC参数理论计算值$(1.75\pm 0.11)\%$相符; (b)存储时间为0.5—10 μs时AFC存储效率变化, 由四次测量数据平均得到 Figure5. (a) The AFC memory with a storage time of 1 μs. The storage efficiency at 1 μs is $(1.71\pm 0.04)\%$, which agrees with the theoretical efficiency of $(1.75\pm 0.11)\%$, estimated from the AFC structure shown in Fig.4(b). (b) The storage efficiency with storage time from 0.5 μs to 10 μs. Each data point is averaged by four measurements.
3.$ ^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$跃迁能级间距与温度的依赖关系实验中烧孔效果不理想导致AFC存储效率不佳. 一个可能的原因是烧孔过程中样品升温导致无法实现高效率烧孔. 为了分析这一可能原因, 首先测定$^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$跃迁的中心吸收频率与样品温度的依赖关系(图6). 在最低工作温度(3.5 K)下, 吸收中心位于517.14812 THz. 图 6 温度导致的$^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$跃迁频率偏移及理论拟合 Figure6. The line shift of $^7{\rm{F}}_0 \rightarrow {}^5{\rm{D}}_0$ depending on the sample temperature. The curve is fitted using two-phonon Raman-process model[39,40], giving $\alpha = (4.22 \pm 0.57)\;{\rm{cm}}^{-1}$ and $T_{\rm D} = (144.1 \pm 6.2)\;{\rm{K}}$ for this crystal.