Abstract
The non-classical light resonance on the cesium D<sub>1</sub> (894.6 nm) line has important applications in solid-state quantum information networks due to its unique advantages. The cesium D<sub>1</sub> line has a simplified hyperfine structure and can be used to realize a light-atom interface. In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D<sub>1</sub> line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal. However, the squeezing level is relatively low, and the tunability that has practical significance for squeezed light has not been further investigated. Theoretically, the increase of the transmittance of output mirror and the decrease of the intra-cavity loss of the OPO can improve the squeezing level. Here, we use super-polished and optimal coating cavity mirrors to improve the nonlinear process in OPO. We prepare 447.3 nm blue light from 894.6 nm fundamental light by a second harmonic generation cavity (SHG). The SHG is a two-mirror standing-wave cavity with a PPKTP crystal as the nonlinear medium. The power of generated blue laser is 32 mW when the incident infrared power is 120 mW. Using the blue light to pump an OPO, we achieve quadrature squeezed vacuum light at cesium D<sub>1</sub> line. The OPO is a two-mirror standing-wave cavity with a PPKTP crystal. The threshold of OPO is reduced to 28 mW. The squeezing level of generated quadrature squeezed vacuum light is increased to 3.3 dB when the pump power is 15 mW. Taking into account the overall detection efficiency, the actual squeezing reaches 5.5 dB. We inject a weak signal beam into the OPO cavity to act as an optical parametric amplifier (OPA), and test the tunability of squeezzed light. The blue light and the squeezed light are tuned by using a low-frequency triangular wave signal to scan the Ti: sapphire laser. Gradually increasing the amplitude of the scanning triangle wave signal, the generated bright squeezed light can be continuously tuned over a range around 80 MHz without losing the stability of the whole system. The generated squeezed light offers the possibility for the efficient coupling between the non-classical source and solid medium in the process of quantum interface.
Highlights
E-mail: junxiang_zhang@zju.edu.cn amplifier, optical parametric amplifier (OPA)) 获得了 12.6 dB 的明亮压缩态光 场. 2019 年, Sun 等 [13] 利用 OPO 获得了 13.8 dB 压缩真空态光场. 目前实验中观测到的最高压缩 度 由 Schnabel 小 组 [14] 保 持, 他们利用一个由 PPKTP 晶体构建的半整块 OPO 获得 1064 nm 压缩真空光, 将压缩真空光注入定制的高量子效 率 (99.5%) 光电探测器中, 测得 15 dB 压缩. 然而, 以上制备的高压缩度非经典光场的工作波长都远 离碱金属原子吸收线
In our previous work, we demonstrated 2.8-dB quadrature squeezed vacuum light at cesium D1 line in an optical parametric oscillator(OPO) with a periodically poled KTP(PPKTP) crystal
1) (School of Optoelectronic Engineering, Xi’an Technological University, Xi’an 710021, China) 2) (Department of Physics, Zhejiang University, Hangzhou 310007, China) ( Received 26 July 2021; revised manuscript received 3 November 2021 )
Summary
为 120 mW, 通过倍频过程产生的 447.3 nm 蓝光 经过光学隔离器后功率为 32 mW. OPO 为近共心两镜驻波腔结构, 由之前的 基频光单共振改为基频光和抽运光双共振, 采用 高光洁度腔镜 (表面质量为 10-5) 以减少散射损 耗. 前腔镜曲率半径为 50 mm, 对 894.6 nm 光高 反, 对 447.3 nm 光透射率为 10%; 后腔镜曲率半 径 为 50 mm, 对 894.6 nm 光透射率由之前的 5%[22] 提高为 9.5%, 对 447.3 nm 光高反, 腔长为 102 mm. OPO 采用一块长度为 10 mm 的 一 类 PPKTP 晶体作为非线性介质, 晶体温度由温度控 制仪及帕尔贴元件精确控制. 将辅助信号光注入 OPO, 利用 Pound-Drever-Hall 稳频技术 [26] 锁定 腔长, OPO 输出的明亮信号光用于后续光路及平 衡零拍探测的调节. Ω = f /γ为归一化频率 , f为谱仪分析频率 , γ = c(T + L)/(2l) 为 OPO 衰 减 速 率 , l 为 腔 长. 式中: h 为平衡零拍探测器的量子效率 (86%); ξ1 为压缩光与本底光的干涉耦合效率 (95%); z 为光 路传输效率 (96%); ρ = T /(T + L) 为 OPO 逸出 效率, T 为 OPO 后腔镜对信号光透射率, L 代表. 总探测效率 ηdet 为 74.5%, 根据 (2) 式, 则 Vsq 为 5.5 dB
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