Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2016YFA0301404) and the National Natural Science Foundation of China (Grant No. 11504217)
Received Date:22 April 2020
Accepted Date:09 May 2020
Available Online:05 June 2020
Published Online:20 September 2020
Abstract:Optical Schr?dinger cat state is not only one of the basic elements of quantum mechanics, but also a pivotal resource of continuous-variable quantum information. The non-Gaussian operation in its preparation can also be a key technology in distilling continuous-variable squeezing and entanglement. In the experimental preparation, a small part of a beam of vacuum squeezing is separated and detected as the trigger of appearance of Schr?dinger cat state. Filter operation in the trigger optical path is important since it affects dark counts of single photon detector, frequency mode matching of trigger mode and signal mode, and preparing rate of the Schr?dinger cat state, etc. In this paper, we describe the design of optical filter in the trigger path and the measurement of the filter cavity length. According to the design, filter cavity length $ {l_{{\rm{FC}}}}$ should satisfy $ {\rm{189}}\;{\text{μm}} > {l_{{\rm{FC}}}} > {\rm{119}}\;{\text{μm}}$. This cavity length is too small to be measured with a ruler. To measure the cavity length, we introduce an optical method, in which Gouy phases of Hermite Gaussian transverse modes TEM00 and TEM10 are used. When the cavity length is scanned, resonant peaks and the corresponding scanning voltages are recorded. From theoretical derivation, the cavity length is related to the filter cavity piezo response to the scanning voltage $ {\varPsi '_{\rm{G}}}$, the slope rate of piezo scanning voltage $ U'$, and the time distance between TEM00 and TEM10 resonant peaks $ \Delta t$. The finally measured cavity length is ${l_{{\rm{FC}}}} = ({\rm{141}} \pm 28)~{\text{μm}}$, which satisfies the design requirement. The measurement error mainly originates from inaccurate fitting of $ {\varPsi '_{\rm{G}}}$ and $ U'$, and readout error of $ \Delta t$. It is shown that the error of $ {\varPsi '_{\rm{G}}}$ is dominant since less data are used in the curve fitting. The measurement error is expected to be reduced if much more data of piezo response to scanning voltage are collected and used to fit $ {\varPsi '_{\rm{G}}}$ with higher order polynomials. The proposed measurement method of short cavity length needs neither wide tuning laser nor any peculiar instrument, and does not depend on any dispersion property of the cavity, and hence it has a certain generality. It can be hopefully used in many other optical systems, such as cavity quantum electrodynamics, where ultrashort cavity plays a central role. Keywords:Schr?dinger cat state/ squeezed state/ filter cavity/ Gouy phase shift
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2.滤波设计在光学薛定谔猫态的实验制备中, 将真空压缩态光场分出5%左右进入单光子探测器, 作为触发光路. 由于OPO具有有限的FSR, 输出的压缩态光场除了有简并的压缩真空态光场外, 还有大量频率非简并的光子对, 这些光子对的频率间隔是ΔνFSR,OPO的整数倍, 在光学薛定谔猫态制备中它们属环境噪声, 或者称为暗计数, 需要过滤掉. 在本文的实验中, 用一个干涉滤波片和一个短腔相结合的方式对OPO输出的压缩态光场进行滤波. 图1为干涉滤波片(蓝色曲线)和滤波腔(绿色曲线)的透射曲线以及OPO腔输出压缩光的频谱曲线(红色曲线). 横轴是激光频率与压缩光中心频率之差$\nu - {\nu _0}$, 单位是OPO的FSR. 为有效滤除非简并光子对, 滤波腔应满足两个条件: 1)带宽(bandwidth, BW) ΔνBW,fc应小于OPO腔的自由光谱范围ΔνFSR,OPO, 即ΔνBW,fc < ΔνFSR,OPO, 这样红色曲线中仅有中间$\nu - {\nu _0} = 0$处的透射峰透过, 而两侧的透射峰被滤波腔衰减; 2)滤波腔的自由光谱范围ΔνFSR,fc应大于干涉滤波片的带宽ΔνBW,if, 即ΔνFSR,fc > ΔνBW,if. 因为滤波腔透射曲线在$\nu - {\nu _0} = 0$两侧的透射峰可能恰好与OPO非简并透射峰对有重叠, 加一个带宽小于ΔνFSR,fc的干涉滤波片, 这样OPO输出的非简并光子对, 即使通过两侧的透射峰输出也会被干涉滤波片衰减. 图 1 OPO输出的压缩光谱线(红色), 以及滤波腔(绿色)和干涉滤波片(蓝色)的透射谱线(横轴是光频率$\nu $与压缩光中心频率${\nu _0}$的差, 单位是OPO的自由光谱范围ΔνFSR,OPO) Figure1. Spectrum of squeezing from OPO (red), and transmission spectra of filter cavity (green) and interference filter (blue). Horizontal axis is difference of optical frequency and central frequency of squeezing, the unit is ΔνFSR,OPO.