基于外差锁腔法的真空双折射精密测量关键技术研究

The birefringence of vacuum under strong electric/magnetic fields was predicted by quantum electrodynamics (QED). Since it is a very weak effect, there is no concrete experimental evidence so far. Therefore, the detection of the quantum-electrodynamical vacuum polarization is still one of the frontier research topics in physics. The current experimental scheme for measuring the magnetic vacuum birefringence is to use a linear-polarized light, passing through the vacuum under a strong magnetic field, hence the rotation of polarization of light induced by the assumed birefringence of vacuum can be detected (i.e., ellipsometry). We plan to use heterodyne laser interferometry to measure the phase difference of two laser beams with mutually perpendicular polarizations, caused by the birefringence of vacuum. Because the phase measurement has a higher resistance to light intensity fluctuation than ellipsometry, we expect that the measurement precision of this scheme can be better than that of experiments using ellipsometry by one order of magnitude. In this project, we will study the high-finesse Fabry-Perot cavity and laser frequency stabilization, ultra-stable optical bench for heterodyne laser interferometry and ultra-precision phase measurement. The precision measurement system for detecting vacuum birefringence will be constructed and a preliminary result of phase difference measurement of 3×10^-5rad can be achieved.

真空在强电/磁场下具有双折射特性是量子电动力学的理论预言，但是由于此效应极其微弱，至今仍然没有得到明确的实验验证；因此，关于量子电动力学真空极化的相关实验仍然是物理前沿研究课题之一。目前关于磁场真空极化的实验方案是让线偏振光通过具有强磁场的真空，然后通过测量光偏振转角（亦即椭偏测量）来决定真空的双折射性。我们将利用外差激光干涉仪直接测量真空双折射性所导致的两束偏振互相垂直激光光束的相位差；由于外差干涉相位测量对于光功率噪声有很高的抗干扰能力，预期测量精度可以比椭偏测量高一个数量级。本项目将进行实验方案与误差建模分析，开展高精细度光学谐振腔与激光稳频控制、超稳激光干涉光学平台研制、高精度外差干涉相位测量等关键技术研究，预计完成精密双折射性测量系统的搭建，并得到3×10^-5rad的相位差测量分辨率，为开展磁场真空双折射性精密测量研究奠定基础。

本项目提出基于高精度外差相位测量的外差锁腔法实验方案，开展强磁场下真空双折射性测量的预先研究。主要研究内容包含：（1）基于外差锁腔法的磁场真空双折射实验方案设计与误差分析；（2）超稳光学平台、激光稳频与锁相控制、高精度相位测量等关键技术。项目取得的研究成果如下：（1）完成总体实验方案设计，提出了一种可实现超高精细度的四镜嵌套式F-P腔实现方案；（2）利用氢氧催化粘结技术完成准一体化超稳光学平台，为精密光学锁相和激光干涉测量打下基础；（3）完成数字式激光稳频和锁相控制系统，残余相位噪声达到6*10^-4rad/sqrt(Hz)；（4）完成数字式高精度相位计，本底噪声达到5*10^-6rad/sqrt(Hz)。以上研究成果为开展对量子电动力学中真空极化效应和轴子探测实验研究奠下基础。