A High-sensitivity Polarimeter Using Dual-Modulation

According to quantum electrodynamics(QED),the interaction between quantum vacuum fluctuation and external field will lead to vacuum birefringence,which state that electromagnetic field will produce birefringence when traveling through a transverse external magnetic or electric field.Theoretically,vacuum birefringence can be detected by measuring polarization state change of light using a polarimeter.But it has not been observed in laboratory until now due to faintness of the effectCombined with current laboratories and international research,we have successfully developed a high-sensitivity polarimeter for magnetic vacuum birefringence(MVB)experiments.Using a dual modulation method(photoelastic modulation and polarization modulation),signal can be modulated to high frequency and avoid low frequency noise.At the same time,a digital demodulation system based on data acquisition system was developed.The noise floor of the demodulation system has reached 2.4×10-8V/Hz1/2,which meet the requirements of our experimentsDuring the experiment,temperature noise and modulation depth of the photoelastic modulator are studied.Using temperature modulation experiment,we found that the signal was affected by the temperature of photoelastic modulator An active temperature control system was developed and the temperature fluctuations are controlled within 1mK.We also found that the sensitivity of polarimeter varied with the modulation depth of the photoelastic modulator and the best sensitivity was found though testing several different modulation depth.The sensitivity of polarimeter finally reached 5×10-9rad/(?),which achieved advanced level in the field and satisfied the requirement of MVB experiment.