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Register a new userNoise characterization for resonantly-enhanced polarimetric vacuum magnetic-birefringence experiments
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In this work we present data characterizing the sensitivity of the Bir{e}fringence Magnetique du Vide (BMV) instrument. BMV is an experiment attempting to measure vacuum magnetic birefringence (VMB) via the measurement of an ellipticity induced in a linearly polarized laser field propagating through a birefringent region of vacuum in the presence of an external magnetic field. Correlated measurements of laser noise alongside the measurement in the main detection channel allow us to separate measured sensing noise from the inherent birefringence noise of the apparatus. To this end we model different sources of sensing noise for cavity-enhanced polarimetry experiments, such as BMV. Our goal is to determine the main sources of noise, clarifying the limiting factors of such an apparatus. We find our noise models are compatible with the measured sensitivity of BMV. In this context we compare the phase sensitivity of separate-arm interferometers to that of a polarimetry apparatus for the discussion of current and future VMB measurements.
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We present the current status and outlook of the optical characterization of the polarimeter at the Bir{e}fringence Magnetique du Vide (BMV) experiment. BMV is a polarimetric search for the QED predicted anisotropy of vacuum in the presence of external electromagnetic fields. The main challenge faced in this fundamental test is the measurement of polarization ellipticity on the order of ${10^{-15}}$ induced in linearly polarized laser field per pass through a magnetic field having an amplitude and length ${B^{2}L=100,mathrm{T}^{2}mathrm{m}}$. This challenge is addressed by understanding the noise sources in precision cavity-enhanced polarimetry. In this paper we discuss the first investigation of dynamical birefringence in the signal-enhancing cavity as a result of cavity mirror motion.
Although experimental efforts have been active for about 30 years now, a direct laboratory observation of vacuum magnetic birefringence, an effect due to vacuum fluctuations, still needs confirmation. Indeed, the predicted birefringence of vacuum is $Delta n = 4.0times 10^{-24}$ @ 1~T. One of the key ingredients when designing a polarimeter capable of detecting such a small birefringence is a long optical path length within the magnetic field and a time dependent effect. To lengthen the optical path within the magnetic field a Fabry-Perot optical cavity is generally used with a finesse ranging from ${cal F} approx 10^4$ to ${cal F} approx7times 10^5$. Interestingly, there is a difficulty in reaching the predicted shot noise limit of such polarimeters. We have measured the ellipticity and rotation noises along with a Cotton-Mouton and a Faraday effect as a function of the finesse of the cavity of the PVLAS polarimeter. The observations are consistent with the idea that the cavity mirrors generate a birefringence-dominated noise whose ellipticity is amplified by the cavity itself. The optical path difference sensitivity at $10;$Hz is $S_{Delta{cal D}}=6times 10^{-19};$m$/sqrt{rm Hz}$, a value which we believe is consistent with an intrinsic thermal noise in the mirror coatings.
We have developed a repeating pulsed magnet system which generates magnetic fields of about 10 T in a direction transverse to an incident beam over a length of 0.8 m with a repetition rate of 0.2 Hz. Its repetition rate is by two orders of magnitude higher than usual pulsed magnets. It is composed of four low resistance racetrack coils and a 30 kJ transportable capacitor bank as a power supply. The system aims at axion-like particle searches with a pulsed light source and vacuum birefringence measurements. We report on the details of the system and its performances.
We measure the current noise of several cryogenic cables in a pulse tube based dilution refrigerator at frequencies between about 1~mHz and 50~kHz. We show that vibration-induced noise can be efficiently suppressed by using vacuum-insulated cables between room temperature and the 2nd pulse tube stage. A noise peak below 4 fA at the 1.4~Hz operation frequency of the pulse tube, and a white noise density of 0.44 fA/sqrt{Hz} in the millihertz range are obtained.
A new experiment to measure vacuum magnetic birefringence (VMB), the OVAL experiment, is reported. We developed an original pulsed magnet that has a high repetition rate and applies the strongest magnetic field among VMB experiments. The vibration isolation design and feedback system enable the direct combination of the magnet with a Fabry-Perot cavity. To ensure the searching potential, a calibration measurement with dilute nitrogen gas and a prototype search for vacuum magnetic birefringence are performed. Based on the results, a strategy to observe vacuum magnetic birefringence is reported.
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