Laboratoire National des Champs Magnétiques Pulsés CNRS – INSA

Personnel involved:
Permanent: R. Battesti (EC)
Magneto optics
Non permanent: J. Mauchain (CDD, IE), F. Bielsa (post doc)
Collaboration: LCAR (UMR 5589, Toulouse)
LMA (UPS 2713 , Villeurbanne)
Magnetic Birefringence of Vacuum (BMV)
1. Introduction
The activity of our group is devoted to
perform experimental test of fundamental
interest using high magnetic fields and optical
techniques.
Since 2001 we are pushing forward
collaboration with LCAR of Toulouse and
LMA of Lyon to measure the vacuum
magnetic birefringence (BMV Project), a QED
prediction. From 2006 to 2008 we have also
performed a measurement at LULI of the
Ecole Polytechnique to search for photon
oscillations into massive particles (Boson
project) always in collaboration with the
LNCMI of Toulouse. Photon oscillations into
massive particle can be at the origin of effects
mimicking an optical activity of vacuum, and
therefore they are strictly related with the
search for the Cotton-Mouton effect of vacuum.
2. BMV project
The BMV project goal is to measure
the birefringence induced in vacuum by an
external magnetic field (Birefringence
Magnétique de Vide), a QED prediction which
dates from 1935 and not yet experimentally
proved. This magnetic birefringence, also
known as Cotton-Mouton effect, has been
measured in gases since the sixties and it exists
in any medium.
The BMV project is a collaboration
between the LNCMI of Toulouse and the
LCAR. Our experiment is now fully
operational in a clean experimental room
hosted by the LNCMI and we have performed
31
our first measurements. We will describe them
in the following.
Our experimental setup is described in
ref [1]. A Nd-YAG laser (�=1064 nm) is
locked on a 2.2 m optical resonant Fabry-
Perot cavity using the Pound-Drever-Hall
technique. The light is polarized by a high
quality polarizer and the transverse magnetic
field is delivered by two especially designed
coils (Xcoil). To increase as much as possible
the effect to be measured we need a very high
finesse cavity. The polarization of the light
transmitted by the Fabry-Perot cavity is
analyzed by a second high quality polarizer.
As far as the optical cavity is
concerned, we used different sets of mirrors in
order to test more and more efficient
configurations. Mirrors are characterised by
their reflectivity from which we can deduce the
nominal finesse of the cavity. We have locked
our laser to cavities of finesse about 5 000 and
about 130 000. We have also at our disposal
mirrors built by the LMA of Lyon with which
we collaborate since the beginning of the
project in 2001. The finesse of the cavity
realized using such mirrors is expected to be
about 650 000.
The experimental finesse is measured
by measuring the lifetime of photons in the
cavity. We shut down the laser and we
measure the intensity of transmitted light
versus time. The transmitted light intensity
decays exponentially. Once light lifetime in the
cavity is known, finesse can be calculated. We
have currently reached a cavity finesse of 131
000 (decay time of 306 μs). The full width at
maximum of the cavity resonance line has a