symmetry discrimination and measurements of parity violating and magnetoelectric dichroisms in a Cs vapor
Jocelyne Guéna, Michel Lintz, Marie-Anne Bouchiat
We detect atomic parity violation (APV) by using stimulated emission, in the form of a chiral optical gain of a cesium vapor on the 7S-6P3/2 probe transition, consecutive to linearly polarized 6S-7S excitation. The method relies on a pump-probe scheme in which the excited 7S state is populated, in zero magnetic field, by an intense, pulsed excitation beam, with a linear polarization [epsilon]ex and a wave vector [k] parallel to the applied electric field [E]. The atoms are then probed by a second, collinear pulsed laser beam, tuned to resonance with the 7S - 6P3/2 transition. The probe beam is amplified by stimulated emission and its linear polarization [epsilon]pr undergoes a small change of direction, due to the presence of the pseudoscaler PV term in the gain. This dichroism, with axes at +/-45° from the ([epsilon]ex,[E]) plane of symmetry of the experiment, is smaller than the parity-conserving, Stark linear dichroism by the ratio of the Stark to the PV 6S-7S transition amplitudes [beta]E/ E1pv, approximately 10^6. As a result the gain axes are tilted by an angle [theta]pv = E1pv/[beta]E, approximately 1µradian.This is the relevant APV quantity, measured by balanced-mode, two channel polarimetry, with dedicated real-time calibration. This detection method has been validated by a measurement of expected sign and magnitude with a 9% accuracy [ref. 1], now improved by more than a factor of two, thanks essentially to a dichroic polarization-tilt magnifier [ref. 2]. Rotational symmetry around the common pump-probe beam axis is used to reduce systematic effects [ref. 3], among which the parity-conserving Jones dichroism due to transverse E and B fields. This latter kind of magneto-optical effects has already been observed in especially chosen liquids thanks to the application of intense (>10T) magnetic fields. Here, symmetry discrimination, applied to the strongly forbidden 6S-7S transition, allows its detection with applied magnetic fields 10^5 to 10^7 times weaker, in a dilute vapor, where the mechanisms involved are clearly understood [ref. 3]. Besides, we discuss a novel kind of magneto-optical effects manifesting parity violation, as possible observables for APV measurements to the 0.1% precision level on the 6S-7S Cs transition. They consist in observing a parity violating circular dichroism in the presence of transverse electric and magnetic fields [ref. 4]. The advantage, as compared to the longitudinal E field configuration, is that the interaction length can be made much longer without increasing the applied potentials. Thanks to the much larger optical density of the excited vapor, stimulated emission gives rise to considerable amplification of the polarimetric signal during the propagation of the probe beam. Even moderate transverse electric fields would allow to reach the triggered superradiance regime, for further enhancing the PV asymmetry. Finally we suggest a means of rotating the transverse direction of the fields so as to restore the cylindrical symmetry and to suppress systematic effects, as in the longitudinal E field configuration. [1] J. Guéna et al.: Phys. Rev. Lett. 90, 143001, (2003) [2] D. Chauvat et al.: Opt. Commun. 138, 249 (1997) [3] M.-A. Bouchiat, J. Guéna, M. Lintz: Eur Phys J D28, 331 (2004) [4] J. Guéna, M. Lintz & M.-A. Bouchiat: arXiv:physics/0403141 & JOSA B, to be published
•Jan 1, 2004•0 citations