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symmetry discrimination and measurements of parity violating and magnetoelectric dichroisms in a Cs vapor

Jocelyne Guéna, Michel Lintz, Marie-Anne Bouchiat

Published Jan 1, 2004• 0 citations
OpenAlex

Abstract

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

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New Manifestation of Atomic Parity Violation in Cesium: A Chiral Optical Gain Induced by Linearly Polarized 6S−7S Excitation

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We have detected, by using stimulated emission, an atomic parity violation (APV) in the form of a chiral optical gain of a cesium vapor on the 7S-6P(3/2) transition, consecutive to linearly polarized 6S-7S excitation. We demonstrate the validity of this detection method of APV, by presenting a 9% accurate measurement of expected sign and magnitude. We stress several advantages of this new approach which fully exploits the cylindrical symmetry of the setup. Future measurements at the percent level will provide an important cross-check of an existing more precise result obtained by a different method.

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Proposal for high-precision atomic-parity-violation measurements by amplification of the asymmetry by stimulated emission in a transverse electric and magnetic field pump-probe experiment

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Amplification by stimulated emission of radiation provides an interesting means for increasing the sensitivity of atomic-parity-violation (APV) measurements in a pump-probe configuration well adapted to the 6S-7S Cs transition. It takes advantage of the large number of atoms excited along the path of the pump beam. In the longitudinal electric field configuration currently exploited in our ongoing APV measurement, this number is limited only by the total voltage sustainable by the Cs vapor. To overcome this limit we consider, both theoretically and experimentally, the possibility of performing the measurements in a transverse electric field configuration requiring a much lower voltage. We discuss the necessarily different nature of the observable and the magnetoelectric optical effects that come into play. These condition modifications of the experimental configuration with, in particular, the application of a transverse magnetic field. We suggest the possibility of rotating the transverse direction of the fields so as to suppress systematic effects. With a long interaction length a precision reaching 0.1% in a quantum-noise-limited measurement can be expected, limited only by the necessity of operating below the threshold of spontaneous superradiant emission of the excited medium. Were we to approach this limit, however, we could greatly amplify the asymmetry using triggered superradiance.

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From linear amplification to triggered superradiance : illustrative examples of stimulated emission and polarization spectroscopy for sensitive detection of a pulsed excited forbidden transition

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Magneto-optical effect near the D1 resonance of spin-polarized cold cesium atoms

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We report our study of the magneto-optical effect in a strictly linear regime on spin-polarized cold cesium atoms. Due to the low intensity and the short illumination period of the probe beam, less than 7.5% of the sample atoms change their states by absorbing probe photons. We produce a medium of atoms at rest in either the $6{S}_{1∕2},F=3,{m}_{F}=0$ or $6{S}_{1∕2},F=3,{m}_{F}=3$ state by optically pumping atoms trapped in a magneto-optical trap. We use the $D1$ resonance with large lower and upper state hyperfine splittings as a probe transition to avoid hyperfine mixing from the Zeeman interaction. Under this idealized situation we measure the Stokes parameters in order to find the polarization rotation and circular dichroism experienced by the probe light. We find that there are qualitative differences between the results for the ${m}_{F}=0$ and ${m}_{F}=3$ cases. While dispersion and consequent Faraday rotation play a dominant role when the atoms are in the ${m}_{F}=0$ state, it is dissipation and circular dichroism that are important when they are in the ${m}_{F}=3$ state. Similarly, while the size of the Faraday rotation and the circular dichroism for the ${m}_{F}=0$ case scales linearly with the applied magnetic field, for the ${m}_{F}=3$ case it is the shift of the probe polarization change versus frequency that is linearly proportional to the magnetic field strength.

Physical Review A•Apr 22, 2005•3 citations
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
We have detected, by using stimulated emission, an atomic parity violation (APV) in the form of a chiral optical gain of a cesium vapor on the 7S-6P(3/2) transition, consecutive to linearly polarized 6S-7S excitation. We demonstrate the validity of this detection method of APV, by presenting a 9% accurate measurement of expected sign and magnitude. We stress several advantages of this new approach which fully exploits the cylindrical symmetry of the setup. Future measurements at the percent level will provide an important cross-check of an existing more precise result obtained by a different method.
Amplification by stimulated emission of radiation provides an interesting means for increasing the sensitivity of atomic-parity-violation (APV) measurements in a pump-probe configuration well adapted to the 6S-7S Cs transition. It takes advantage of the large number of atoms excited along the path of the pump beam. In the longitudinal electric field configuration currently exploited in our ongoing APV measurement, this number is limited only by the total voltage sustainable by the Cs vapor. To overcome this limit we consider, both theoretically and experimentally, the possibility of performing the measurements in a transverse electric field configuration requiring a much lower voltage. We discuss the necessarily different nature of the observable and the magnetoelectric optical effects that come into play. These condition modifications of the experimental configuration with, in particular, the application of a transverse magnetic field. We suggest the possibility of rotating the transverse direction of the fields so as to suppress systematic effects. With a long interaction length a precision reaching 0.1% in a quantum-noise-limited measurement can be expected, limited only by the necessity of operating below the threshold of spontaneous superradiant emission of the excited medium. Were we to approach this limit, however, we could greatly amplify the asymmetry using triggered superradiance.
A pump-probe atomic parity violation (APV) experiment performed in a longitudinal electric field, provides a signal breaking mirror symmetry while preserving the cylindrical symmetry of the set-up. The excited vapor acts on the probe beam as a linear dichroic amplifier, imprinting a very specific signature on the detected signal. Our differential polarimeter is oriented to yield a null result unless the excited atoms are endowed with a chirality of some kind. Ideally, only the APV (E-odd) and the calibration (E-even) signals contribute to a chiral atomic response, a situation highly favourable to the detection of a tiny effect. In the present work we give a thorough analysis of possible unwanted defects like stray transverse fields or misalignments which would spoil the ideal set-up and lead to chiral systematics. A possible way to suppress such effects is to perform global rotations of the experiment by incremental steps of 45 degrees, leaving both stray fields and misalignments unaltered. The conspiration of at least two defects is necessary to affect the E-odd polarimeter signal. The transverse nature of the defects manifests itself by an azimuthal cosine square modulation. The harmful systematics are those which survive the averaging over four successive configurations. They require the presence of a stray transverse electric field, which can be determined and eventually minimized by auxiliary measurements of the systematic effects, amplified by applying a known external magnetic field. Transverse stray magnetic fields must be compensated by a similar procedure. We also propose statistical correlation tests as diagnoses of the aforementioned systematic effects.
This paper presents our experimental strategy for detecting an electric-field controlled pulsed-excited highly forbidden transition such as the 6S-7S transition of the Cs atom. The amplification of a pulsed probe beam close to resonance for one hfs component of the 7S – 6P3/2 transition is monitored. Furthermore the polarization modification experienced by the probe beam as a result of its interaction with the vapor is meticulously analysed. In convincing examples we point out the high sensitivity and complete selectivity of the method for the angular anisotropy of the 7S state, such as orientation or alignment created in the 6S-7S excitation process. We illustrate the flexibility of the method by varying the pump-probe delay, or by changing the relative positions of the beams or the magnitude of the E-field that directly controls the optical gain. A smooth transition from linear amplification to triggered superradiance is observed which necessitates readjustment of the probe intensity for optimal detection. We demonstrate the vapor-induced enhancement of the amplification anisotropy, expected at high optical density.
We report here the first observation and measurement of the transition magnetic moment for a strongly forbidden S-S transition of an alkali atom. By using single-photon excitation of the 6S1/2-7S1/2 caesium transition in a quasi-static electric field, we have detected an electronic polarization of the excited state, specific to an interference effect occurring in mixed electric magnetic dipole transitions. Our measurements, independent of the vapour density, indicate that the magnetic dipole amplitude M 1 has the same size as the electric dipole amplitude E1 , induced by an electric field, parallel to the light wave polarization, and equal to 2.62 ± 0.21 V/cm. We also obtain the relative sign of M 1 and E1. Using a theoretical value of E 1 and a well-defined phase convention, we deduce for the magnetic moment of the transition < 7S, 1/ 2 | μ z | 6S, 1/ 2 > = - (4.24 ± 0.34) 10-5 |μB|, in good agreement with a theoretical estimate. This result can also be expressed in terms of the oscillator strength in zero electric field f6S-7S = 4.05 × 10-15, or of the single-photon M1 decay rate Γ 7S-6S = 0.93 x 10-6 s-1.
We report our study of the magneto-optical effect in a strictly linear regime on spin-polarized cold cesium atoms. Due to the low intensity and the short illumination period of the probe beam, less than 7.5% of the sample atoms change their states by absorbing probe photons. We produce a medium of atoms at rest in either the 6S1∕2,F=3,mF=06{S}_{1∕2},F=3,{m}_{F}=06S1∕2​,F=3,mF​=0 or 6S1∕2,F=3,mF=36{S}_{1∕2},F=3,{m}_{F}=36S1∕2​,F=3,mF​=3 state by optically pumping atoms trapped in a magneto-optical trap. We use the D1D1D1 resonance with large lower and upper state hyperfine splittings as a probe transition to avoid hyperfine mixing from the Zeeman interaction. Under this idealized situation we measure the Stokes parameters in order to find the polarization rotation and circular dichroism experienced by the probe light. We find that there are qualitative differences between the results for the mF=0{m}_{F}=0mF​=0 and mF=3{m}_{F}=3mF​=3 cases. While dispersion and consequent Faraday rotation play a dominant role when the atoms are in the mF=0{m}_{F}=0mF​=0 state, it is dissipation and circular dichroism that are important when they are in the mF=3{m}_{F}=3mF​=3 state. Similarly, while the size of the Faraday rotation and the circular dichroism for the mF=0{m}_{F}=0mF​=0 case scales linearly with the applied magnetic field, for the mF=3{m}_{F}=3mF​=3 case it is the shift of the probe polarization change versus frequency that is linearly proportional to the magnetic field strength.