M. Lintz

New manifestation of atomic parity violation in cesium: a chiral optical gain induced by linearly polarized 6S-7S excitation.

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.

Parity violation in forbidden transitions: Detection of the electroweak alignment or polarization in the upper state by stimulated emission

Abstract We present a new method for detecting the parity violation induced by weak neutral currents in a forbidden atomic transition such as the 6 S-7 S Cs transition. The 7 S atoms are produced by pulsed excitation and detected through the subsequent transient gain which appears at the 7 S-6 P frequency. A right-left asymmetry in this gain is expected as a result of parity violation, with possible amplification of this asymmetry when the vapor becomes optically thick. With respect to previous observation conditions, substantial improvement in signal/noise ratio is expected.

Sensitive pulsed pump-probe atomic polarimetry for parity-violation measurements in caesium

We describe an ongoing experiment to measure parity violation in atomic caesium, based on detection by stimulated emission. Our goal is to measure to 1 a left-right asymmetry of to test electroweak theory and look for new physics beyond the Standard Model. The Cs highly forbidden transition, , is excited in a vapour (5-10 mtorr) by a pump laser pulse in a longitudinal electric field . The PV asymmetry resulting from the weak interaction during optical excitation is converted into an anisotropy in the gain of a probe laser pulse which stimulates the allowed transition , and manifests itself as a tiny -odd rotation of the probes linear polarization. Differential polarimetry allows dark-field detection of the rotation angle with a baseline defined to better than and discrimination between true and pseudo-rotation. Lineshape-independent angle calibration is performed using a parity-conserving -even anisotropy. To isolate the parity-violating effect, we exploit the symmetry of revolution of the experiment by (i) rotating pump and probe linear polarizations around the beam axis and (ii) reversing in a cylindrically symmetric cell. After describing the apparatus and data acquisition procedure, we summarize the current experimental status and short-term prospects.

Two-beam linear magneto-optical spectroscopy of atomic transitions between short lived states

Two-beam, linear magneto-optical spectroscopy is a powerful tool for studying short-lived states. We present both measurements and a quantitative theoretical analysis of magneto-rotation observed in the forward scattering of a linearly polarised laser beam passing through an amplifying atomic medium placed in a longitudinal magnetic field. The probed transition connects two short-lived, excited atomic levels, the upper state (here the 7S1/2 level of cesium) being prepared initially via another transition from ground state, excited by a linearly polarised pump beam. The probe polarisation undergoes three different magneto-optical processes: optical rotation, with separate contributions from the two transitions, and linear dichroism due to Hanle precession of the upper state alignment. Complete resolution of the hyperfine structures and ninety degree switching of the probe polarisation enable us to isolate all of these processes. To lowest order in optical thickness the relative intensities and lineshapes are well interpreted.

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

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.

Cylindrical symmetry discrimination of magnetoelectric optical systematic effects in a pump-probe atomic parity violation experiment

A pump-probe atomic parity violation (APV) experiment performed in a longitudinal electric field E⃗l, has the advantage of providing a signal which breaks mirror symmetry but preserves cylindrical symmetry of the set-up, i.e. this signal remains invariant when the pump and probe linear polarizations are simultaneously rotated about their common direction of propagation. 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 a chirality of some kind is acting on the excited atoms. Ideally, only the APV (E⃗l-odd) and the calibration (E⃗l-even) signals should participate in such a chiral atomic response, a situation highly favourable to sensitive detection of a tiny effect. In the present work, we give a thorough analysis of possible undesirable defects such as spurious transverse fields or misalignments, which may spoil the ideal configuration and generate a chiral response leading to possible systematics. We study a possible way to get rid of such defects by performing global rotations of the experiment by incremental angular steps ϕ, leaving both stray fields and misalignments unaltered. Our analysis shows that at least two defects are necessary for the E⃗l-odd polarimeter output to be affected; a cos(2ϕ) modulation in the global rotations reveals the transverse nature of the defects. The harmful systematic effects are those which subsist after we average over four configurations obtained by successive rotations of 45°. They require the presence of a stray transverse electric field. By doing auxiliary atomic measurements made in known, applied, magnetic fields which amplify the systematic effect, it is possible to measure the transverse E-field and to minimize it. Transverse magnetic fields must also be carefully compensated following a similar procedure.We discuss the feasibility of reducing the systematic uncertainty below the one percent level. We also propose statistical correlation tests as diagnoses of the aforementioned systematic effects.

Differential-mode atomic polarimetry with pulsed lasers: high-precision zero adjustment

We discuss the precise zero adjustment of a balanced differential-mode polarimeter containing an atomic sample for dark-field detection of a small atomic parity-violation anisotropy odd under reversal of an applied E field. Our sample is endowed with an E-even parity-conserving anisotropy, possibly large, but oriented to produce no imbalance. An imbalance arises both from defects in the preparation of the sample and from small probe-beam anisotropies coupling with those of the sample. An adjustment procedure allows identification and reduction of each defect. The residual E-even imbalance is maintained at the noise level by active compensation.

Doppler-free inhibited fluorescence spectroscopy of a forbidden three-level system

Abstract In a three-level system driven by two lasers, one of which connects a populated state to a state of same parity through a forbidden transition, stimulated emission is detected through its competition with fluorescence in a different transition sharing the same upper level. “Inhibited fluorescence” appears as a sensitive sub-Doppler spectroscopy well suited to this parity-forbidden three-level system. It is applied to the 6S 1 2 −7S 3 2 −6P 3 2 configuration of atomic cesium, in view of its interest for par ity violation measurements. The 6P 3 2 hfs is fully resolved and the collisional damping of the 7S−6P optical coherence is measured. A theoretical model reproduces the spectra satisfactorily.

Theory of transition probability saturation for photodissociation of Cs2 and photoionization of the 5D5/2 product atoms

Transition probability saturation has been used as a means of investigation of cesium photoionization via photodissociative states of Cs2 around 550 nm. We present here a theoretical interpretation of the data obtained with polarized light. It allows for dispersion of the cross-sections due to the multi-level structure of the dissociated molecules and of the 5D5/2 product atoms. Thehfs in 5D5/2 is also taken into account. From best data fits we obtain determinations of the averaged cross-sections, free of uncertainty due to Cs or Cs2 densities. The 5D5/2 ionization cross-section agrees with theoretical prediction.

Measurement of the radiative lifetime of the cesium 5D5/2 level using pulsed excitation and delayed probe absorption

The 5D5/2 level of cesium is populated by pulsed 540 nm photodissociation of Cs2, and selectively detected via the 6P3/2 population that results by radiative decay. It is monitored by absorption of a delayed probe pulse. Its time evolution yields a value of 1260±80 ns for the radiative lifetime of the Cs 5D5/2 level, in good agreement with the calculation by Theodosiou [15].