| Because photons are bosons, the two-photon transition (6s 2S0)→(5d6 d 3D1), Delta E=35934 cm-1, in atomic barium is strictly forbidden when the photons are degenerate in energy (lambda=556 nm) and collinearly propagating. To search for exchange-antisymmetric states of photons, we have driven this transition in an atomic beam of barium with two independent laser light fields, both tuned to the energy midpoint. The lasers are locked to an in-vacuum optical power-build-up cavity, with a build-up factor of ∼100. The atoms interact with the laser light at the cavity waist. Population of the 5d6d 3D 1 state is measured by detection of fluorescence in several decay channels with wavelengths in the 420--450 nm range. The excitation-rate and detection efficiency of the apparatus is calibrated with the same transition in the allowed regime, that is, by detuning the lasers +/-1 GHz away from the midpoint, while remaining resonant with the two-photon transition.; No statistically significant transitions above background were observed. We limit the probability nu of photons being in exchange-antisymmetric states to nu < 7.4 x 10-10 at the 90% confidence level. |
