Quantum Nondemolition Measurements using Cold Trapped Atoms

We have investigated possible implementations of optical quantum nondemolition measurements, using rubidium atoms in a magneto-optical trap as a nonlinear medium. Using a Λ-type three-level system in the D1 line of ⁸⁷Rb, the observed performances are quantitatively the best obtained so far for a single back action evading measurement. Moreover, the magneto-optical trap and the quantum nondemolition effect are both running continuously at the same time and mutual perturbations have been avoided by using a ``dark spot'' technique. This experiment demonstrates clearly the interest of using cold atoms for controlling the quantum fluctuations of light.