Nondestructive measurement of the transition probability in a Sr optical lattice clock

We present the experimental demonstration of nondestructive probing of the ¹S₀-³P₀ clock transition probability in an optical lattice clock with ⁸⁷Sr atoms. It is based on the phase shift induced by the atoms on a weak off-resonant laser beam. The method we propose is a differential measurement of this phase shift on two modulation sidebands with opposite detuning with respect to the ¹S₀-¹P₁ transition, allowing a detection limited by the photon shot noise. We have measured an atomic population of 10⁴ atoms with a signal-to-noise ratio of 100 per cycle, while keeping more than 95% of the atoms in the optical lattice with a depth of 0.1 mK. The method proves simple and robust enough to be operated as part of the whole clock setup. This detection scheme enables us to reuse atoms for subsequent clock state interrogations, dramatically reducing the loading time and thereby improving the clock frequency stability.