S-wave collisional frequency shift of a fermion clock

We report an s-wave collisional frequency shift of an atomic clock based on fermions. In contrast to bosons, the fermion clock shift is insensitive to the population difference of the clock states, set by the first pulse area in Ramsey spectroscopy, θ₁. The fermion shift instead depends strongly on the second pulse area θ₂. It allows the shift to be canceled, nominally at θ₂=π/2, but correlations perturb the null to slightly larger θ₂. The frequency shift is relevant for optical lattice clocks and increases with the spatial inhomogeneity of the clock excitation field, naturally larger at optical frequencies.