  Infinite-layer nickelates present a new family of potential unconventional
superconductors. A key open question is the superconducting pairing symmetry.
We present low-temperature measurements of the London penetration depth in
optimally doped La_{0.8}Sr_{0.2}NiO_{2}, Pr_{0.8}Sr_{0.2}NiO_{2}, and
Nd_{0.8}Sr_{0.2}NiO_{2}. For La and Pr-nickelates, the superfluid density shows
a quadratic temperature dependence, indicating nodal superconductivity in the
presence of disorder. Nd-nickelate exhibits complex low-temperature behavior,
which we attribute to magnetic impurities. These results are consistent with
d-wave pairing.
