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Arnold uses uni-directional path tracing and so 'soft' caustics originating at glossy surfaces are perfectly possible, as well as caustics coming from big sources of indirect light.
Caustics can be enabled in the 'Advanced' tab of the Standard Surface shader.
Arnold will require a high number of Diffuse samples to achieve a clean image when using caustics. Care should be taken when enabling this feature.
A tutorial can be found here (MtoA).
As this is a frequently asked question we have provided both the question and answer in this thread.