Implementing fast, ray-traced soft shadows in a game engine 11 years ago
This technique is limited to "unrealistic" point lights (except for stars). To improve the shadow quality it is necessary to work with light fields. This can be done either by voxel fields (I think nvidia tried something like this) or depth fields. I use the later one with the advantage that it can be easily combined with other techniques and can be updated in real time. For almost static environments like the one in the example I would advise to use precomputed emission maps. There is also something wrong with the shadow map (right image) I suspect a blur step is missing.