What you see is the "default" light. AFAIK you can get no pointer to the default light to change its position, color, intensity or the like. If you want to have control you will have to add your own light to the root entity which then replaces the "default" light by some internal magic.
So you do something like this:
Qt3DCore::QEntity *pLightEntity=new Qt3DCore::QEntity(m_pRootEntity);