An Atheist, doesn't believe in Heaven. Doesn't believe in God. And doesn't believe in any life after death. They believe, simply, in mathematics, and sciences to prove theories. They do no believe in anything that could happen to a spirit when it is dead. Once someone dies, they simply cease to exist. No more is their spirit on Earth. No more does it even exist. That is the way of the Atheists.
Information for this belief was given by actual Atheists. They had a much, much, more complex way of saying it though.