e viewed as a single-event source X' = {0} with probabil- ity p(0) = 1, corresponding to zero information (I(0) = - log.[p(0)] = - log.(1) = 0) and zero entropy =kg(I(0)) = 0. The difference in memory entropy before and after erasure is, thus, H = H' - H = -k loge 2. This is the amount of entropy that is communicated to the environment. These values represent a lower bound because the physical erasure process must also generate some form of heat, for instance passing a current through a memory transistor gate. The key conclusion of Landauer's principle can be summarized into the following statement: Information is physical. This means that the creation, erasure, manipulation, or processing of information involves physical laws. This represents a new awareness, which contrasts with the classi- cal background of Shannon's information theory, and calls for more questions, answers, and paradoxes. These are to be found in quantum information theory (QIT), which will be addressed shortly. At this stage, we must at least be convinced that there is more to information than a mere mathematical or ethereal definition, as we have been used and trained through education to believe intuitively. Landauer's principle brings up another interesting question, which concerns com- puting. When one performs hand calculations on a blackboard, one is obliged at some point to erase it, while saving the last results and other useful parameters somewhere in the corner of the blackboard. Very large blackboards, with different folding or sliding panels, could fit a full math course. But the information must be cleared for the next course. This shows that processing information (computation) requires information era- sure, because of the finite size of the memory. A computer that would keep and store the information regarding all of its intermediate calculation steps (namely the memory contents at each step, or the full record of the memory changes) would rapidly "choke on its own garbage." Such a necessary erasure yet obliterates the inform