Classical capacity of free-space optical communication

A prominent landmark in the extension of Shannon information theory to the quantum domain is the realization that any particular physical system can store only a finite amount of information. As a consequence, the quantity of information that can be exchanged in any communication process is limited by the available physical resources. Two of the main results in this regard are the Holevo bound and the Holevo-Schumacher-Westmoreland (HSW) theorem. The former limits the amount of information that can be retrieved after storage in a quantum system. For an encoding alphabet whose jth “letter” is the state σj , the mutual information between the encoded and the decoded message—independent of the decoding strategy—cannot exceed the Holevo information χ, defined by