The Holevo capacity of a quantum channel is the supremum of its Holevo information over finite ensembles of input states. Using entropy in bits, it is
 |
where 
and 
.
The unassisted classical capacity of a memoryless channel is the regularized quantity
.
Input states in an 
-use
ensemble may be entangled across uses, so equality 
requires an additivity argument. A noiseless
-dimensional channel has Holevo capacity
.
