In quantum information theory, the idea of a typical subspace plays an important role in the proofs of many coding theorems (the most prominent example being Schumacher compression). Its role is analogous to that of the typical set in classical information theory.
Unconditional Quantum Typicality
The projector onto the typical subspace of is defined as
where we have "overloaded" the symbol to refer also to the set of -typical sequences:
The three important properties of the typical projector are as follows:
where the first property holds for arbitrary and sufficiently large .
Conditional Quantum Typicality
Consider an ensemble of states. Suppose that each state has the following spectral decomposition:
Consider a density operator which is conditional on a classical sequence :
We define the weak conditionally typical subspace as the span of vectors (conditional on the sequence ) such that the sample conditional entropy of their classical labels is close to the true conditional entropy of the distribution :
The projector onto the weak conditionally typical subspace of is as follows:
where we have again overloaded the symbol to refer to the set of weak conditionally typical sequences:
The three important properties of the weak conditionally typical projector are as follows:
where the first property holds for arbitrary and sufficiently large , and the expectation is with respect to the distribution .
- Mark M. Wilde, "From Classical to Quantum Shannon Theory", arXiv:1106.1445.