Efficient Decoherence Control Algorithms

2 Nice Error Bases

A quantum error-correcting codes must be able to deal with a continuum of errors. However, quantum mechanics is linear; hence, it suffices to consider a discrete set of errors. A convenient choice for such a set of error is given by a nice error basis. A nice error basis E = {Eg |g ∈ G0} is a set of unitary d × d  matrices indexed by a finite group G0  of order d2  such that

1.
tr(E1) = d  for the element of E indexed by the identity element of G0  ;
2.
tr(A†B) = 0  for distinct A, B  in E ;
3.
a scalar multiple of the product AB  is in E for all A,B  in E .

In particular, the matrices of a nice error basis form an orthonormal basis with respect to the trace inner product          -1    †
⟨A |B ⟩ = d tr(A  B)  , a very valuable property.