Returns the unique Choi matrix corresponding to a Kraus representation of a channel.

Quantum channel E: L(H1) -> L(H2) may be described by a collection of operators A_i, called Kraus operators, such that

$$ E(\rho) = \sum_i A_i \rho A_i^\dagger. $$

Kraus representation is not unique. Alternatively, E may be specified by its Choi matrix J(E) defined as

$$ J(E) = (E \otimes I)(|\phi\rangle\langle\phi|) $$

where $|\phi\rangle = \sum_i|i\rangle|i\rangle$ is the unnormalized maximally entangled state and I: L(H1) -> L(H1) is the identity map. Choi matrix is unique for a given channel.

The computation of the Choi matrix from a Kraus representation is essentially a reconstruction of a matrix from its eigendecomposition. It has the cost of O(kd**4) where k is the number of Kraus operators and d is the dimension of the input and output Hilbert space.

kraus_operators Sequence of Kraus operators specifying a quantum channel.

Choi matrix of the channel specified by kraus_operators.