Returns a mixture representing a state vector with only some qubits kept.

The input state vector can have any shape, but if it is one-dimensional it will be interpreted as qubits, since that is the most common case, and fail if the dimension is not size 2 ** n. States in the output mixture will retain the same type of shape as the input state vector.

If the state vector cannot be factored into a pure state over keep_indices then eigendecomposition is used and the output mixture will not be unique.

state_vector The state vector to take the partial trace over.
keep_indices Which indices to take the partial trace of the state_vector on.
atol The tolerance for determining that a factored state is pure.

A single-component mixture in which the factored state vector has probability '1' if the partially traced state is pure, or else a mixture of the default eigendecomposition of the mixed state's partial trace.

ValueError If the input state_vector is one dimension, but that dimension size is not a power of two.
IndexError If any indexes are out of range.