cirq.density_matrix_from_state_vector

Returns the density matrix of the state vector.

cirq.density_matrix_from_state_vector(
    state_vector: np.ndarray,
    indices: Optional[Iterable[int]] = None,
    qid_shape: Optional[Tuple[int, ...]] = None
) -> np.ndarray

Calculate the density matrix for the system on the given qubit indices, with the qubits not in indices that are present in state vector traced out. If indices is None the full density matrix for state_vector is returned. We assume state_vector follows the standard Kronecker convention of numpy.kron (big-endian).

For example:

state_vector = np.array([1/np.sqrt(2), 1/np.sqrt(2)], dtype=np.complex64) indices = None gives us

$$
\rho = \begin{bmatrix}
        0.5 & 0.5 \\
        0.5 & 0.5
\end{bmatrix}
$$

state_vector A sequence representing a state vector in which the ordering mapping to qubits follows the standard Kronecker convention of numpy.kron (big-endian).
indices list containing indices for qubits that you would like to include in the density matrix (i.e.) qubits that WON'T be traced out. follows the standard Kronecker convention of numpy.kron.
qid_shape specifies the dimensions of the qudits for the input state_vector. If not specified, qubits are assumed and the state_vector must have a dimension a power of two.

A numpy array representing the density matrix.

ValueError if the size of state_vector is not a power of 2 and the shape is not given or if the shape is given and state_vector has a size that contradicts this shape.
IndexError if the indices are out of range for the number of qubits corresponding to state_vector.