cirq.StepResult.density_matrix

StepResult.density_matrix(indices: Iterable[int] = None) → numpy.ndarray

Returns the density matrix of the wavefunction.

Calculate the density matrix for the system on the given qubit
indices, with the qubits not in indices that are present in self.state
traced out. If indices is None the full density matrix for self.state
is returned, given self.state follows standard Kronecker convention
of numpy.kron.

For example:
    self.state = np.array([1/np.sqrt(2), 1/np.sqrt(2)],
        dtype=np.complex64)
    indices = None
    gives us

ho = egin{bmatrix}

0.5 & 0.5

0.5 & 0.5

\end{bmatrix}

Args:
    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.

Returns:
    A numpy array representing the density matrix.

Raises:
    ValueError: if the size of the state represents more than 25 qubits.
    IndexError: if the indices are out of range for the number of qubits
        corresponding to the state.