cirq.SimulationTrialResult.density_matrix

SimulationTrialResult.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.final_state traced out. If indices is None the full density
matrix for self.final_state is returned, given self.final_state
follows the standard Kronecker convention of numpy.kron.

For example:
    self.final_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.