def density_matrix_of(self, qubits: List[ops.Qid] = None) -> np.ndarray:
r"""Returns the density matrix of the state.
Calculate the density matrix for the system on the list, qubits.
Any qubits not in the list that are present in self.state_vector() will
be traced out. If qubits is None the full density matrix for
self.state_vector() is returned, given self.state_vector() follows
standard Kronecker convention of numpy.kron.
For example:
self.state_vector() = np.array([1/np.sqrt(2), 1/np.sqrt(2)],
dtype=np.complex64)
qubits = None
gives us
$$
\rho = \begin{bmatrix}
0.5 & 0.5 \\
0.5 & 0.5
\end{bmatrix}
$$
Args:
qubits: list containing qubit IDs 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.
"""
return qis.density_matrix_from_state_vector(
self.state_vector(),
[self.qubit_map[q]
for q in qubits] if qubits is not None else None,
qid_shape=self._qid_shape,
)
def density_matrix_from_state_vector(*args, **kwargs):
return qis.density_matrix_from_state_vector(*args, **kwargs)