# This code is part of Qiskit.
#
# (C) Copyright IBM 2018, 2019.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""
Simulator command to snapshot internal simulator representation.
"""
from qiskit import QuantumCircuit
from qiskit.providers.aer.extensions import Snapshot
[docs]class SnapshotDensityMatrix(Snapshot):
"""Snapshot instruction for density matrix method of Qasm simulator."""
def __init__(self, label, num_qubits):
"""Create a density matrix state snapshot instruction.
Args:
label (str): the snapshot label.
num_qubits (int): the number of qubits to snapshot.
Raises:
ExtensionError: if snapshot is invalid.
"""
super().__init__(label,
snapshot_type='density_matrix',
num_qubits=num_qubits)
def snapshot_density_matrix(self, label, qubits=None):
"""Take a density matrix snapshot of simulator state.
Args:
label (str): a snapshot label to report the result
qubits (list or None): the qubits to apply snapshot to. If None all
qubits will be snapshot [Default: None].
Returns:
QuantumCircuit: with attached instruction.
Raises:
ExtensionError: if snapshot is invalid.
"""
snapshot_register = Snapshot.define_snapshot_register(self, label, qubits)
return self.append(
SnapshotDensityMatrix(label, num_qubits=len(snapshot_register)),
snapshot_register)
QuantumCircuit.snapshot_density_matrix = snapshot_density_matrix