add SaveState instruction

qiskit/providers/aer/backends/qasm_simulator.py

@@ -295,14 +295,14 @@ class QasmSimulator(AerBackend):
             'initialize', 'delay', 'pauli', 'mcx_gray',
             # Custom instructions
             'kraus', 'roerror', 'snapshot', 'save_expval', 'save_expval_var',
-            'save_probabilities', 'save_probabilities_dict',
+            'save_probabilities', 'save_probabilities_dict', 'save_state',
             'save_density_matrix', 'save_statevector',
             'save_amplitudes', 'save_amplitudes_sq', 'save_stabilizer'
         ]),
         'custom_instructions': sorted([
             'roerror', 'kraus', 'snapshot', 'save_expval', 'save_expval_var',
             'save_probabilities', 'save_probabilities_dict',
-            'save_density_matrix', 'save_statevector',
+            'save_state', 'save_density_matrix', 'save_statevector',
             'save_amplitudes', 'save_amplitudes_sq', 'save_stabilizer']),
         'gates': []
     }
@@ -481,7 +481,7 @@ def _method_configuration(method=None):
             config.custom_instructions = sorted([
                 'roerror', 'snapshot', 'kraus', 'superop', 'save_expval', 'save_expval_var',
                 'save_probabilities', 'save_probabilities_dict', 'save_density_matrix',
-                'save_amplitudes_sq'])
+                'save_amplitudes_sq', 'save_state'])
             config.basis_gates = sorted([
                 'u1', 'u2', 'u3', 'u', 'p', 'r', 'rx', 'ry', 'rz', 'id', 'x',
                 'y', 'z', 'h', 's', 'sdg', 'sx', 't', 'tdg', 'swap', 'cx',
@@ -511,7 +511,7 @@ def _method_configuration(method=None):
             config.custom_instructions = sorted([
                 'roerror', 'snapshot', 'save_expval', 'save_expval_var',
                 'save_probabilities', 'save_probabilities_dict',
-                'save_amplitudes_sq', 'save_stabilizer'])
+                'save_amplitudes_sq', 'save_stabilizer', 'save_state'])
             config.basis_gates = sorted([
                 'id', 'x', 'y', 'z', 'h', 's', 'sdg', 'sx', 'cx', 'cy', 'cz',
                 'swap', 'delay',

qiskit/providers/aer/backends/statevector_simulator.py

@@ -130,7 +130,7 @@ class StatevectorSimulator(AerBackend):
             'initialize', 'kraus', 'roerror', 'delay', 'pauli',
             'save_expval', 'save_density_matrix', 'save_statevector',
             'save_probs', 'save_probs_ket', 'save_amplitudes',
-            'save_amplitudes_sq'
+            'save_amplitudes_sq', 'save_state'
         ],
         'gates': []
     }

qiskit/providers/aer/backends/unitary_simulator.py

@@ -133,7 +133,7 @@ class UnitarySimulator(AerBackend):
             'rzz', 'rzx', 'ccx', 'cswap', 'mcx', 'mcy', 'mcz', 'mcsx',
             'mcp', 'mcu1', 'mcu2', 'mcu3', 'mcrx', 'mcry', 'mcrz',
             'mcr', 'mcswap', 'unitary', 'diagonal', 'multiplexer', 'delay', 'pauli',
-            'save_unitary'
+            'save_unitary', 'save_state'
         ],
         'gates': []
     }

qiskit/providers/aer/library/__init__.py

@@ -33,6 +33,7 @@
 .. autosummary::
     :toctree: ../stubs/
 
+    SaveState
     SaveExpectationValue
     SaveExpectationValueVariance
     SaveProbabilities
@@ -53,6 +54,7 @@
 .. autosummary::
     :toctree: ../stubs/
 
+    save_state
     save_expectation_value
     save_expectation_value_variance
     save_probabilities
@@ -95,12 +97,14 @@
     state.
 """
 
-__all__ = ['SaveExpectationValue', 'SaveExpectationValueVariance',
+__all__ = ['SaveState',
+           'SaveExpectationValue', 'SaveExpectationValueVariance',
            'SaveProbabilities', 'SaveProbabilitiesDict',
            'SaveStatevector', 'SaveStatevectorDict', 'SaveDensityMatrix',
            'SaveAmplitudes', 'SaveAmplitudesSquared', 'SaveUnitary',
            'SaveStabilizer']
 
+from .save_state import SaveState, save_state
 from .save_expectation_value import (
     SaveExpectationValue, save_expectation_value,
     SaveExpectationValueVariance, save_expectation_value_variance)
@@ -111,5 +115,5 @@
 from .save_density_matrix import SaveDensityMatrix, save_density_matrix
 from .save_amplitudes import (SaveAmplitudes, save_amplitudes,
                               SaveAmplitudesSquared, save_amplitudes_squared)
-from .save_stabilizer import (SaveStabilizer, save_stabilizer)
-from .save_unitary import (SaveUnitary, save_unitary)
+from .save_stabilizer import SaveStabilizer, save_stabilizer
+from .save_unitary import SaveUnitary, save_unitary

qiskit/providers/aer/library/save_state.py

@@ -0,0 +1,85 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2018, 2021.
+#
+# 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 instruction to save simulator state.
+"""
+
+from qiskit.circuit import QuantumCircuit
+from .save_data import SaveSingleData, default_qubits
+
+
+class SaveState(SaveSingleData):
+    """Save simulator state
+
+    The format of the saved state depends on the simulation method used.
+    """
+    def __init__(self, num_qubits,
+                 label=None,
+                 pershot=False,
+                 conditional=False):
+        """Create new instruction to save the simualtor state.
+
+        The format of the saved state depends on the simulation method used.
+
+        Args:
+            num_qubits (int): the number of qubits of the
+            label (str or None): Optional, the key for retrieving saved data
+                                 from results. If None the key will be the
+                                 state type of the simulator.
+            pershot (bool): if True save a list of states for each
+                            shot of the simulation rather than a single
+                            state [Default: False].
+            conditional (bool): if True save data conditional on the current
+                                classical register values [Default: False].
+
+        .. note::
+
+            This save instruction must always be performed on the full width of
+            qubits in a circuit, otherwise an exception will be raised during
+            simulation.
+        """
+        if label is None:
+            label = '_method_'
+        super().__init__('save_state', num_qubits, label,
+                         pershot=pershot,
+                         conditional=conditional)
+
+
+def save_state(self, label=None, pershot=False, conditional=False):
+    """Save the current simulator quantum state.
+
+    Args:
+        label (str or None): Optional, the key for retrieving saved data
+                             from results. If None the key will be the
+                             state type of the simulator.
+        pershot (bool): if True save a list of statevectors for each
+                        shot of the simulation [Default: False].
+        conditional (bool): if True save pershot data conditional on the
+                            current classical register values
+                            [Default: False].
+
+    Returns:
+        QuantumCircuit: with attached instruction.
+
+    .. note:
+
+        This instruction is always defined across all qubits in a circuit.
+    """
+    qubits = default_qubits(self)
+    instr = SaveState(len(qubits),
+                      label=label,
+                      pershot=pershot,
+                      conditional=conditional)
+    return self.append(instr, qubits)
+
+
+QuantumCircuit.save_state = save_state