Support for measurements operations

CHANGELOG.md

@@ -4,6 +4,12 @@
 
 ### Improvements 🛠
 
+* Measurement operations are now added to the PennyLane template when a `QuantumCircuit`
+  is converted using `load`. Additionally, one can override any existing terminal
+  measurements by providing a list of PennyLane 
+  `measurements <https://docs.pennylane.ai/en/stable/introduction/measurements.html>`_ themselves.
+  [(#405)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/405)
+
 ### Breaking changes 💔
 
 ### Deprecations 👋
@@ -16,6 +22,8 @@
 
 This release contains contributions from (in alphabetical order):
 
+Utkarsh Azad
+
 ---
 # Release 0.34.0
 

pennylane_qiskit/converter.py

@@ -20,7 +20,8 @@
 
 import numpy as np
 from qiskit import QuantumCircuit
-from qiskit.circuit import Parameter, ParameterExpression
+from qiskit.circuit import Parameter, ParameterExpression, Measure, Barrier
+from qiskit.circuit.library import GlobalPhaseGate
 from qiskit.exceptions import QiskitError
 from sympy import lambdify
 
@@ -45,7 +46,7 @@ def _check_parameter_bound(param: Parameter, var_ref_map: Dict[Parameter, Any]):
             a dictionary mapping qiskit parameters to trainable parameter values
     """
     if isinstance(param, Parameter) and param not in var_ref_map:
-        raise ValueError("The parameter {} was not bound correctly.".format(param))
+        raise ValueError(f"The parameter {param} was not bound correctly.")
 
 
 def _extract_variable_refs(params: Dict[Parameter, Any]) -> Dict[Parameter, Any]:
@@ -87,9 +88,7 @@ def _check_circuit_and_bind_parameters(
         QuantumCircuit: quantum circuit with bound parameters
     """
     if not isinstance(quantum_circuit, QuantumCircuit):
-        raise ValueError(
-            "The circuit {} is not a valid Qiskit QuantumCircuit.".format(quantum_circuit)
-        )
+        raise ValueError(f"The circuit {quantum_circuit} is not a valid Qiskit QuantumCircuit.")
 
     if params is None:
         return quantum_circuit
@@ -120,8 +119,8 @@ def map_wires(qc_wires: list, wires: list) -> dict:
         return dict(zip(qc_wires, wires))
 
     raise qml.QuantumFunctionError(
-        "The specified number of wires - {} - does not match "
-        "the number of wires the loaded quantum circuit acts on.".format(len(wires))
+        f"The specified number of wires - {len(wires)} - does not match "
+        "the number of wires the loaded quantum circuit acts on."
     )
 
 
@@ -143,18 +142,22 @@ def execute_supported_operation(operation_name: str, parameters: list, wires: li
         operation(*parameters, wires=wires)
 
 
-def load(quantum_circuit: QuantumCircuit):
+def load(quantum_circuit: QuantumCircuit, measurements=None):
     """Loads a PennyLane template from a Qiskit QuantumCircuit.
     Warnings are created for each of the QuantumCircuit instructions that were
     not incorporated in the PennyLane template.
 
     Args:
         quantum_circuit (qiskit.QuantumCircuit): the QuantumCircuit to be converted
+        measurements (list[pennylane.measurements.MeasurementProcess]): the list of PennyLane
+            `measurements <https://docs.pennylane.ai/en/stable/introduction/measurements.html>`_
+            that overrides the terminal measurements that may be present in the input circuit.
 
     Returns:
         function: the resulting PennyLane template
     """
 
+    # pylint:disable=fixme, too-many-branches
     def _function(params: dict = None, wires: list = None):
         """Returns a PennyLane template created based on the input QuantumCircuit.
         Warnings are created for each of the QuantumCircuit instructions that were
@@ -177,8 +180,11 @@ def _function(params: dict = None, wires: list = None):
 
         wire_map = map_wires(qc_wires, wires)
 
+        # Stores the measurements encountered in the circuit
+        mid_circ_meas, terminal_meas = [], []
+
         # Processing the dictionary of parameters passed
-        for op, qargs, cargs in qc.data:
+        for idx, (op, qargs, _) in enumerate(qc.data):
             # the new Singleton classes have different names than the objects they represent, but base_class.__name__ still matches
             instruction_name = getattr(op, "base_class", op.__class__).__name__
 
@@ -224,16 +230,43 @@ def _function(params: dict = None, wires: list = None):
                 gate = dagger_map[instruction_name]
                 qml.adjoint(gate)(wires=operation_wires)
 
+            elif isinstance(op, Measure):
+                # Store the current operation wires
+                op_wires = set(operation_wires)
+                # Look-ahead for more gate(s) on its wire(s)
+                meas_terminal = True
+                for next_op, next_qargs, __ in qc.data[idx + 1 :]:
+                    # Check if the subsequent whether next_op is measurement interfering
+                    if not isinstance(next_op, (Barrier, GlobalPhaseGate)):
+                        next_op_wires = set(wire_map[hash(qubit)] for qubit in next_qargs)
+                        # Check if there's any overlapping wires
+                        if next_op_wires.intersection(op_wires):
+                            meas_terminal = False
+                            break
+
+                # Allows for queing the mid-circuit measurements
+                if not meas_terminal:
+                    mid_circ_meas.append(qml.measure(wires=operation_wires))
+                else:
+                    terminal_meas.extend(operation_wires)
+
             else:
                 try:
                     operation_matrix = op.to_matrix()
                     pennylane_ops.QubitUnitary(operation_matrix, wires=operation_wires)
                 except (AttributeError, QiskitError):
                     warnings.warn(
-                        __name__ + ": The {} instruction is not supported by PennyLane,"
-                        " and has not been added to the template.".format(instruction_name),
+                        f"{__name__}: The {instruction_name} instruction is not supported by PennyLane,"
+                        " and has not been added to the template.",
                         UserWarning,
                     )
+        # Use the user-provided measurements
+        if measurements:
+            if qml.queuing.QueuingManager.active_context():
+                return [qml.apply(meas) for meas in measurements]
+            return measurements
+
+        return tuple(mid_circ_meas + list(map(qml.measure, terminal_meas))) or None
 
     return _function
 

tests/test_converter.py

@@ -5,6 +5,7 @@
 from qiskit import ClassicalRegister, QuantumCircuit, QuantumRegister
 from qiskit import extensions as ex
 from qiskit.circuit import Parameter
+from qiskit.circuit.library import EfficientSU2
 from qiskit.exceptions import QiskitError
 from qiskit.quantum_info.operators import Operator
 
@@ -771,10 +772,9 @@ def test_map_wires_exception_mismatch_in_number_of_wires(self, recorder):
 class TestConverterWarnings:
     """Tests that the converter.load function emits warnings."""
 
-    def test_barrier_not_supported(self, recorder):
+    def test_template_not_supported(self, recorder):
         """Tests that a warning is raised if an unsupported instruction was reached."""
-        qc = QuantumCircuit(3, 1)
-        qc.barrier()
+        qc = EfficientSU2(3, reps=1)
 
         quantum_circuit = load(qc)
 
@@ -785,7 +785,7 @@ def test_barrier_not_supported(self, recorder):
         # check that the message matches
         assert (
             record[-1].message.args[0]
-            == "pennylane_qiskit.converter: The Barrier instruction is not supported by"
+            == "pennylane_qiskit.converter: The Gate instruction is not supported by"
             " PennyLane, and has not been added to the template."
         )
 
@@ -818,11 +818,10 @@ def test_qasm_from_file(self, tmpdir, recorder):
 
         quantum_circuit = load_qasm_from_file(qft_qasm)
 
-        with pytest.warns(UserWarning) as record:
-            with recorder:
-                quantum_circuit()
+        with recorder:
+            quantum_circuit()
 
-        assert len(recorder.queue) == 6
+        assert len(recorder.queue) == 10
 
         assert recorder.queue[0].name == "PauliX"
         assert recorder.queue[0].parameters == []
@@ -868,11 +867,10 @@ def test_qasm_(self, recorder):
 
         quantum_circuit = load_qasm(qasm_string)
 
-        with pytest.warns(UserWarning) as record:
-            with recorder:
-                quantum_circuit(params={})
+        with recorder:
+            quantum_circuit(params={})
 
-        assert len(recorder.queue) == 2
+        assert len(recorder.queue) == 6
 
         assert recorder.queue[0].name == "PauliX"
         assert recorder.queue[0].parameters == []
@@ -1110,3 +1108,74 @@ def cost(x, y):
         ]
 
         assert np.allclose(jac, jac_expected)
+
+    def test_meas_circuit_in_qnode(self, qubit_device_2_wires):
+        """Tests loading a converted template in a QNode with measurements."""
+
+        angle = 0.543
+
+        qc = QuantumCircuit(2, 2)
+        qc.h(0)
+        qc.measure(0, 0)
+        qc.rz(angle, [0])
+        qc.cx(0, 1)
+        qc.measure_all()
+
+        measurements = [qml.expval(qml.PauliZ(0)), qml.vn_entropy([1])]
+        quantum_circuit = load(qc, measurements=measurements)
+
+        @qml.qnode(qubit_device_2_wires)
+        def circuit_loaded_qiskit_circuit():
+            return quantum_circuit()
+
+        @qml.qnode(qubit_device_2_wires)
+        def circuit_native_pennylane():
+            qml.Hadamard(0)
+            qml.measure(0)
+            qml.RZ(angle, wires=0)
+            qml.CNOT([0, 1])
+            return [qml.expval(qml.PauliZ(0)), qml.vn_entropy([1])]
+
+        assert circuit_loaded_qiskit_circuit() == circuit_native_pennylane()
+
+        quantum_circuit = load(qc, measurements=None)
+
+        @qml.qnode(qubit_device_2_wires)
+        def circuit_loaded_qiskit_circuit2():
+            meas = quantum_circuit()
+            return [qml.expval(m) for m in meas]
+
+        @qml.qnode(qubit_device_2_wires)
+        def circuit_native_pennylane2():
+            qml.Hadamard(0)
+            m0 = qml.measure(0)
+            qml.RZ(angle, wires=0)
+            qml.CNOT([0, 1])
+            return [qml.expval(m) for m in [m0, qml.measure(0), qml.measure(1)]]
+
+        assert circuit_loaded_qiskit_circuit2() == circuit_native_pennylane2()
+
+    def test_diff_meas_circuit(self):
+        """Tests mid-measurements are recognized and returned correctly."""
+
+        angle = 0.543
+
+        qc = QuantumCircuit(3, 3)
+        qc.h(0)
+        qc.measure(0, 0)
+        qc.rx(angle, [0])
+        qc.cx(0, 1)
+        qc.measure(1, 1)
+
+        qc1 = QuantumCircuit(3, 3)
+        qc1.h(0)
+        qc1.measure(2, 2)
+        qc1.rx(angle, [0])
+        qc1.cx(0, 1)
+        qc1.measure(1, 1)
+
+        qtemp, qtemp1 = load(qc), load(qc1)
+        assert qtemp()[0] == qml.measure(0) and qtemp1()[0] == qml.measure(2)
+
+        qtemp2 = load(qc, measurements=[qml.expval(qml.PauliZ(0))])
+        assert qtemp()[0] != qtemp2()[0] and qtemp2()[0] == qml.expval(qml.PauliZ(0))