add qasm measurement

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 ### Improvements 🛠
 
+* Updated `load_qasm` to take the optional kwarg `measurements` which get performed at the end of the loaded circuit and `load_qasm` can now detect mid-circuit measurements from `qasm`.
+[(#555)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/555)
+
 ### Breaking changes 💔
 
 ### Deprecations 👋
@@ -16,6 +19,8 @@
 
 This release contains contributions from (in alphabetical order):
 
+Mashhood Khan
+
 ---
 # Release 0.36.0
 

pennylane_qiskit/converter.py

@@ -20,6 +20,7 @@
 from functools import partial, reduce
 
 import numpy as np
+import qiskit.qasm2
 from qiskit import QuantumCircuit
 from qiskit.circuit import Parameter, ParameterExpression, ParameterVector
 from qiskit.circuit import Measure, Barrier, ControlFlowOp, Clbit
@@ -584,14 +585,17 @@ def _function(*args, params: dict = None, wires: list = None, **kwargs):
     return _function
 
 
-def load_qasm(qasm_string: str):
+def load_qasm(qasm_string: str, measurements=None):
     """Loads a PennyLane template from a QASM string.
     Args:
         qasm_string (str): the name of the QASM string
+        measurements (None | pennylane.measurements.MeasurementProcess | list[pennylane.measurements.MeasurementProcess]):
+            the PennyLane `measurements <https://docs.pennylane.ai/en/stable/introduction/measurements.html>`_
+            that override the terminal measurements that may be present in the input circuit
     Returns:
         function: the new PennyLane template
     """
-    return load(QuantumCircuit.from_qasm_str(qasm_string), measurements=[])
+    return load(qiskit.qasm2.loads(qasm_string), measurements=measurements)
 
 
 def load_qasm_from_file(file: str):

tests/test_converter.py

@@ -1165,8 +1165,8 @@ def test_qasm_(self, recorder):
         with recorder:
             quantum_circuit(params={})
 
-        # only X and CNOT queued (not 4 x qml.measure)
-        assert len(recorder.queue) == 2
+        # X and CNOT queued with 4 x qml.measure
+        assert len(recorder.queue) == 6
 
         assert recorder.queue[0].name == "PauliX"
         assert recorder.queue[0].parameters == []
@@ -1176,6 +1176,66 @@ def test_qasm_(self, recorder):
         assert recorder.queue[1].parameters == []
         assert recorder.queue[1].wires == Wires([2, 0])
 
+        for i in range(2, 6):
+            assert recorder.queue[i].name == "MidMeasureMP"
+            assert recorder.queue[i].wires == Wires([i - 2])
+
+    def test_qasm_measure(self):
+        """Tests that measurements specified as an argument are added to the converted circuit."""
+        qasm_string = 'include "qelib1.inc";' "qreg q[2];" "creg c[2];" "h q[0];" "cx q[0], q[1];"
+        dev = qml.device("default.qubit")
+        measurements = [qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliZ(1))]
+        loaded_circuit = load_qasm(qasm_string, measurements=measurements)
+
+        # loaded circuit with measurements
+        @qml.qnode(dev)
+        def quantum_circuit1():
+            return loaded_circuit()
+
+        # native pennylane measurements
+        @qml.qnode(dev)
+        def quantum_circuit2():
+            load_qasm(qasm_string)()
+            return [qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliZ(1))]
+
+        assert quantum_circuit1() == quantum_circuit2()
+
+    def test_qasm_mid_circuit_measure(self):
+        """Tests that the QASM primitive measure is correctly converted."""
+        qasm_string = (
+            'include "qelib1.inc";'
+            "qreg q[2];"
+            "creg c[2];"
+            "h q[0];"
+            "measure q[0] -> c[0];"
+            "rz(0.24) q[0];"
+            "cx q[0], q[1];"
+            "measure q -> c;"
+        )
+        dev = qml.device("default.qubit")
+        loaded_circuit = load_qasm(qasm_string)
+
+        # loaded circuit
+        @qml.qnode(dev)
+        def quantum_circuit1():
+            mid_measure, m0, m1 = loaded_circuit()
+            qml.cond(mid_measure == 0, qml.RX)(np.pi / 2, 0)
+            return qml.expval(mid_measure), qml.expval(m1)
+
+        # native pennylane circuit
+        @qml.qnode(dev)
+        def quantum_circuit2():
+            qml.Hadamard(0)
+            mid_measure = qml.measure(0)
+            qml.RZ(0.24, 0)
+            qml.CNOT([0, 1])
+            m0 = qml.measure([0])
+            m1 = qml.measure([1])
+            qml.cond(mid_measure == 0, qml.RX)(np.pi / 2, 0)
+            return qml.expval(mid_measure), qml.expval(m1)
+
+        assert quantum_circuit1() == quantum_circuit2()
+
 
 class TestConverterIntegration:
     def test_use_loaded_circuit_in_qnode(self, qubit_device_2_wires):