[SDS-617] Fix measurement statements in ProjectQ

src/quantuminspire/projectq/backend_qx.py

@@ -485,8 +485,8 @@ def _store(self, cmd: Command) -> None:
             self._qasm += f"\nbarrier q[{qb_str}]"
             return
 
-        # when we find a gate after measurements we don't have fsp
-        # add any delayed measurement statements
+        # When fsp is enabled and we have skipped measurements when we find a gate after these measurements,
+        # switch to non-fsp and add any delayed measurement statements.
         if self._full_state_projection and len(self._measured_ids) != 0:
             self._switch_fsp_to_nonfsp()
 
@@ -638,11 +638,6 @@ def _run(self) -> None:
         if self._qasm == "":
             return
 
-        # Finally: add measurement commands for all qubits if no measurements are given.
-        # Only for simulation backends, measurements after all gate operations will perform properly in FSP.
-        if not self._measured_ids and self._is_simulation_backend:
-            self.__add_measure_all_qubits()
-
         self._finalize_qasm()
         self._execute_cqasm()
         self._filter_result_by_measured_qubits()
@@ -743,13 +738,15 @@ def __init__(self, qubit_id: int) -> None:
     def _get_measured_qubit_iterator(self, measurement_block_index: int) -> Iterator[int]:
         """
         Get an iterator for the measured qubits. The iterator returns the indexes of the qubits that are measured.
+        With fsp, the simulator will return all qubits as measured, but when there were measurements in the algorithm
+        we want to get these measured qubits only.
 
         :param measurement_block_index: measurement block index for multi measurement results.
 
         :return:
             Iterator which iterated the indexes of the measured qubits
         """
-        if self._full_state_projection:
+        if self._full_state_projection and len(self._measured_ids) != 0:
             bit_iterator = map(lambda bit: self._physical_to_simulated(self._logical_to_physical(bit)),
                                self._measured_ids)
         else:
@@ -788,11 +785,3 @@ def receive(self, command_list: List[Command]) -> None:
                 self._run()
                 self._flushed = True
                 self._reset()
-
-    def __add_measure_all_qubits(self) -> None:
-        """ Adds measurements at the end of the quantum algorithm for all allocated qubits. """
-        qubits_reference = self.main_engine.active_qubits.copy()
-        qubits_counts = len(qubits_reference)
-        for _ in range(qubits_counts):
-            q = qubits_reference.pop()
-            Measure | q

src/tests/quantuminspire/projectq/test_backend_qx.py

@@ -21,7 +21,7 @@
 import coreapi
 from collections import OrderedDict
 from functools import reduce
-from unittest import mock, TestCase
+from unittest import TestCase
 from unittest.mock import MagicMock, patch
 
 from projectq.meta import LogicalQubitIDTag
@@ -596,6 +596,7 @@ def test_receive(self, function_mock):
         command = MagicMock(gate=NOT, qubits=[[MagicMock(id=0)]], control_qubits=[MagicMock(id=1)])
         command_list = [command_alloc0, command_alloc1, command, MagicMock(gate=FlushGate())]
         self.qi_backend.main_engine = MagicMock()
+        self.qi_backend.main_engine.mapper.current_mapping = {0: 0, 1: 1}  # bits 0 and 1 are logical bits 0 and 1
         with patch('sys.stdout', new_callable=io.StringIO):
             self.qi_backend.receive(command_list)
         self.assertEqual(self.qi_backend.qasm, "")
@@ -611,6 +612,7 @@ def test_reuse_after_flush_raises_runtime_error(self, function_mock):
         api = MockApiClient()
         backend = QIBackend(quantum_inspire_api=api)
         backend.main_engine = MagicMock()
+        backend.main_engine.mapper.current_mapping = {0: 0, 1: 1}  # bits 0 and 1 are logical bits 0 and 1
         with patch('sys.stdout', new_callable=io.StringIO):
             self.assertRaisesRegex(RuntimeError, "Same instance of QIBackend used for circuit after Flush.",
                                    backend.receive, command_list)
@@ -624,6 +626,7 @@ def test_receive_multiple_flush(self, function_mock):
         command_list = [command_alloc0, command_alloc1, command, MagicMock(gate=FlushGate()),
                         MagicMock(gate=FlushGate())]
         self.qi_backend.main_engine = MagicMock()
+        self.qi_backend.main_engine.mapper.current_mapping = {0: 0, 1: 1}  # bits 0 and 1 are logical bits 0 and 1
         with patch('sys.stdout', new_callable=io.StringIO):
             self.qi_backend.receive(command_list)
         self.assertEqual(self.qi_backend.qasm, "")
@@ -918,32 +921,22 @@ def test_run_has_correct_output(self):
         self.assertTrue(std_output.startswith('version 1.0\n# cQASM generated by Quantum Inspire'))
         self.assertTrue('qubits 0' in std_output)
 
-    def test_run_raises_error_no_result(self):
-        with patch('sys.stdout', new_callable=io.StringIO):
-            self.qi_backend.qasm = "_"
-            self.qi_backend.measured_ids = [0, 1]
-            self.qi_backend.allocation_map = [(0, 0), (1, 1)]
-            self.qi_backend.main_engine = MagicMock()
-            self.qi_backend.main_engine.mapper.current_mapping = {0: 0, 1: 1}
-            result_mock = MagicMock()
-            result_mock.get.return_value = [OrderedDict()]
-            self.api.execute_qasm.return_value = result_mock
-            self.assertRaisesRegex(ProjectQBackendError, 'Result from backend contains no histogram data!',
-                                   self.qi_backend.run)
-        self.api.execute_qasm.assert_called_once()
-
-    @patch('quantuminspire.projectq.backend_qx.Measure')
-    def test_run_no_measurements(self, measure_mock):
+    def _run_raises_error_no_result(self, return_val):
         with patch('sys.stdout', new_callable=io.StringIO):
             self.qi_backend.qasm = "_"
             self.qi_backend.measured_ids = []
             self.qi_backend.allocation_map = [(0, 0), (1, 1)]
             self.qi_backend.main_engine = MagicMock()
-            self.qi_backend.main_engine.active_qubits = [0, 1]
             self.qi_backend.main_engine.mapper.current_mapping = {0: 0, 1: 1}
             result_mock = MagicMock()
-            result_mock.get.return_value = [{}]
+            result_mock.get.return_value = return_val
             self.api.execute_qasm.return_value = result_mock
             self.assertRaisesRegex(ProjectQBackendError, 'Result from backend contains no histogram data!',
                                    self.qi_backend.run)
         self.api.execute_qasm.assert_called_once()
+
+    def test_run_raises_error_no_result_as_ordered_dict(self):
+        self._run_raises_error_no_result([OrderedDict()])
+
+    def test_run_raises_error_no_result_as_empty_dict(self):
+        self._run_raises_error_no_result([{}])