remove unused qubits from layout when split-transpilation is enable i…

…n VQE
Qiskit · Jun 11, 2021 · bab18b5 · bab18b5
1 parent 8948a8f
commit bab18b5
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Showing 2 changed files with 18 additions and 6 deletions.
diff --git a/qiskit/opflow/converters/circuit_sampler.py b/qiskit/opflow/converters/circuit_sampler.py
@@ -313,18 +313,30 @@ def sample_circuits(
                     common_circuit = operator[1].to_circuit_op().reduce().to_circuit()
                     # 1. transpile a common circuit
                     transpiled_common_circuit = self.quantum_instance.transpile(common_circuit)[0]
-                    layout = transpiled_common_circuit._layout
 
                     # 2. transpile diff circuits
                     diff_circuits = []
                     for circuit in circuits:
                         diff_circuit = circuit.copy()
                         del diff_circuit.data[0 : len(common_circuit)]
                         diff_circuits.append(diff_circuit)
-                    orig_layout = self.quantum_instance.compile_config["initial_layout"]
-                    self.quantum_instance.compile_config["initial_layout"] = layout
+
+                    if transpiled_common_circuit._layout is not None:
+                        layout = transpiled_common_circuit._layout.copy()
+                        used_qubits = set([])
+                        for diff_circuit in diff_circuits:
+                            for used_qubit in diff_circuit.qubits:
+                                used_qubits.add(used_qubit)
+                        for q in list(layout.get_virtual_bits().keys()):
+                            if q not in used_qubits:
+                                del layout[q]
+                        orig_layout = self.quantum_instance.compile_config["initial_layout"]
+                        self.quantum_instance.compile_config["initial_layout"] = layout
+
                     diff_circuits = self.quantum_instance.transpile(diff_circuits)
-                    self.quantum_instance.compile_config["initial_layout"] = orig_layout
+
+                    if transpiled_common_circuit._layout is not None:
+                        self.quantum_instance.compile_config["initial_layout"] = orig_layout
 
                     # 3. combine
                     transpiled_circuits = []

diff --git a/qiskit/transpiler/layout.py b/qiskit/transpiler/layout.py
@@ -123,11 +123,11 @@ def _set_type_checked_item(self, virtual, physical):
 
     def __delitem__(self, key):
         if isinstance(key, int):
-            del self._p2v[key]
             del self._v2p[self._p2v[key]]
+            del self._p2v[key]
         elif isinstance(key, Qubit):
-            del self._v2p[key]
             del self._p2v[self._v2p[key]]
+            del self._v2p[key]
         else:
             raise LayoutError(
                 "The key to remove should be of the form"