Fix global phase update in BasisTranslator Pass (Qiskit#14078)

* add missing branch for global phase update in basis_translator

* Adding test

* reno

Author: alexanderivrii

crates/accelerate/src/basis/basis_translator/mod.rs

@@ -762,38 +762,46 @@ fn replace_node(
             )?;
         }
 
-        if let Param::ParameterExpression(old_phase) = target_dag.global_phase() {
-            let bound_old_phase = old_phase.bind(py);
-            let bind_dict = PyDict::new(py);
-            for key in target_dag.global_phase().iter_parameters(py)? {
-                let key = key?;
-                bind_dict.set_item(&key, parameter_map.get_item(&key)?)?;
-            }
-            let mut new_phase: Bound<PyAny>;
-            if bind_dict.values().iter().any(|param| {
-                param
-                    .is_instance(PARAMETER_EXPRESSION.get_bound(py))
-                    .is_ok_and(|x| x)
-            }) {
-                new_phase = bound_old_phase.clone();
-                for key_val in bind_dict.items() {
-                    new_phase =
-                        new_phase.call_method1(intern!(py, "assign"), key_val.downcast()?)?;
+        match target_dag.global_phase() {
+            Param::ParameterExpression(old_phase) => {
+                let bound_old_phase = old_phase.bind(py);
+                let bind_dict = PyDict::new(py);
+                for key in target_dag.global_phase().iter_parameters(py)? {
+                    let key = key?;
+                    bind_dict.set_item(&key, parameter_map.get_item(&key)?)?;
                 }
-            } else {
-                new_phase = bound_old_phase.call_method1(intern!(py, "bind"), (bind_dict,))?;
-            }
-            if !new_phase.getattr(intern!(py, "parameters"))?.is_truthy()? {
-                new_phase = new_phase.call_method0(intern!(py, "numeric"))?;
-                if new_phase.is_instance(&PyComplex::type_object(py))? {
-                    return Err(TranspilerError::new_err(format!(
-                        "Global phase must be real, but got {}",
-                        new_phase.repr()?
-                    )));
+                let mut new_phase: Bound<PyAny>;
+                if bind_dict.values().iter().any(|param| {
+                    param
+                        .is_instance(PARAMETER_EXPRESSION.get_bound(py))
+                        .is_ok_and(|x| x)
+                }) {
+                    new_phase = bound_old_phase.clone();
+                    for key_val in bind_dict.items() {
+                        new_phase =
+                            new_phase.call_method1(intern!(py, "assign"), key_val.downcast()?)?;
+                    }
+                } else {
+                    new_phase = bound_old_phase.call_method1(intern!(py, "bind"), (bind_dict,))?;
                 }
+                if !new_phase.getattr(intern!(py, "parameters"))?.is_truthy()? {
+                    new_phase = new_phase.call_method0(intern!(py, "numeric"))?;
+                    if new_phase.is_instance(&PyComplex::type_object(py))? {
+                        return Err(TranspilerError::new_err(format!(
+                            "Global phase must be real, but got {}",
+                            new_phase.repr()?
+                        )));
+                    }
+                }
+                let new_phase: Param = new_phase.extract()?;
+                dag.add_global_phase(&new_phase)?;
             }
-            let new_phase: Param = new_phase.extract()?;
-            dag.add_global_phase(&new_phase)?;
+
+            Param::Float(_) => {
+                dag.add_global_phase(target_dag.global_phase())?;
+            }
+
+            _ => {}
         }
     }
 

releasenotes/notes/fix-global-phase-in-basis-translator-d08d665198589828.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a problem in :class:`.BasisTranslator` transpiler pass, where the global
+    phase of the DAG was not updated correctly. 
+    Fixed `#14074 <https://github.com/Qiskit/qiskit/issues/14074>`__.

test/python/transpiler/test_basis_translator.py

@@ -1008,6 +1008,15 @@ def test_global_phase(self):
         )
         self.assertEqual(Operator(dag_to_circuit(out_dag)), Operator(expected))
 
+    def test_rx_to_rz(self):
+        """Verify global phase is updated correctly in basis translation.
+        See https://github.com/Qiskit/qiskit/issues/14074."""
+        theta = 0.5 * pi
+        circ = QuantumCircuit(1)
+        circ.rx(theta, 0)
+        out_circ = BasisTranslator(std_eqlib, ["h", "rz"])(circ)
+        self.assertEqual(Operator(circ), Operator(out_circ))
+
     def test_skip_target_basis_equivalences_1(self):
         """Test that BasisTranslator skips gates in the target_basis - #6085"""
         circ = QuantumCircuit()