Fix global phase handling in circuit.repeat (#5588) (#5592)

* global phase should not be propagated in repeat

* add reno

(cherry picked from commit 7e40ed1)

Co-authored-by: Julien Gacon <gaconju@gmail.com>

qiskit/circuit/quantumcircuit.py

@@ -413,8 +413,7 @@ def repeat(self, reps):
             QuantumCircuit: A circuit containing ``reps`` repetitions of this circuit.
         """
         repeated_circ = QuantumCircuit(*self.qregs, *self.cregs,
-                                       name=self.name + '**{}'.format(reps),
-                                       global_phase=reps * self.global_phase)
+                                       name=self.name + '**{}'.format(reps))
 
         # benefit of appending instructions: decomposing shows the subparts, i.e. the power
         # is actually `reps` times this circuit, and it is currently much faster than `compose`.

releasenotes/notes/fix-global-phase-upon-repeat-f742f4c0201fea97.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fix the global phase of the output of ``circuit.repeat``. If a circuit with global
+    phase is appended to another circuit, the global phase is currently not propagated.
+    The simulators rely on this, since the phase otherwise gets applied multiple times.
+    This sets the global phase of ``circuit.repeat`` to 0 instead of multiplying the existing
+    phase times the number of repetitions.

test/python/circuit/test_circuit_operations.py

@@ -14,13 +14,15 @@
 """Test Qiskit's QuantumCircuit class."""
 
 from ddt import ddt, data
+import numpy as np
 from qiskit import BasicAer
 from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
 from qiskit import execute
 from qiskit.circuit import Gate, Instruction, Parameter
 from qiskit.circuit.exceptions import CircuitError
 from qiskit.test import QiskitTestCase
 from qiskit.circuit.library.standard_gates import SGate
+from qiskit.quantum_info import Operator
 
 
 @ddt
@@ -403,6 +405,14 @@ def test_repeat(self):
             rep = qc.repeat(3)
             self.assertEqual(rep, ref)
 
+    @data(0, 1, 4)
+    def test_repeat_global_phase(self, num):
+        """Test the global phase is properly handled upon repeat."""
+        phase = 0.123
+        qc = QuantumCircuit(1, global_phase=phase)
+        expected = np.exp(1j * phase * num) * np.identity(2)
+        np.testing.assert_array_almost_equal(Operator(qc.repeat(num)).data, expected)
+
     def test_power(self):
         """Test taking the circuit to a power works."""
         qc = QuantumCircuit(2)