Checks if DiagonalGate has modulus 1 (Qiskit#8098)

* Checks if DiagonalGate has modulus 1 - Fixes Qiskit#3842

Co-authored-by: ewinston <ewinston@users.noreply.github.com>
Co-authored-by: hhyap <hhyap@users.noreply.github.com>

* reno

* Fix release-note typo

Co-authored-by: ewinston <ewinston@users.noreply.github.com>
Co-authored-by: hhyap <hhyap@users.noreply.github.com>
Co-authored-by: Jake Lishman <jake@binhbar.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/extensions/quantum_initializer/diagonal.py

@@ -55,7 +55,7 @@ def __init__(self, diag):
                 raise QiskitError(
                     "Not all of the diagonal entries can be converted to complex numbers."
                 ) from ex
-            if not np.abs(z) - 1 < _EPS:
+            if not np.abs(np.abs(z) - 1) < _EPS:
                 raise QiskitError("A diagonal entry has not absolute value one.")
         # Create new gate.
         super().__init__("diagonal", int(num_action_qubits), diag)

releasenotes/notes/3842-14af3f8d922a7253.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Extra validation was added to :class:`.DiagonalGate` to check the argument has modulus one.

test/python/circuit/test_diagonal_gate.py

@@ -18,8 +18,10 @@
 
 from qiskit import QuantumCircuit, QuantumRegister, BasicAer, execute
 
+from qiskit import QiskitError
 from qiskit.test import QiskitTestCase
 from qiskit.compiler import transpile
+from qiskit.extensions.quantum_initializer import DiagonalGate
 from qiskit.quantum_info.operators.predicates import matrix_equal
 
 
@@ -54,6 +56,13 @@ def test_diag_gate(self):
                 unitary_desired = _get_diag_gate_matrix(diag)
                 self.assertTrue(matrix_equal(unitary, unitary_desired, ignore_phase=False))
 
+    def test_mod1_entries(self):
+        """Test that diagonal raises if entries do not have modules of 1."""
+        from qiskit.quantum_info.operators.predicates import ATOL_DEFAULT, RTOL_DEFAULT
+
+        with self.assertRaises(QiskitError):
+            DiagonalGate([1, 1 - 2 * ATOL_DEFAULT - RTOL_DEFAULT])
+
 
 def _get_diag_gate_matrix(diag):
     return np.diagflat(diag)