CR Hamiltonian experiment without cr_gate (#794)

* Bug fix; instantiate CR Ham expr without cr_gate

* fix another bug with extra unittest

qiskit_experiments/library/characterization/cr_hamiltonian.py

@@ -157,22 +157,21 @@ def __init__(
         Raises:
             QiskitError: When ``qubits`` length is not 2.
         """
-        super().__init__(qubits, analysis=CrossResonanceHamiltonianAnalysis(), backend=backend)
+        # backend parameters required to run this experiment
+        # random values are populated here but these are immediately updated after backend is set
+        # this is to keep capability of generating circuits just for checking
+        self._dt = 1
+        self._cr_channel = 0
+        self._granularity = 1
+        self._cr_gate = cr_gate
 
         if len(qubits) != 2:
             raise QiskitError(
                 "Length of qubits is not 2. Please provide index for control and target qubit."
             )
 
+        super().__init__(qubits, analysis=CrossResonanceHamiltonianAnalysis(), backend=backend)
         self.set_experiment_options(flat_top_widths=flat_top_widths, **kwargs)
-        self._cr_gate = cr_gate
-
-        # backend parameters required to run this experiment
-        # random values are populated here but these are immediately updated after backend is set
-        # this is to keep capability of generating circuits just for checking
-        self._dt = 1
-        self._cr_channel = 0
-        self._granularity = 1
 
     @classmethod
     def _default_experiment_options(cls) -> Options:

releasenotes/notes/fix-instantiate-crhamtomo-without-crgate-1b5bdd957b9fad69.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Instantiating the :class:`.CrossResonanceHamiltonian` experiment without ``cr_gate``
+    and with ``backend`` has been raising AttributeError. The bug has been fixed.

test/test_cross_resonance_hamiltonian.py

@@ -135,6 +135,28 @@ def test_circuit_generation_no_backend(self):
         # Not raise an error
         expr.circuits()
 
+    def test_instance_with_backend_without_cr_gate(self):
+        """Calling set backend method without setting cr gate."""
+        backend = FakeBogota()
+        setattr(
+            backend.configuration(),
+            "timing_constraints",
+            {"granularity": 16},
+        )
+
+        # not raise an error
+        exp = cr_hamiltonian.CrossResonanceHamiltonian(
+            qubits=(0, 1),
+            flat_top_widths=[1000],
+            backend=backend,
+        )
+        ref_config = backend.configuration()
+        self.assertEqual(exp._dt, ref_config.dt)
+
+        # These properties are set when cr_gate is not provided
+        self.assertEqual(exp._cr_channel, ref_config.control((0, 1))[0].index)
+        self.assertEqual(exp._granularity, ref_config.timing_constraints["granularity"])
+
     @data(
         [1e6, 2e6, 1e3, -3e6, -2e6, 1e4],
         [-1e6, -2e6, 1e3, 3e6, 2e6, 1e4],