Fix bug with carrier_freq being a JAX tracer if envelope is constant …

…in Signal (#247)

qiskit_dynamics/signals/signals.py

@@ -96,8 +96,12 @@ def __init__(
 
         if isinstance(envelope, Array):
             # if envelope is constant and the carrier is zero, this is a constant signal
-            if carrier_freq == 0.0:
-                self._is_constant = True
+            try:
+                # try block is for catching JAX tracer errors
+                if carrier_freq == 0.0:
+                    self._is_constant = True
+            except Exception:  # pylint: disable=broad-except
+                pass
 
             if envelope.backend == "jax":
                 self._envelope = lambda t: envelope * jnp.ones_like(Array(t).data)

releasenotes/notes/carrier-freq-0-19ad4362c874944f.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    In the case that ``envelope`` is a constant, the :meth:`Signal.__init__` method has been updated
+    to not attempt to evaluate ``carrier_freq == 0.0`` if ``carrier_freq`` is a JAX tracer. In this
+    case, it is not possible to determine if the :class:`Signal` instance is constant. This resolves
+    an error that was being raised during JAX tracing if ``carrier_freq`` is abstract.

test/dynamics/signals/test_signals.py

@@ -933,6 +933,35 @@ def eval_const(a):
         jit_grad_eval = jit(grad(eval_const))
         self.assertAllClose(jit_grad_eval(3.0), 1.0)
 
+        # validate that is_constant is being properly set
+        def eval_const_conditional(a):
+            a = Array(a)
+            sig = Signal(a)
+
+            if sig.is_constant:
+                return 5.0
+            else:
+                return 3.0
+
+        jit_eval = jit(eval_const_conditional)
+        self.assertAllClose(jit_eval(1.0), 5.0)
+
+    def test_jit_grad_carrier_freq_construct(self):
+        """Test jit/gradding through a function that constructs a signal and takes carrier frequency
+        as an argument.
+        """
+
+        def eval_sig(a, v, t):
+            a = Array(a)
+            v = Array(v)
+            return Array(Signal(a, v)(t)).data
+
+        jit_eval = jit(eval_sig)
+        self.assertAllClose(jit_eval(1.0, 1.0, 1.0), 1.0)
+
+        jit_grad_eval = jit(grad(eval_sig))
+        self.assertAllClose(jit_grad_eval(1.0, 1.0, 1.0), 1.0)
+
     def test_signal_list_jit_eval(self):
         """Test jit-compilation of SignalList evaluation."""
         call_jit = jit(lambda t: Array(self.signal_list(t)).data)