Define max_step separately for CavityQED (#199)

The CavityQED processor uses discrete pulses and different gates may vary a lot in terms of the gate time. This often results in a piece of pulses being neglected by the adaptive ODE solver. Ideally, the max_step option should be set as half of the shortest gate time but this information is lost during the compilation and can only be estimated from the compiled pulses. This PR allows the calculation of max_step to be defined for each type of Processor.

src/qutip_qip/device/cavityqed.py

@@ -206,6 +206,18 @@ def get_final_circuit_state(self, final_processor_state: Qobj) -> Qobj:
             range(1, len(final_processor_state.dims[0]))
         )
 
+    def _get_max_step(self):
+        """
+        Define the maximal sampling step for the solver.
+        """
+        full_tlist = self.get_full_tlist()
+        if full_tlist is not None:
+            # For this model, discrete pulses are used.
+            # Hence we use half of the gate time as the step size.
+            return np.min(np.diff(full_tlist)) / 2
+        else:
+            return 0.0
+
 
 class CavityQEDModel(Model):
     """

src/qutip_qip/device/processor.py

@@ -1200,20 +1200,15 @@ def run_state(
         # A better solution is to use the gate, which
         # is however, much harder to implement at this stage, see also
         # https://github.com/qutip/qutip-qip/issues/184.
-        full_tlist = self.get_full_tlist()
-        if full_tlist is not None:
-            total_circuit_time = (full_tlist)[-1]
-        else:
-            total_circuit_time = 0.0
         if is_qutip5:
             options = kwargs.get("options", qutip.Options())
             if options.get("max_step", 0.0) == 0.0:
-                options["max_step"] = total_circuit_time / 25
+                options["max_step"] = self._get_max_step()
             options["progress_bar"] = False
         else:
             options = kwargs.get("options", qutip.Options())
             if options.max_step == 0.0:
-                options.max_step = total_circuit_time / 10
+                options.max_step = self._get_max_step()
             options.progress_bar = False
         kwargs["options"] = options
         # choose solver:
@@ -1228,6 +1223,17 @@ def run_state(
 
         return evo_result
 
+    def _get_max_step(self):
+        """
+        Define the maximal sampling step for the solver.
+        """
+        full_tlist = self.get_full_tlist()
+        if full_tlist is not None:
+            total_circuit_time = (full_tlist)[-1]
+        else:
+            total_circuit_time = 0.0
+        return total_circuit_time / 10
+
     def load_circuit(self, qc):
         """
         Translate an :class:`.QubitCircuit` to its