Add automatic determination of t_span for DynamicsBackend.solve (#353)

Co-authored-by: Daniel Puzzuoli <dan.puzzuoli@gmail.com>

Author: arthurostrauss

qiskit_dynamics/backend/dynamics_backend.py

@@ -345,7 +345,7 @@ def _set_solver(self, solver):
     def solve(
         self,
         solve_input: List[Union[QuantumCircuit, Schedule, ScheduleBlock]],
-        t_span: ArrayLike,
+        t_span: Optional[ArrayLike] = None,
         y0: Optional[Union[ArrayLike, QuantumState, BaseOperator]] = None,
         convert_results: Optional[bool] = True,
         validate: Optional[bool] = True,
@@ -361,7 +361,8 @@ def solve(
         ``y0`` is not specified, it will be set from ``self.options.initial_state``.
 
         Args:
-            t_span: Time interval to integrate over.
+            t_span: Time interval to integrate over. Defaults to ``None``, in which case the
+                    interval is set to ``[[0, input.duration] for input in solve_input]]``.
             y0: Initial state.
             solve_input: Time evolution of the system in terms of quantum circuits or qiskit
                          pulse schedules.
@@ -382,7 +383,8 @@ def solve(
             y0 = self.options.initial_state
         if isinstance(y0, str) and y0 == "ground_state":
             y0 = Statevector(self._dressed_states[:, 0])
-
+        if t_span is None:
+            t_span = [[0, sched.duration * self.dt] for sched in schedules]
         solver_results = self.options.solver.solve(
             t_span=t_span,
             y0=y0,

releasenotes/notes/default_tspan_backend_solve-b1ad23f6ea291474.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    ``DynamicsBackend.solve()`` method can now work without specifying a ``t_span`` argument. 
+    The default ``t_span`` is set to be ``[0, solve_input.duration]`` for each provided ``solve_input``.
+    This allows users to solve the dynamics of a quantum circuit without having to specify its
+    duration in advance.

test/dynamics/backend/test_dynamics_backend.py

@@ -347,9 +347,11 @@ def test_solve(self):
         input_variety = [x_sched0, x_circ0]
 
         # solve for all combinations of input types and initial states
-        for solve_input, (y0, expected_result) in product(input_variety, y0_and_expected_results):
+        for solve_input, (y0, expected_result), t_span in product(
+            input_variety, y0_and_expected_results, ([0, n_samples * backend.dt], None)
+        ):
             solver_results = backend.solve(
-                t_span=[0, n_samples * backend.dt],
+                t_span=t_span,
                 y0=y0,
                 solve_input=[solve_input],
             )
@@ -358,6 +360,7 @@ def test_solve(self):
             for solver_result in solver_results:
                 self.assertTrue(solver_result.success)
                 self.assertAllClose(solver_result.y[-1], expected_result, atol=1e-8, rtol=1e-8)
+                self.assertEqual(solver_result.t[-1], n_samples * backend.dt)
 
     def test_pi_pulse_initial_state(self):
         """Test simulation of a pi pulse with a different initial state."""