Deprecate PulseSimulator backend (#1728)

* Deprecate PulseSimulator backend

This commit deprecates the PulseSimulator backend for qiskit aer. For
the past several releases development on the pulse simulator has mostly
been frozen as the combined efforts for improving pulse level simulation
have been working on Qiskit Dynamics instead. This commit marks the
legacy PulseSimulator as deprecated and points existing users to Qiskit
Dynamics instead.

Closes #1722

* handle deprecation of pulse simulation in tests

---------

Co-authored-by: Hiroshi Horii <horii@jp.ibm.com>
Co-authored-by: Hiroshi Horii <hhorii@users.noreply.github.com>

qiskit_aer/aerprovider.py

@@ -13,6 +13,8 @@
 # pylint: disable=invalid-name
 """Provider for Qiskit Aer backends."""
 
+import warnings
+
 from qiskit.providers import ProviderV1 as Provider
 from qiskit.providers.providerutils import filter_backends
 
@@ -59,6 +61,14 @@ def __init__(self):
             AerProvider._BACKENDS = backends
 
     def get_backend(self, name=None, **kwargs):
+        if name == "pulse_simulator":
+            warnings.warn(
+                "The Pulse simulator backend in Qiskit Aer is deprecated and will "
+                "be removed in a future release. Instead the qiskit-dynamics "
+                "library should be used instead for simulating at the pulse level.",
+                DeprecationWarning,
+                stacklevel=2,
+            )
         return super().get_backend(name=name, **kwargs)
 
     def backends(self, name=None, filters=None, **kwargs):

qiskit_aer/backends/pulse_simulator.py

@@ -54,7 +54,14 @@
 
 
 class PulseSimulator(AerBackend):
-    r"""Pulse schedule simulator backend.
+    r"""Deprecated: Pulse schedule simulator backend.
+
+    .. warning::
+
+       This simulator is deprecated having been superseded by the
+       `Qiskit Dynamics <https://qiskit.org/documentation/dynamics/>`__ library.
+       If you need to perform pulse level simulation you should use the
+       Qiskit Dynamics library instead.
 
     The ``PulseSimulator`` simulates continuous time Hamiltonian dynamics of a quantum system,
     with controls specified by pulse :class:`~qiskit.Schedule` objects, and the model of the
@@ -154,6 +161,13 @@ def __init__(self,
                  defaults=None,
                  provider=None,
                  **backend_options):
+        warn(
+            "The Pulse simulator backend in Qiskit Aer is deprecated and will "
+            "be removed in a future release. Instead the qiskit-dynamics "
+            "library should be used instead for simulating at the pulse level.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
 
         if configuration is None:
             configuration = BackendConfiguration.from_dict(

releasenotes/notes/deprecate-pulse-simulator-27cde3ece112c346.yaml

@@ -0,0 +1,10 @@
+---
+deprecations:
+  - |
+    The :class:`~.PulseSimulator` backend has been deprecated and will be
+    removed in a future release. If you're using the :class:`~.PulseSimulator`
+    backend to perform pulse level simulation, instead you should use the
+    `Qiskit Dynamics <https://qiskit.org/documentation/dynamics/>`__ library
+    instead to perform the simulation. Qiskit Dynamics provides a more
+    flexible and robust pulse level simulation framework than the
+    :class:`~.PulseSimulator` backend.

test/terra/backends/test_config_pulse_simulator.py

@@ -40,7 +40,8 @@ def test_from_backend(self):
         """Test that configuration, defaults, and properties are correclty imported."""
 
         athens_backend = FakeAthens()
-        athens_sim = PulseSimulator.from_backend(athens_backend)
+        with self.assertWarns(DeprecationWarning):
+            athens_sim = PulseSimulator.from_backend(athens_backend)
 
         self.assertEqual(athens_backend.properties(), athens_sim.properties())
         # check that configuration is correctly imported
@@ -79,7 +80,8 @@ def test_from_backend_system_model(self):
         """Test that the system model is correctly imported from the backend."""
 
         athens_backend = FakeAthens()
-        athens_sim = PulseSimulator.from_backend(athens_backend)
+        with self.assertWarns(DeprecationWarning):
+            athens_sim = PulseSimulator.from_backend(athens_backend)
 
         # u channel lo
         athens_attr = athens_backend.configuration().u_channel_lo
@@ -97,7 +99,8 @@ def test_set_system_model_options(self):
         """Test setting of options that need to be changed in multiple places."""
 
         athens_backend = FakeAthens()
-        athens_sim = PulseSimulator.from_backend(athens_backend)
+        with self.assertWarns(DeprecationWarning):
+            athens_sim = PulseSimulator.from_backend(athens_backend)
 
         # u channel lo
         set_attr = [[UchannelLO(0, 1.0 + 0.0j)]]
@@ -118,7 +121,8 @@ def test_from_backend_parametric_pulses(self):
         Results don't matter, just need to check that it runs.
         """
 
-        backend = PulseSimulator.from_backend(FakeAthens())
+        with self.assertWarns(DeprecationWarning):
+            backend = PulseSimulator.from_backend(FakeAthens())
 
         qc = QuantumCircuit(2)
         qc.x(0)
@@ -132,7 +136,8 @@ def test_parametric_pulses_error(self):
         """Verify error is raised if a parametric pulse makes it into the digest."""
 
         fake_backend = FakeAthens()
-        backend = PulseSimulator.from_backend(fake_backend)
+        with self.assertWarns(DeprecationWarning):
+            backend = PulseSimulator.from_backend(fake_backend)
 
         # reset parametric_pulses option
         backend.set_option('parametric_pulses', fake_backend.configuration().parametric_pulses)
@@ -150,7 +155,8 @@ def test_set_meas_levels(self):
         """Test setting of meas_levels."""
 
         athens_backend = FakeAthens()
-        athens_sim = PulseSimulator.from_backend(athens_backend)
+        with self.assertWarns(DeprecationWarning):
+            athens_sim = PulseSimulator.from_backend(athens_backend)
 
         # test that a warning is thrown when meas_level 0 is attempted to be set
         with warnings.catch_warnings(record=True) as w:
@@ -187,7 +193,8 @@ def test_set_system_model_from_backend(self):
             armonk_sim = PulseSimulator.from_backend(backend=armonk_backend,
                                                      system_model=system_model)
 
-            self.assertEqual(len(w), 1)
+            self.assertEqual(len(w), 2)
+            self.assertTrue('will be removed in a future release' in str(w[0].message))
             self.assertTrue('inconsistencies' in str(w[-1].message))
 
         # check that system model properties have been imported
@@ -211,7 +218,8 @@ def test_set_system_model_in_constructor(self):
 
             test_sim = PulseSimulator(system_model=system_model)
 
-            self.assertEqual(len(w), 0)
+            self.assertEqual(len(w), 1)
+            self.assertTrue('will be removed in a future release' in str(w[0].message))
 
         # check that system model properties have been imported
         self.assertEqual(test_sim.configuration().dt, system_model.dt)
@@ -226,7 +234,8 @@ def test_set_system_model_after_construction(self):
         system_model.u_channel_lo = [[UchannelLO(0, 1.0 + 0.0j)]]
 
         # first test setting after construction with no hamiltonian
-        test_sim = PulseSimulator()
+        with self.assertWarns(DeprecationWarning):
+            test_sim = PulseSimulator()
 
         with warnings.catch_warnings(record=True) as w:
             # Cause all warnings to always be triggered.
@@ -243,7 +252,8 @@ def test_set_system_model_after_construction(self):
         # next, construct a pulse simulator with a config containing a Hamiltonian and observe
         # warnings
         armonk_backend = FakeArmonk()
-        test_sim = PulseSimulator(configuration=armonk_backend.configuration())
+        with self.assertWarns(DeprecationWarning):
+            test_sim = PulseSimulator(configuration=armonk_backend.configuration())
 
         # add system model and verify warning is raised
         with warnings.catch_warnings(record=True) as w:
@@ -261,7 +271,8 @@ def test_set_system_model_after_construction(self):
     def test_validation_num_acquires(self):
         """Test that validation fails if 0 or >1 acquire is given in a schedule."""
 
-        test_sim = PulseSimulator.from_backend(FakeArmonk())
+        with self.assertWarns(DeprecationWarning):
+            test_sim = PulseSimulator.from_backend(FakeArmonk())
         test_sim.set_options(
             meas_level=2,
             qubit_lo_freq=test_sim.defaults().qubit_freq_est,
@@ -298,7 +309,8 @@ def test_run_simulation_from_backend(self):
         dt = 2.2222222222222221e-10
         armonk_backend.configuration().dt = dt
 
-        armonk_sim = PulseSimulator.from_backend(armonk_backend)
+        with self.assertWarns(DeprecationWarning):
+            armonk_sim = PulseSimulator.from_backend(armonk_backend)
         armonk_sim.set_options(
             meas_level=2,
             meas_return='single',