Example added BasicSim docs that shows default shots

CHANGELOG.md

@@ -10,8 +10,16 @@
 
 ### Documentation 📝
 
+* The documentation for `qiskit.basicsim` and `qiskit.aer` device have been updated with a mention that 
+  they must be used in finite-shot mode.
+  [(#549)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/549)
+
 ### Bug fixes 🐛
 
+* The `qiskit.basicsim` and `qiskit.aer` device (with the `aer_simulator` backend) now raise a warning 
+  when `shots=None` and change the number of shots to `shots=1024`.
+  [(#549)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/549)
+
 ### Contributors ✍️
 
 This release contains contributions from (in alphabetical order):

doc/devices/aer.rst

@@ -13,6 +13,9 @@ You can instantiate a ``'qiskit.aer'`` device for PennyLane with:
     import pennylane as qml
     dev = qml.device('qiskit.aer', wires=2)
 
+This device must be used in finite-shots mode with the ``aer_simulator`` backend. If ``shots=None`` in this setting, 
+it will be overwritten to ``shots=1024``.
+
 This device can then be used just like other devices for the definition and evaluation of QNodes within PennyLane.
 A simple quantum function that returns the expectation value of a measurement and depends on three classical input
 parameters would look like:

doc/devices/basicsim.rst

@@ -14,11 +14,12 @@ that can be accessed in this plugin through:
 
 This device uses the Qiskit ``BasicSimulator`` backend from the
 `basic_provider <https://docs.quantum.ibm.com/api/qiskit/providers_basic_provider>`_ module in Qiskit.
+This device must be used in finite-shots mode. If ``shots=None``, it will be overwritten to ``shots=1024``.
 
 .. note::
 
     The `Qiskit Aer <https://qiskit.github.io/qiskit-aer/>`_ device
     provides a fast simulator that is also capable of simulating
     noise. It is available as :ref:`"qiskit.aer" <aer device page>`, but the backend must be
     installed separately with ``pip install qiskit-aer``.
-
+

pennylane_qiskit/basic_aer.py

@@ -55,7 +55,7 @@ class BasicAerDevice(QiskitDevice):
 
     short_name = "qiskit.basicaer"
 
-    def __init__(self, wires, shots=1024, backend="qasm_simulator", **kwargs):
+    def __init__(self, wires, shots=None, backend="qasm_simulator", **kwargs):
 
         max_ver = Version("0.46", partial=True)
 
@@ -93,10 +93,10 @@ class BasicSimulatorDevice(QiskitDevice):
     analytic_warning_message = (
         "The plugin does not currently support analytic calculation of expectations, variances "
         "and probabilities with the BasicProvider backend {}. Such statistics obtained from this "
-        "device are estimates based on samples."
+        "device are estimates based on samples. The simulation will be run with shots=1024."
     )
 
-    def __init__(self, wires, shots=1024, backend="basic_simulator", **kwargs):
+    def __init__(self, wires, shots=None, backend="basic_simulator", **kwargs):
 
         min_version = Version("1.0.0")
 

pennylane_qiskit/qiskit_device.py

@@ -144,6 +144,7 @@ class QiskitDevice(QubitDevice, abc.ABC):
         "The analytic calculation of expectations, variances and "
         "probabilities is only supported on statevector backends, not on the {}. "
         "Such statistics obtained from this device are estimates based on samples."
+        "The simulation will be run with shots=1024."
     )
 
     _eigs = {}

tests/test_qiskit_device.py

@@ -108,7 +108,9 @@ class TestSupportForV1andV2:
     )
     def test_v1_and_v2_mocked(self, dev_backend):
         """Test that device initializes with no error mocked"""
-        dev = qml.device("qiskit.remote", wires=10, backend=dev_backend, use_primitives=True)
+        dev = qml.device(
+            "qiskit.remote", wires=10, backend=dev_backend, use_primitives=True
+        )
         assert dev._backend == dev_backend
 
     @pytest.mark.parametrize(
@@ -120,7 +122,9 @@ def test_v1_and_v2_mocked(self, dev_backend):
     )
     def test_v1_and_v2_manila(self, dev_backend):
         """Test that device initializes with no error with V1 and V2 backends by Qiskit"""
-        dev = qml.device("qiskit.remote", wires=5, backend=dev_backend, use_primitives=True)
+        dev = qml.device(
+            "qiskit.remote", wires=5, backend=dev_backend, use_primitives=True
+        )
 
         @qml.qnode(dev)
         def circuit(x):
@@ -156,7 +160,9 @@ def test_device_with_transpilation_options(self, device, wires, device_options):
         assert dev.transpile_args == device_options
 
     @pytest.mark.parametrize("transpile_options", test_transpile_options)
-    def test_transpilation_option_update(self, device, wires, device_options, transpile_options):
+    def test_transpilation_option_update(
+        self, device, wires, device_options, transpile_options
+    ):
         """Test that the transpilation options are updated as expected."""
         dev = device(wires, device_options)
         assert dev.transpile_args == device_options
@@ -167,22 +173,46 @@ def test_transpilation_option_update(self, device, wires, device_options, transp
 class TestAnalyticWarningHWSimulator:
     """Tests the warnings for when the analytic attribute of a device is set to true"""
 
-    def test_warning_raised_for_hardware_backend_analytic_expval(self, recorder):
+    testcases_warning_raised_for_hardware_backend_analytic_expval = [
+        (
+            "qiskit.aer",
+            "aer_simulator",
+            (
+                "The analytic calculation of expectations, variances and "
+                "probabilities is only supported on statevector backends, not on the aer_simulator. "
+                "Such statistics obtained from this device are estimates based on samples."
+                "The simulation will be run with shots=1024."
+            ),
+        ),
+        (
+            "qiskit.basicsim",
+            "basic_simulator",
+            (
+                "The plugin does not currently support analytic calculation of expectations, variances "
+                "and probabilities with the BasicProvider backend basic_simulator. Such statistics obtained from this "
+                "device are estimates based on samples. The simulation will be run with shots=1024."
+            ),
+        ),
+    ]
+
+    @pytest.mark.parametrize(
+        "device_backend_msg",
+        testcases_warning_raised_for_hardware_backend_analytic_expval,
+    )
+    def test_warning_raised_for_hardware_backend_analytic_expval(
+        self, device_backend_msg, recorder
+    ):
         """Tests that a warning is raised if the analytic attribute is true on
         hardware simulators when calculating the expectation"""
 
         with pytest.warns(UserWarning) as record:
-            dev = qml.device("qiskit.aer", backend="aer_simulator", wires=2, shots=None)
+            shortname, backend, msg = device_backend_msg
+            dev = qml.device(shortname, backend=backend, wires=2, shots=None)
 
         # check that only one warning was raised
         assert len(record) == 1
         # check that the message matches
-        assert (
-            record[0].message.args[0] == "The analytic calculation of "
-            "expectations, variances and probabilities is only supported on "
-            f"statevector backends, not on the {dev.backend.name}. Such statistics obtained from this "
-            "device are estimates based on samples."
-        )
+        assert record[0].message.args[0] == msg
 
     @pytest.mark.parametrize("method", ["unitary", "statevector"])
     def test_no_warning_raised_for_software_backend_analytic_expval(
@@ -191,7 +221,9 @@ def test_no_warning_raised_for_software_backend_analytic_expval(
         """Tests that no warning is raised if the analytic attribute is true on
         statevector simulators when calculating the expectation"""
 
-        _ = qml.device("qiskit.aer", backend="aer_simulator", method=method, wires=2, shots=None)
+        _ = qml.device(
+            "qiskit.aer", backend="aer_simulator", method=method, wires=2, shots=None
+        )
 
         # check that no warnings were raised
         assert len(recwarn) == 0