Provide a workaround to #422, Sampler failing when no measurements (#426

)

* Provide a workaround to #422, Sampler failing when no measurements

* Add test for workaround to Issue #422.

* Redo previous commit after running tox.

* Fix tox error from previous build.

* Fix tox error from last build.

* Un-modify the metadata in a notebook

With this, the current PR will touch one fewer file

* Update test/cutting/test_cutting_roundtrip.py

Co-authored-by: Jim Garrison <garrison@ibm.com>

* Update test/cutting/test_cutting_roundtrip.py

Co-authored-by: Jim Garrison <garrison@ibm.com>

* black

---------

Co-authored-by: Ibrahim <ibrahim.shehzad@ibm.com>
Co-authored-by: Ibrahim Shehzad <75153717+Ibrahim-Shehzad@users.noreply.github.com>
(cherry picked from commit f596307)

circuit_knitting/cutting/cutting_experiments.py

@@ -276,14 +276,21 @@ def _append_measurement_circuit(
     if not inplace:
         qc = qc.copy()
 
+    # If the circuit has no measurements, the Sampler will fail.  So, we
+    # measure one qubit as a temporary workaround to
+    # https://github.com/Qiskit-Extensions/circuit-knitting-toolbox/issues/422
+    pauli_indices = cog.pauli_indices
+    if not pauli_indices:
+        pauli_indices = [0]
+
     # Append the appropriate measurements to qc
-    obs_creg = ClassicalRegister(len(cog.pauli_indices), name="observable_measurements")
+    obs_creg = ClassicalRegister(len(pauli_indices), name="observable_measurements")
     qc.add_register(obs_creg)
     # Implement the necessary basis rotations and measurements, as
     # in BackendEstimator._measurement_circuit().
     genobs_x = cog.general_observable.x
     genobs_z = cog.general_observable.z
-    for clbit, subqubit in enumerate(cog.pauli_indices):
+    for clbit, subqubit in enumerate(pauli_indices):
         # subqubit is the index of the qubit in the subsystem.
         # actual_qubit is its index in the system of interest (if different).
         actual_qubit = qubit_locations[subqubit]

test/cutting/test_cutting_roundtrip.py

@@ -42,6 +42,7 @@
 from qiskit.extensions import UnitaryGate
 from qiskit.quantum_info import PauliList, random_unitary
 from qiskit.primitives import Estimator
+from qiskit_aer.primitives import Sampler
 
 from circuit_knitting.utils.simulation import ExactSampler
 from circuit_knitting.cutting import (
@@ -179,3 +180,30 @@ def test_cutting_exact_reconstruction(example_circuit):
     logger.info("Max error: %f", np.max(np.abs(exact_expvals - simulated_expvals)))
 
     assert np.allclose(exact_expvals, simulated_expvals, atol=1e-8)
+
+
+def test_sampler_with_identity_subobservable(example_circuit):
+    """This test ensures that the sampler does not throw an error if you pass it a subcircuit with no observable measurements.
+
+    Tests temporary workaround to Issue #422.
+
+    This test passes if no exceptions are raised.
+
+    """
+
+    qc = example_circuit
+    observable_to_test = PauliList(
+        ["IIZ"]
+    )  # Without the workaround to Issue #422, this observable causes a Sampler error.
+    subcircuits, bases, subobservables = partition_problem(
+        qc, "AAB", observables=observable_to_test
+    )
+    subexperiments, coefficients = generate_cutting_experiments(
+        subcircuits, subobservables, num_samples=np.inf
+    )
+    samplers = {label: Sampler() for label in subexperiments.keys()}
+    results = {
+        label: sampler.run(subexperiments[label]).result()
+        for label, sampler in samplers.items()
+    }
+    _ = results