fix: Explicit target for identity gate (#38)

Qubit count validation was failing due to identity gate target not
being counted.

Author: speller26

src/braket/default_simulator/gate_operations.py

@@ -49,18 +49,21 @@ def from_braket_instruction(instruction) -> GateOperation:
 class Identity(GateOperation):
     """Identity gate"""
 
+    def __init__(self, targets):
+        self._targets = tuple(targets)
+
     @property
     def matrix(self) -> np.ndarray:
         return np.eye(2)
 
     @property
     def targets(self) -> Tuple[int, ...]:
-        return ()
+        return self._targets
 
 
 @from_braket_instruction.register(braket_instruction.I)
 def _i(instruction) -> Identity:
-    return Identity()
+    return Identity([instruction.target])
 
 
 class Hadamard(GateOperation):

src/braket/default_simulator/operation.py

@@ -29,9 +29,10 @@ class Operation(ABC):
     def targets(self) -> Tuple[int, ...]:
         """ Tuple[int, ...]: The indices of the qubits the operation applies to.
 
-        For an index to be a target, the operation must have a nontrivial (i.e. non-identity) action
-        on that index. For example, a tensor product observable with a Z factor on qubit j acts
-        trivially on j, so j would not be a target.
+        Note: For an index to be a target of an observable, the observable must have a nontrivial
+        (i.e. non-identity) action on that index. For example, a tensor product observable with a
+        Z factor on qubit j acts trivially on j, so j would not be a target. This does not apply to
+        gate operations.
         """
 
 

test/unit_tests/braket/default_simulator/test_gate_operations.py

@@ -19,7 +19,7 @@
 from braket.ir.jaqcd import shared_models
 
 testdata = [
-    (instruction.I(target=4), (), gate_operations.Identity),
+    (instruction.I(target=4), (4,), gate_operations.Identity),
     (instruction.H(target=13), (13,), gate_operations.Hadamard),
     (instruction.X(target=11), (11,), gate_operations.PauliX),
     (instruction.Y(target=10), (10,), gate_operations.PauliY),

test/unit_tests/braket/default_simulator/test_operation_helpers.py

@@ -42,7 +42,7 @@
 invalid_hermitian_matrices = [(np.array([[1, 0], [0, 1j]])), (np.array([[1, 2], [3, 4]]))]
 
 gate_testdata = [
-    gate_operations.Identity(),
+    gate_operations.Identity([0]),
     gate_operations.Hadamard([0]),
     gate_operations.PauliX([0]),
     gate_operations.PauliY([0]),

test/unit_tests/braket/default_simulator/test_simulation.py

@@ -51,7 +51,7 @@
     ),
     ([gate_operations.V([0])], 1, [0.5 + 0.5j, 0.5 - 0.5j], [0.5, 0.5],),
     ([gate_operations.Vi([0])], 1, [0.5 - 0.5j, 0.5 + 0.5j], [0.5, 0.5],),
-    ([gate_operations.Identity()], 1, [1, 0], [1, 0]),
+    ([gate_operations.Identity([0])], 1, [1, 0], [1, 0]),
     ([gate_operations.Unitary([0], [[0, 1], [1, 0]])], 1, [0, 1], [0, 1]),
     (
         [gate_operations.PauliX([0]), gate_operations.PhaseShift([0], 0.15)],

test/unit_tests/braket/default_simulator/test_simulator.py

@@ -101,6 +101,21 @@ def test_simulator_run_bell_pair(bell_ir, batch_size):
     assert result.additionalMetadata == AdditionalMetadata(action=bell_ir)
 
 
+def test_simulator_identity():
+    simulator = DefaultSimulator()
+    shots_count = 1000
+    result = simulator.run(
+        Program.parse_raw(
+            json.dumps({"instructions": [{"type": "i", "target": 0}, {"type": "i", "target": 1}]})
+        ),
+        qubit_count=2,
+        shots=shots_count,
+    )
+    counter = Counter(["".join(measurement) for measurement in result.measurements])
+    assert counter.keys() == {"00"}
+    assert counter["00"] == shots_count
+
+
 @pytest.mark.xfail(raises=ValueError)
 def test_simulator_run_no_results_no_shots(bell_ir):
     simulator = DefaultSimulator()