Fix BasisEmbedding to allow only-1 bitstrings

.github/CHANGELOG.md

@@ -215,6 +215,10 @@
 
 <h3>Bug fixes</h3>
 
+* Fixes a bug where `BasisEmbedding` would not accept inputs whose bits are all ones 
+  or all zeros. 
+  [(#1114)](https://github.com/PennyLaneAI/pennylane/pull/1114)
+
 * The `ExpvalCost` class raises an error if instantiated
   with non-expectation measurement statistics.
   [(#1106)](https://github.com/PennyLaneAI/pennylane/pull/1106)

pennylane/templates/embeddings/basis.py

@@ -51,7 +51,7 @@ def _preprocess(features, wires):
 
         features = list(qml.math.toarray(features))
 
-        if set(features) != {0, 1}:
+        if not set(features).issubset({0, 1}):
             raise ValueError(f"Basis state must only consist of 0s and 1s; got {features}")
 
         return features

tests/templates/test_embeddings.py

@@ -332,10 +332,13 @@ def circuit(x=None):
 class TestBasisEmbedding:
     """ Tests the BasisEmbedding method."""
 
-    def test_state(self):
-        """Checks the state."""
+    @pytest.mark.parametrize("state", [[0, 1],
+                                       [1, 1],
+                                       [1, 0],
+                                       [0, 0]])
+    def test_state(self, state):
+        """Checks that the correct state is prepared."""
 
-        state = np.array([0, 1])
         n_qubits = 2
         dev = qml.device('default.qubit', wires=n_qubits)
 
@@ -345,7 +348,8 @@ def circuit(x=None):
             return [qml.expval(qml.PauliZ(i)) for i in range(n_qubits)]
 
         res = circuit(x=state)
-        assert np.allclose(res, [1, -1])
+        expected = [1 if s == 0 else -1 for s in state]
+        assert np.allclose(res, expected)
 
     def test_too_many_input_bits_exception(self):
         """Verifies that exception thrown if there are more features than qubits."""