Added optional parameter n_qubits to get_sparse_operator for FermionOperators (#72)

* Test for n_qubits param for get_sparse_operator with FermionOperators

* Added optional param n_qubits to get_sparse_operator

Author: idk3

src/fermilib/transforms/_conversion.py

@@ -34,7 +34,7 @@ def get_sparse_operator(operator, n_qubits=None):
     if isinstance(operator, InteractionOperator):
         return get_sparse_interaction_operator(operator)
     elif isinstance(operator, FermionOperator):
-        return jordan_wigner_sparse(operator)
+        return jordan_wigner_sparse(operator, n_qubits)
     elif isinstance(operator, QubitOperator):
         if n_qubits is None:
             n_qubits = count_qubits(operator)

src/fermilib/transforms/_conversion_test.py

@@ -115,5 +115,14 @@ def test_sparse_matrix_linearity(self):
                          [0, 3, 5, 6, 9, 10, 12, 15])
 
 
+class GetSparseOperatorFermionTest(unittest.TestCase):
+
+    def test_sparse_matrix_zero_n_qubit(self):
+        sparse_operator = get_sparse_operator(FermionOperator.zero(), 4)
+        sparse_operator.eliminate_zeros()
+        self.assertEqual(len(list(sparse_operator.data)), 0)
+        self.assertEqual(sparse_operator.shape, (16, 16))
+
+
 if __name__ == '__main__':
     unittest.main()