test: Add example from issue #1357 in tests

tests/test_hamiltonians_trotter.py

@@ -3,7 +3,7 @@
 import numpy as np
 import pytest
 
-from qibo import hamiltonians
+from qibo import hamiltonians, symbols
 from qibo.backends import NumpyBackend
 from qibo.quantum_info import random_hermitian, random_statevector
 
@@ -143,6 +143,14 @@ def test_trotter_hamiltonian_three_qubit_term(backend):
     backend.assert_allclose(final_state, target_state)
 
 
+def test_symbolic_hamiltonian_circuit_different_dts(backend):
+    """Issue: https://github.com/qiboteam/qibo/issues/1357."""
+    ham = hamiltonians.SymbolicHamiltonian(symbols.Z(0))
+    a = ham.circuit(0.1)
+    b = ham.circuit(0.1)
+    assert np.linalg.norm(ham.circuit(0.2).unitary() - (a + b).unitary()) == 0
+
+
 def test_old_trotter_hamiltonian_errors():
     """Check errors when creating the deprecated ``TrotterHamiltonian`` object."""
     with pytest.raises(NotImplementedError):