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tests/test_expectation.py

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+import math
+import random
+
+import pytest
+from qibo import Circuit, construct_backend, gates, hamiltonians
+from qibo.symbols import X, Z
+
+
+def build_observable(nqubits):
+    """Helper function to construct a target observable."""
+    hamiltonian_form = 0
+    for i in range(nqubits):
+        hamiltonian_form += 0.5 * X(i % nqubits) * Z((i + 1) % nqubits)
+
+    hamiltonian = hamiltonians.SymbolicHamiltonian(form=hamiltonian_form)
+    return hamiltonian, hamiltonian_form
+
+
+def build_circuit(nqubits, nlayers, seed=42):
+    """Helper function to construct a layered quantum circuit."""
+    random.seed(seed)
+
+    circ = Circuit(nqubits)
+    for _ in range(nlayers):
+        for q in range(nqubits):
+            circ.add(gates.RY(q=q, theta=random.uniform(-math.pi, math.pi)))
+            circ.add(gates.RZ(q=q, theta=random.uniform(-math.pi, math.pi)))
+        [circ.add(gates.CNOT(q % nqubits, (q + 1) % nqubits) for q in range(nqubits))]
+    circ.add(gates.M(*range(nqubits)))
+    return circ
+
+
+@pytest.mark.parametrize("nqubits", [2, 5, 10])
+def test_observable_expval(backend, nqubits):
+    numpy_backend = construct_backend("numpy")
+    ham, ham_form = build_observable(nqubits)
+    circ = build_circuit(nqubits=nqubits, nlayers=1)
+
+    exact_expval = numpy_backend.calculate_expectation_state(
+        hamiltonian=ham,
+        state=circ().state(),
+        normalize=False,
+    )
+
+    tn_expval = backend.expectation(circuit=circ, observable=ham_form)
+
+    assert math.isclose(exact_expval, tn_expval, abs_tol=1e-7)