def test_operation_to_choi(channel):
"""Verifies that cirq.operation_to_choi correctly computes the Choi matrix."""
n_qubits = cirq.num_qubits(channel)
actual = cirq.operation_to_choi(channel)
expected = compute_choi(channel)
assert np.isclose(np.trace(actual), 2 ** n_qubits)
assert np.all(actual == expected)
def test_choi_for_completely_dephasing_channel():
"""Checks cirq.operation_to_choi on the completely dephasing channel."""
assert np.all(cirq.operation_to_choi(cirq.phase_damp(1)) == np.diag([1, 0, 0, 1]))