def mean_photon(state, wires, params):
    """Computes the expectation value of the ``qml.NumberOperator``
    observable in Strawberry Fields.

    Args:
        state (strawberryfields.backends.states.BaseState): the quantum state
        wires (Sequence[int]): the measured mode
        params (Sequence): sequence of parameters (not used)

    Returns:
        float, float: mean photon number and its variance
    """
    return state.mean_photon(wires[0])
