  We report on experimental implementation of the optimal universal asymmetric
1->2 quantum cloning machine for qubits encoded into polarization states of
single photons. Our linear optical machine performs asymmetric cloning by
partially symmetrizing the input polarization state of signal photon and a
blank copy idler photon prepared in a maximally mixed state. We show that the
employed method of measurement of mean clone fidelities exhibits strong
resilience to imperfect calibration of the relative efficiencies of
single-photon detectors used in the experiment. Reliable characterization of
the quantum cloner is thus possible even when precise detector calibration is
difficult to achieve.
