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Determination of an ultra-low mean photon number is an important issue, either for precise optical measurement or understanding the interaction between individual atoms and photons inside an optical cavity. By utilizing a homemade balanced optical heterodyne detection system, the cw minimum measurable power of 3.6 fW has been reached, whereas the minimum mean photon number of = 0.0014 in our micro-cavity with finesse of 2 × 105and optical length of 48 μm can be realized.