  We propose to realize microwave quantum illumination in weak coupling regime
based on multimode optomechanical systems. In our proposal the multimode
together with a frequency-mismatch process could reduce mechanical thermal
noise. Therefore, we achieve a significant reduction of error probability than
conventional detector in weak coupling regime. Moreover, we optimize the
signal-to-noise ratio for limited bandwidth by tuning the delay time of
entangled wave-packets.
