Correlation of matched electric fields in four-level atoms via electromagnetically induced transparency

We investigated the matching of two continuous fields in a closed four-level system quantum mechanically. When the atomic coherence is prepared in the dressed state by two strong external coupling fields, fluctuations of phase and amplitude between the two probe fields are selectively absorbed, while these fields are mainly transparent in the media. Such selective absorption establishes the phase and amplitude correlation between the two weak probe fields over a wide range of frequencies. However, the correlation remains limited by the standard quantum limit, because of the addition of the anti-phase atomic noise during the four-wave mixing process.