Ultra-strong nonreciprocal amplification with hot atoms

Optical nonreciprocity plays an important role in optical communication and signal processing. Here, we propose and demonstrate a magnetic-free optical nonreciprocal amplification based on a far-detuned four-wave mixing process in hot atoms. The absorption of the probe field can be significantly reduced by the large detuning. In our experiment, with just one pump field, an ultra-strong forward gain of 45 dB is achieved while the backward one keeps a near unit transmission. The experimental realization of ultra-strong nonreciprocal amplification may have applications for the generation of strong nonreciprocal quantum correlations.