In the three-level medium a ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ may be generated by atomic coherence effects which involve negligible saturation and, hence, are essentially "spontaneous-emission-free." This offers considerable experimental advantages for the generation of squeezed states of light via four-wave mixing or optical bistability.

• Received 28 June 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.1288