  To achieve high degree of quantum noise squeezing, an optical cavity is often
employed to enhance the interaction time between light and matter. Here, we
propose to utilize the effect of coherent population trapping (CPT) to directly
generate squeezed light without any optical cavity. Combined with the slow
propagation speed of light in a CPT medium, a coherent state passing through an
atomic ensemble with a high optical density (OD) can evolve into a highly
squeezed state even in a single passage. Our study reveals that noise squeezing
of more than $10$ dB can be achieved with an OD of 1,000, which is currently
available in experiments. A larger OD can further increase the degree of
squeezing. As the light intensity and two-photon detuning are key factors in
the CPT interaction, we also demonstrate that the minimum variance at a given
OD can be reached for a wide range of these two factors, showing the proposed
scheme is flexible and robust. Furthermore, there is no need to consider the
phase-matching condition in the CPT scheme. Our introduction of high OD in
atomic media not only brings a long light-matter interaction time comparable to
optical cavities, but also opens new avenue in the generation of squeezed light
for quantum interface.
