Multilevel dark states in an inhomogeneously broadened open atomic system

We report an experimental observation of coherent population trapping in multilevel dark states prepared in the ground hyperfine states of ${}^{87}\mathrm{Rb}$ atoms confined in a room-temperature vapor cell. The dark states are created by a linearly polarized coupling laser that is tuned to the ${}^{87}\mathrm{Rb}$ $D_1$ $F=2\mathrm{}\leftrightarrow F^{\prime }=1\mathrm{}$ or 2 transition and establishes Λ-type transition chains. The Doppler shift for the Λ-type transition chains is compensated by the left and right circularly polarized components of the coupling laser. The multilevel dark states that consist of three or two magnetic sublevels among the five degenerate magnetic sublevels in the $F=2\mathrm{}$ ground hyperfine state are probed by a weak probe laser scanned across the same $D_1$ transition and their existence is manifested by a strong dependence of the probe excitation on the probe polarization. Our experiment demonstrates the importance of coherent population trapping in a laser coupled degenerate multilevel open atomic system, which may play an important role in diverse physical phenomena such as atom cooling and trapping, optical pumping, and multiple laser excitation and ionization of atoms and molecules.