Magnetically induced optical transparency with an ultranarrow spectrum

Magnetically induced optical transparency (MIOT) is a technique to realize the narrow transmission spectrum in a cavity quantum electrodynamics system, which is demonstrated in a recent experiment of cold Sr-88 atoms in an optical cavity [M. N. Winchester, M. A. Norcia, J. R. K. Cline, and J. K. Thompson, Phys. Rev. Lett. 118, 263601 (2017)]. In this experiment, MIOT induces a new narrow transmission window for the probe beam, which is highly immune to the fluctuation of the cavity mode frequency. The linewidth of this transmission window approaches the decay rate of the electronic P-3(1) state [about (2 pi) 7.5 kHz] and is much less than the uncertainty of the cavity mode frequency [about (2 pi) 150 kHz]. In this paper, we propose an approach to further reduce the linewidth of this MIOT-induced transmission window, with the help of two Raman beams which couple the electronic (3)(P1) state to the S-3(1) state, and the S-3(1) state to the P-3(0) state, respectively. With this approach, one can reduce the transmission linewidth by orders of magnitude. Moreover, the peak value of the relative transmission power or the transmission rate of the probe beam is almost unchanged by the Raman beams, and the peak position is insensitive to the average frequency of the two Raman beams.