Control of Light Pulse Propagation with Only a Few Cold Atoms in a High-Finesse Microcavity

Propagation of a light pulse through a high-$Q$ optical microcavity containing a few cold atoms ($\overline{N}<10$) in its cavity mode is investigated experimentally. With less than ten cold rubidium atoms launched into an optical microcavity, up to 170 ns propagation lead time (“superluminal”), and 440 ns propagation delay time (subluminal) are observed. Comparison of the experimental data with numerical simulations as well as future experiments are discussed.

Figure 1

The experimental setup. The inset shows the transmittance and refractive index for light propagation through the cavity. The transmittance splits due to atom-cavity coherent coupling. The corresponding dispersive property results in abnormal propagation. According to the slope of the dispersion curve, subluminal or superluminal propagation takes place at ${\omega }_1$ or ${\omega }_2$.

Figure 2

(a) The transmitted probe signal when atoms enter the cavity one after another. (b) The transmitted probe signal when a group of atoms enters the cavity.

Figure 3

Transmitted probe pulse profiles observed with heterodyne detection method. Solid square: with atoms in the cavity. Open circle: without atom in the cavity. (a) When the detuning is $\delta /2\pi =-53\mathrm{M}\mathrm{H}\mathrm{z}$, 440 ns delay is observed. (b) When $\delta /2\pi =-45\mathrm{M}\mathrm{H}\mathrm{z}$, 170 ns advance time is observed. (c) The delay/advance time as a function of the effective atom number calculated using a Monte Carlo simulation. The intersections between the curves and the horizontal lines (the observed delay or advance time) determine the effective number of atoms in the cavity to be $\overline{N}=9.4$.