Ultraslow helical optical bullets and their acceleration in magneto-optically controlled coherent atomic media

We propose a scheme to produce ultraslow (3+1)-dimensional helical optical solitons, also called helical optical bullets, in a resonant three-level Λ-type atomic system via quantum coherence. We show that, due to the effect of electromagnetically induced transparency, the helical optical bullets can propagate with an ultraslow velocity up to 10^-5 c (c is the light speed in vacuum) in the longitudinal direction and a slow rotational motion (with velocity 10^-7 c) in transverse directions. The generation power of such optical bullets can be lowered to microwatts, and their stability can be achieved by using a Bessel optical lattice potential formed by a far-detuned laser field. We also show that the transverse rotational motion of the optical bullets can be accelerated by applying a time-dependent Stern-Gerlach magnetic field. Because of the ultraslow velocity in the longitudinal direction, a significant acceleration of the rotational motion of optical bullets may be observed for a very short medium length.