A dark state of Chern bands: Designing flat bands with higher Chern number

We introduce a scheme by which flat bands with higher Chern number $\vert C\vert>1$ can be designed in ultracold gases through a coherent manipulation of Bloch bands. Inspired by quantum-optics methods, our approach consists in creating a “dark Bloch band” by coupling a set of source bands through resonant processes. Considering a $\mathrm{\Lambda }$ system of three bands, the Chern number of the dark band is found to follow a simple sum rule in terms of the Chern numbers of the source bands: $C_D\text{\hspace{0.17em}⁣}=\text{\hspace{0.17em}⁣}{C}_1+C_2-C_3$. Altogether, our dark-state scheme realizes a nearly flat Bloch band with predictable and tunable Chern number $C_D$. We illustrate our method based on a $\mathrm{\Lambda }$ system, formed of the bands of the Harper-Hofstadter model, which leads to a nearly flat Chern band with $C_D\text{\hspace{0.17em}⁣}=\text{\hspace{0.17em}⁣}2$. We explore a realistic sequence to load atoms into the dark Chern band, as well as a probing scheme based on Hall drift measurements. Dark Chern bands offer a practical platform where exotic fractional quantum Hall states could be realized in ultracold gases.