Topological charge pumping with subwavelength Raman lattices

Recent experiments demonstrated deeply subwavelength lattices using atoms with $N$ internal states Raman-coupled with lasers of wavelength $\lambda$. The resulting unit cell was $\lambda \mathrm{/}2N$ in extent, an $N$-fold reduction compared to the usual $\lambda \mathrm{/}2$ periodicity of an optical lattice. For resonant Raman coupling, this lattice consists of $N$ independent sinusoidal potentials (with period $\lambda \mathrm{/}2$) displaced by $\lambda \mathrm{/}2N$ from each other. We show that detuning from Raman resonance induces tunneling between these potentials. Periodically modulating the detuning couples the $s$- and $p$-bands of the potentials, creating a pair of coupled subwavelength Rice–Mele chains. This operates as a novel topological charge pump that counter-intuitively can give half the displacement per pump cycle of each individual Rice–Mele chain separately. We analytically describe this behavior in terms of infinite-system Chern numbers, and numerically identify the associated finite-system edge states.