Topology vs localization in synthetic dimensions

Motivated by recent developments in quantum simulation of synthetic dimensions, e.g., in optical lattices of ultracold atoms, we discuss here d-dimensional periodic, gapped quantum systems for d ≤ 4, with a focus on the topology of the occupied energy states. We perform this analysis by asking whether the spectral subspace below the gap can be spanned by smooth and periodic Bloch functions, corresponding to localized Wannier functions in position space. By constructing these Bloch functions inductively in the dimension, we show that if they are required to be orthonormal, then, in general, their existence is obstructed by the first two Chern classes of the underlying Bloch bundle, with the second Chern class characterizing, in particular, the four-dimensional situation. If the orthonormality constraint is relaxed, we show how m occupied energy bands can be spanned by a Parseval frame comprising at most m + 2 Bloch functions.