Expansion of higher-dimensional cubical complexes with application to quantum locally testable codes

We introduce a higher-dimensional "cubical" chain complex and apply it to the design of quantum locally testable codes. Our cubical chain complex can be constructed for any dimension $t$, and in a precise sense generalizes the Sipser-Spielman construction of expander codes (case $t=1$) and the constructions by Dinur et. al and Panteleev and Kalachev of a square complex (case $t$=2), which have been applied to the design of classical locally testable and quantum low-density parity check codes respectively. For $t=4$ our construction gives a family of quantum locally testable codes conditional on a conjecture about robustness of four-tuples of random linear maps. These codes have linear dimension, inverse poly-logarithmic relative distance and soundness, and polylogarithmic-size parity checks. Our complex can be built in a modular way from two ingredients. Firstly, the geometry (edges, faces, cubes, etc.) is provided by a set $G$ of size $N$, together with pairwise commuting sets of actions $A_1,\ldots,A_t$ on it. Secondly, the chain complex itself is obtained by associating local coefficient spaces based on codes, with each geometric object, and introducing local maps on those coefficient spaces. We bound the cycle and co-cycle expansion of the chain complex. The assumptions we need are two-fold: firstly, each Cayley graph $Cay(G,A_j)$ needs to be a good (spectral) expander, and secondly, the families of codes and their duals both need to satisfy a form of robustness (that generalizes the condition of agreement testability for pairs of codes). While the first assumption is easy to satisfy, it is currently not known if the second can be achieved.