Visualising Quantum Product Codes

Abstract: The hypergraph product (HGP) is a famous code construction technique with an equally famous canonical visualisation. This visual perspective provides much more than simply a way to build intuition: HGP codes can be defined graphically, properties can be demonstrated graphically, and approaches to fault-tolerant logic have been developed graphically. In recent years two powerful generalisations of this product -- the lifted and balanced products -- have been proposed and employed to great success, but a unified graphical approach to describing these codes has been absent. In these notes I review the canonical approach to visualising HGP codes and then show how, via the addition of a third dimension, it can be generalised to apply to both the lifted and the balanced product. In the process we obtain clear intuition into various properties of these codes such as i) why it is hard to bound $k$ and $d$ ii) the issues with finding a canonical logical basis in the general case and iii) how the two products are related, and how they differ. I have attempted to structure these notes plainly and directly, so that the visual intuition can be easily obtained by those who want it while the rigorous justification is still available to those who demand it.