Looped pipelines enabling effective 3D qubit lattices in a strictly 2D device

Many quantum computing platforms are based on a two-dimensional (2D) physical layout.  In the paper 'Looped pipelines enabling effective 3D qubit lattices in a strictly 2D device' published in PRX Quantum 4, the authors (Zhenyu Cai, Adam Siegel and Professor Simon Benjamin) explore the concept of looped pipelines, which permits one to obtain many of the advantages of a three-dimensional (3D) lattice while operating a strictly 2D device.

The concept leverages qubit shuttling, a well-established feature in platforms like semiconductor spin qubits and trapped-ion qubits.  The looped-pipeline architecture has similar hardware requirements to other shuttling approaches, but can process a stack of qubit arrays instead of just one - even a stack of limited height is enabling for diverse schemes ranging from NISQ-era error mitigation through to fault-tolerant codes.  For the former, protocols involving multiple states can be implemented with a space-time resource cost comparable to preparing one noisy copy.  For the latter, one can realise a far broader variety of code structures - as an example, the authors considered layered 2D codes within which transversal CNOTs were available. 

Under reasonable assumptions this approach can reduce the space-time cost of magic state distillation by 2 orders of magnitude.  Numerical modelling using experimentally motivated noise models verifies that the architecture provides this benefit without significant reduction to the code's threshold.