Kenny Campbell

Quantum data centres (QDCs) are a promising way of scaling up quantum computers. In a single-processor quantum computer, the number of high-quality computational qubits is limited by cross talk and difficulties in addressing individual qubits when many qubits share the same housing. QDCs circumvent these challenges by linking together multiple small quantum processing units (QPUs) over short distances. With this architecture, the intra-QPU noise is kept small, but additional noise is introduced due to the latency of and imperfections in the inter-QPU links. Understanding the trade-offs between these different types of noise is essential for guiding future efforts in QDC manufacture and compilation. We develop and use a classical simulator to emulate the execution of different quantum circuits on an imperfect QDC. An individual inter-QPU CNOT gate is first-considered and then a selection of larger circuits are investigated. In both cases, we implement inter-QPU gates using cat-comm and three variants of TP-comm, which we call 1TP-comm, 2TP-comm and TP-safe, respectively. We find that 1TP-comm and cat-comm yield different output fidelities despite both schemes having the same number of gates, measurements and inter-QPU entanglements. We also determine the relative impacts of entanglement error, intra-QPU gate error and memory depolarisation.