Network-assisted collective operations for efficient distributed quantum computing

Abstract: We propose protocols for the distribution of collective quantum operations between remote quantum processing units (QPUs), a requirement for distributed quantum computing. Using only local operations and classical communication (LOCC), these protocols allow for collective multicontrolled and multitarget gates to be executed in network architectures similar to those used for high-performance computing. The types of gates that can be implemented following this scheme are discussed. The Bell pair cost for a single distributed multicontrolled gate is estimated, arriving to a single additional Bell pair over the theoretically optimal calculation with pre-shared entanglement, demonstrating better scalability when compared to current proposals based on entanglement swapping through a network, and bounds are calculated for general diagonal gates. A recipe is provided for the lumped distribution of gates such as arbitrarily-sized Toffoli and multicontrolled Z, and $R_{zz}(\theta)$ gates. Finally, we provide an exact implementation of a distributed Grover's search algorithm using this protocol to partition the circuit, with Bell pair cost growing linearly with the number of Grover iterations and the number of partitions.