Published papers

Fault-tolerant quantum computers are expected to be offered as cloud services due to their significant resource and infrastructure requirements. Quantum multiprogramming, which runs multiple quantum jobs in parallel, is a promising approach to maximize the utilization of such systems. A key challenge in this setting is the need for an online scheduler capable of handling jobs submitted dynamically while other programs are already running. In this study, we formulate the online job scheduling problem for fault-tolerant quantum computing systems based on lattice surgery and propose an efficient scheduler to address it. To meet the responsiveness required in an online environment, our scheduler approximates lattice surgery programs, originally represented as polycubes, by using simpler cuboid representations. This approximation enables efficient scheduling while improving overall throughput. In addition, we incorporate a defragmentation mechanism into the scheduling process, demonstrating that it can further enhance the utilization of quantum processing unit.

Distributed quantum computation is a promising approach to building a fault-tolerant quantum computer at practical scale. To complete program execution within the minimum time, computational resources and communication resources should be assigned to reduce communication cost. Since remote lattice surgery operation between spatially separated quantum computers takes longer than local lattice surgery, minimizing delay due to the remote lattice surgery is a crucial issue to improve performance of distributed quantum computation. This delay is driven by three related issues: assignment of variable qubits to specific nodes; placement of those qubits within the nodes (resulting in contention for the limited number of local qubit movement paths); and contention for inter-node communication resources (both network interface communication qubits and channels) during concurrent operations. Focusing on the second problem, we propose placing qubits near the network interface based on their inter-node communication needs to avoid intra-node path collisions. We evaluate execution time of the VBE ripple-carry adder in a 2 \times 2 grid topology, comparing our proposed qubit assignment approach to random placement. Our results suggest that the proposed model has the potential to reduce resource contention and to improve total execution time.