Lattice Surgery Aware Resource Analysis for the Mapping and Scheduling of Quantum Circuits for Scalable Modular Architectures

Abstract

Quantum computing platforms are evolving to a point where placing high numbers of qubits into a single core comes with certain difficulties such as fidelity, crosstalk, and high power consumption of dense classical electronics. Utilizing distributed cores, each hosting logical data qubits and logical ancillas connected via classical and quantum communication channels, offers a promising alternative. However, building such a system for logical qubits requires additional optimizations, such as minimizing the amount of state transfer between cores for inter-core two-qubit gates and optimizing the routing of magic states distilled in a magic state factory. In this work, we investigate such a system and its statistics in terms of classical and quantum resources. First, we restrict our quantum gate set to a universal gate set consisting of CNOT, H, T, S, and Pauli gates. We then developed a framework that can take any quantum circuit, transpile it to our gate set using Qiskit, and then partition the qubits using the KaHIP graph partitioner to balanced partitions. Afterwards, we built an algorithm to map these graphs onto the 2D mesh of quantum cores by converting the problem into a Quadratic Assignment Problem with Fixed Assignment (QAPFA) to minimize the routing of leftover two-qubit gates between cores and the total travel of magic states from the magic state factory. Following this stage, the gates are scheduled using an algorithm that takes care of the timing of the gate set. As a final stage, our framework reports detailed statistics such as the number of classical communications, the number of EPR pairs and magic states consumed, and timing overheads for pre- and post- processing for inter-core state transfers. These results help to quantify both classical and quantum resources that are used in distributed logical quantum computing architectures.