A Quantum Network Processor Unit for Distributed Quantum Computing

TL;DR

This work addresses the scalability bottleneck in distributed quantum computing by proposing a decoupled quantum architecture in which a Quantum Network Processing Unit (QNPU) handles inter-node communication, distinct from the local Quantum Processing Unit (QPU). It introduces DistQASM, an extension of OpenQASM for distributed programs, and a dedicated QNPU ISA and microarchitecture, including a cycle-level simulator for evaluation. The key contributions are a four-layer quantum supercomputer stack, a hardware-accelerated communication stack with EPR resource management, and a two-tier microarchitectural design (scalar and superscalar) that significantly speeds up communication-intensive workloads (e.g., BV, QFT, QAOA) by parallelizing remote operations. The results demonstrate that decoupling computation from communication and employing a wider superscalar QNPU benchmarked against monolithic designs substantially improves distributed quantum performance, supporting scalable, potentially heterogeneous quantum architectures for future quantum supercomputers.

Abstract

As quantum computing progresses, the need for scalable solutions to address large-scale computational problems has become critical. Quantum supercomputers are the next upcoming frontier by enabling multiple quantum processors to collaborate effectively to solve large-scale computational problems. The emergence of quantum supercomputers necessitates an efficient interface to manage the quantum communication protocols between quantum processors. In this paper, we propose the Quantum Network Processing Unit (QNPU), which enables quantum applications to efficiently scale beyond the capacity of individual quantum processors, serving as a critical building block for future quantum supercomputers. The QNPU works alongside the Quantum Processing Unit (QPU) in our decoupled processing units architecture, where the QPU handles local quantum operations while the QNPU manages quantum communication between nodes. We design a comprehensive instruction set architecture (ISA) for the QNPU with high-level communication protocol abstractions, implemented via micro-operations that manage EPR resources, quantum operations, and classical communication. To facilitate programming, we introduce DistQASM, which extends OpenQASM with distributed quantum operations. We then propose a microarchitecture featuring both scalar and superscalar QNPU designs to enhance performance for communication-intensive quantum workloads. Finally, we evaluate the performance of our proposed QNPU design with distributed quantum workloads and demonstrate that the QNPU significantly improves the efficiency of communication between quantum nodes, paving the way for quantum supercomputing.

Figures (9)

• Figure 1: Organization of quantum nodes in a quantum supercomputer. Each quantum node integrates a Quantum Processing Unit (QPU) and a Quantum Network Processing Unit (QNPU). Both units contain a classical controller and a quantum chip (qubits). Black solid lines between classical controllers are classical links, and blue dashed lines between quantum chips are quantum links. While only two nodes are shown for clarity, a quantum supercomputer can scale to multiple nodes with similar interconnection patterns forming different topologies.
• Figure 2: The concept of EPR pair buffering in a distributed quantum computing system.
• Figure 3: Two communication protocols. (a) TP-Comm version for implementing a remote CNOT. (b) CAT-Comm version for implementing a remote CNOT.
• Figure 4: Layered architecture for quantum supercomputers. The design consists of four distinct layers, each providing specific functionalities and interfacing with adjacent layers.
• Figure 5: Decoupled processing units for quantum supercomputers. Each quantum node separates computation and communication resources: QPU contains a compute zone with data qubits for local quantum computation, while QNPU contains a communication zone with qubits for EPR pair prefetching and quantum communication protocol execution. The orange dashed line indicates EPR pair establishment between QNPUs, and blue dashed lines show qubit movement capabilities between the computation zone and communication zone.