A Fault-Tolerant Million Qubit-Scale Distributed Quantum Computer

A million qubit-scale quantum computer is essential to realize the quantum supremacy. Modern large-scale quantum computers integrate multiple quantum computers located in dilution refrigerators (DR) to overcome each DR's unscaling cooling budget. However, a large-scale multi-DR quantum computer introduces its unique challenges (i.e., slow and erroneous inter-DR entanglement, increased qubit scale), and they make the baseline error handling mechanism ineffective by increasing the number of gate operations and the inter-DR communication latency to decode and correct errors. Without resolving these challenges, it is impossible to realize a fault-tolerant large-scale multi-DR quantum computer.In this paper, we propose a million qubit-scale distributed quantum computer which uses a novel error handling mechanism enabling fault-tolerant multi-DR quantum computing. First, we apply a low-overhead multi-DR error syndrome measurement (ESM) sequence to reduce both the number of gate operations and the error rate. Second, we apply a scalable multi-DR error decoding unit (EDU) architecture to maximize both the decoding speed and accuracy. Our multi-DR error handling SW-HW co-design improves the ESM latency, ESM errors, EDU latency, and EDU accuracy by 3.7 times, 2.4 times, 685 times, and 6.1 · 10¹⁰ times, respectively.With our scheme applied to assumed voltage-scaled CMOS and mature ERSFQ technologies, we successfully build a fault-tolerant million qubit-scale quantum computer.