Demonstrating Coherent Quantum Routers for Bucket-Brigade Quantum Random Access Memory on a Superconducting Processor

TL;DR

This paper demonstrates coherent quantum routers on a superconducting processor, achieving high fidelity and scalability, which are crucial for practical quantum memory and large-scale quantum computing.

Contribution

The authors experimentally realize quantum routers using a transition composite gate scheme with inherent error mitigation, advancing scalable and coherent QRAM implementation.

Findings

Achieved up to 95.74% fidelity for individual quantum routers

Validated a two-layer quantum routing network with 82.40% average fidelity

Introduced address encoding in non-adjacent qutrit states for error detection

Abstract

Quantum routers (QRouters) are essential components of bucket-brigade quantum random access memory (QRAM), enabling quantum applications such as Grover's search and quantum machine learning. Despite significant theoretical advances, achieving scalable and coherent QRouters experimentally remains challenging. Here, we demonstrate coherent quantum routers using a superconducting quantum processor, laying a practical foundation for scalable QRAM systems. The quantum router at the core of our implementation utilizes the transition composite gate (TCG) scheme, wherein auxiliary energy levels temporarily mediate conditional interactions, substantially reducing circuit depth compared to traditional gate decompositions. Moreover, by encoding routing addresses in the non-adjacent qutrit states $|0\rangle$ and $|2\rangle$, our design inherently enables eraser-detection capability, providing efficient post-selection to mitigate routing errors. Experimentally, we achieve individual QRouter fidelities up to 95.74%, and validate scalability through a two-layer quantum routing network achieving an average fidelity of 82.40%. Our results represent a significant advancement in quantum routing technology, providing enhanced fidelity, built-in error resilience, and practical scalability crucial for the development of future QRAM and large-scale quantum computing architectures.