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arXiv:2503.04295 · PRX Quantum

Implementation of a Quantum Addressable Router Using Superconducting Qubits

Connie Miao, Sébastien Leger, Ziqian Li, Gideon Lee, Liang Jiang, David Schuster

Published Aug 20, 2025• 3 citations
arXivHTMLPDF

Abstract

The implementation of a quantum router capable of performing both quantum signal routing and quantum addressing (a Q 2 -router) represents a key step toward building quantum networks and quantum random access memories. We realize a Q 2 -router that uses fixed-frequency transmon qubits to implement a routing protocol based on two native controlled- i gates. These gates leverage a large Z Z interaction to selectively route information according to a quantum address. We find an estimated average routing fidelity of 95.3 % , with errors arising primarily from decoherence or state preparation and measurement. We present a comprehensive calibration and characterization of both the c- i gates and the overall routing protocol through randomized benchmarking techniques and state tomography.

DOI
10.1103/pq3x-cmw9

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The implementation of a quantum router capable of performing both quantum signal routing and quantum addressing (a Q 2 -router) represents a key step toward building quantum networks and quantum random access memories. We realize a Q 2 -router that uses fixed-frequency transmon qubits to implement a routing protocol based on two native controlled- i gates. These gates leverage a large Z Z interaction to selectively route information according to a quantum address. We find an estimated average routing fidelity of 95.3 % , with errors arising primarily from decoherence or state preparation and measurement. We present a comprehensive calibration and characterization of both the c- i gates and the overall routing protocol through randomized benchmarking techniques and state tomography.
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Quantum algorithms on near-term quantum processors are typically executed using shallow quantum circuits composed of one- and two-qubit gates. However, as circuit depth and gate number increase, gate imperfections and qubit decoherence begin to dominate, limiting algorithmic complexity. An alternative approach is to explore gates involving more than two qubits. In previous work (X. Wu et al., Physical Review X 14, 041030 (2024)), we demonstrated a new superconducting qubit architecture with user-selectable two-qubit interactions via a reconfigurable router, used to connect pairs of qubits. Here, we leverage this novel architecture to realize programmable and efficient multi-qubit operations involving more than two qubits, resulting in faster preparation of multi-qubit entangled states with good fidelities. We also successfully apply model-free reinforcement learning to perform multi-qubit gates, including training a two-qubit controlled-Z gate as well as three-qubit controlled-SWAP and controlled-controlled-phase (Fredkin and Toffoli) gates. Higher nnnth-order gates may also be feasible, using our high-connectivity router design. This could provide a more efficient and higher-fidelity implementation of complex quantum algorithms and a more practical approach to quantum computation.
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The control and handling of errors arising from cross talk and unwanted interactions in multiqubit systems is an important issue in quantum information processing architectures. We introduce a benchmarking protocol that provides information about the amount of addressability present in the system and implement it on coupled superconducting qubits. The protocol consists of randomized benchmarking experiments run both individually and simultaneously on pairs of qubits. A relevant figure of merit for the addressability is then related to the differences in the measured average gate fidelities in the two experiments. We present results from two similar samples with differing cross talk and unwanted qubit-qubit interactions. The results agree with predictions based on simple models of the classical cross talk and Stark shifts.
Quantum random access memory (QRAM) enables efficient classical data access for quantum computers -- a prerequisite for many quantum algorithms to achieve quantum speedup. Despite various proposals, the experimental realization of QRAM remains largely unexplored. Here, we experimentally investigate the circuit-based bucket-brigade QRAM with a superconducting quantum processor. To facilitate the experimental implementation, we introduce a hardware-efficient gate decomposition scheme for quantum routers, which effectively reduces the depth of the QRAM circuit by more than 30% compared to the conventional controlled-SWAP-based implementation. We further propose an error mitigation method to boost the QRAM query fidelity. With these techniques, we are able to experimentally implement the QRAM architectures with two and three layers, achieving query fidelities up to 0.800 ±\pm± 0.026 and 0.604$\pm$0.005, respectively. Additionally, we study the error propagation mechanism and the scalability of our QRAM implementation, providing experimental evidence for the noise resilience nature of the bucket-brigade QRAM architecture. Our results highlight the potential of superconducting quantum processors for realizing a scalable QRAM architecture.