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arXiv:2609.18081

Remote Adiabatic Controlled-Z Gate between Distant Superconducting Qubits

Si-Yu Xu, Yi-Rong Jin, Wen-Gang Zhang, Hai-Feng Yu

quant-phSubmitted Sep 16, 2026• Announced Sep 17, 2026• 0 citations
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Abstract

Direct entangling gates between spatially separated qubits are a key capability for modular quantum computing. They enable nonlocal quantum circuits to be executed across different processor modules, thereby alleviating single-chip scaling constraints and expanding the effective connectivity of the system. Here, we propose and numerically investigate a remote adiabatic controlled-Z gate between two frequency-tunable superconducting qubits connected by a multi-mode coaxial cable. The cable-mediated interaction enables the gate to be implemented without additional tunable couplers, preserving a simple circuit architecture. For a 30-cm cable, simulations incorporating ten standing-wave modes yield an optimized gate infidelity of $4.71\times10^{-7}$ with a duration of 376.7 ns in the absence of decoherence. The gate maintains high fidelity under qubit-frequency fluctuations and millimeter-scale cable-length variations. Calculations for 50- and 100-cm cable situations demonstrate its potential for even larger cryogenic systems. These results provide a promising approach to extending high-fidelity quantum operations beyond individual chips and toward scalable, interconnected superconducting quantum processors.

Comments
12 pages, 11 figures

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Direct entangling gates between spatially separated qubits are a key capability for modular quantum computing. They enable nonlocal quantum circuits to be executed across different processor modules, thereby alleviating single-chip scaling constraints and expanding the effective connectivity of the system. Here, we propose and numerically investigate a remote adiabatic controlled-Z gate between two frequency-tunable superconducting qubits connected by a multi-mode coaxial cable. The cable-mediated interaction enables the gate to be implemented without additional tunable couplers, preserving a simple circuit architecture. For a 30-cm cable, simulations incorporating ten standing-wave modes yield an optimized gate infidelity of 4.71×10−74.71\times10^{-7}4.71×10−7 with a duration of 376.7 ns in the absence of decoherence. The gate maintains high fidelity under qubit-frequency fluctuations and millimeter-scale cable-length variations. Calculations for 50- and 100-cm cable situations demonstrate its potential for even larger cryogenic systems. These results provide a promising approach to extending high-fidelity quantum operations beyond individual chips and toward scalable, interconnected superconducting quantum processors.
Tunable Nonlocal ZZZZZZ Interaction for Remote Controlled-Z Gates Between Distributed Fixed-Frequency Qubits
Scaling superconducting quantum processors toward fault-tolerant operation will likely require architectures that extend beyond monolithic chips. Modular processors connected by low-loss superconducting links provide a promising route, but implementing entangling gates between remote fixed-frequency qubits remains challenging. Here we propose a distributed architecture in which two synchronously controlled double-transmon couplers mediate the interaction between fixed-frequency transmons in separate packages connected by a 25-cm coaxial cable. The scheme activates a tunable nonlocal ZZZZZZ interaction on demand while suppressing residual static coupling, allowing the superconducting link to function as a gate-native interconnect rather than solely as a state-transfer channel. Circuit-level simulations show an on/off ratio exceeding 10610^6106 and a remote controlled-Z gate with a projected coherent fidelity of 99.99%99.99\%99.99% under experimentally relevant parameters. Open-system simulations further indicate that, within the representative Markovian noise model considered here, endpoint-qubit decoherence is the largest contribution to gate infidelity, while photon loss in the retained cable modes remains smaller but non-negligible. These results identify DTC-mediated tunable nonlocal coupling as a promising gate primitive for modular superconducting processors based on fixed-frequency qubits.
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Abstract High-fidelity quantum state transfer and remote entanglement between superconducting fixed-frequency qubits have not yet been realized. In this study, we propose an alternative remote cross-resonance gate. Considering multiple modes of a superconducting coaxial cable connecting qubits, we must find conditions under which the cross-resonance gate operates with a certain accuracy even in the presence of qubit frequency shifts due to manufacturing errors. For 0.25- and 0.5 m cables, remote cross-resonance gates with a concurrence of > 99.9 % in entanglement generation are obtained even with ±10 MHz frequency shifts. For a 1 m cable with a narrow mode spacing, a concurrence of 99.5% is achieved by reducing the coupling between the qubits and cable. The optimized echoed raised-cosine pulse duration is 150–400 ns, which is similar to the operation time of cross-resonance gates between neighboring qubits on a chip. The dissipation through the cable modes does not considerably affect the obtained results. Such high-precision quantum interconnects pave the way not only for scaling up quantum computer systems but also for nonlocal connections on a chip.
Superconducting qubit systems, one of the leading candidates for universal quantum computing, face scalability challenges such as frequency crowding, wiring complexity, and packaging problems. Distributed quantum computing offers a viable strategy for constructing larger quantum information processing systems. Yet, direct universal quantum gates between remote qubits-critical to distributed architectures-remain unrealized. Here, we demonstrate direct high-fidelity entangling gates between two remote superconducting quantum processors separated by a 30 cm distance, utilizing standing-wave modes in their connecting coaxial cable. We achieve cross-entropy benchmarking fidelities of (99.15±0.02)% and (98.03±0.04)% for the controlled-not and controlled-z gates, respectively, outperforming state transfer and feedback-based protocols in fidelity and efficiency. This advancement significantly enhances the prospect of universal distributed quantum information processing, which is the critical step toward future large-scale quantum systems.