Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physical Review Research

Coherent errors in a superconducting six-qubit network with beyond-nearest-neighbor connectivity

Jay Deshmukh, Srijita Das, Binoy Nambiar, Madhavi Chand, Kishor V. Salunkhe, Gautham Umasankar, Aneesh Kamat, Kishan Bhowal, Gaurav Agarwal, Lakshay Bharadwaj -, Eeshita Chawla, Panya Jain, Meghan P. Patankar, Rajamani Vijay

Published Jun 16, 2026• 0 citations
Publisher

Abstract

Enhancing qubit connectivity in quantum processors is an important milestone for more efficient implementation of algorithms and novel error-correction protocols. While all-to-all connectivity has been demonstrated in quantum processors based on ion traps and neutral atoms, the exploration of enhanced connectivity in the superconducting qubit architecture has been a more recent endeavor. Here, we demonstrate the operation of a network comprising six superconducting transmon qubits with a novel, beyond-nearest-neighbor connectivity. We use a circular ring resonator with six qubits placed on the periphery at 60 ∘ spacing and capacitively coupled to it in a three-dimensional circuit-QED architecture. The ring resonator acts as a two-path coupler between each pair of qubits, and we present results demonstrating that each qubit has finite coupling to three other qubits with negligible coupling to the remaining two qubits. In this architecture, with fixed-frequency qubits and always-on coupling, we benchmark the single-qubit and two-qubit gates using randomized benchmarking. We find that our gate fidelities are strongly affected by the enhanced connectivity. We characterize the spread in qubit frequencies due to cross-Kerr terms and quantify its detrimental effect on the performance of single- and two-qubit gates. Finally, we analyze the effect of the spectator qubits using numerical simulations and propose a path toward optimizing this system and achieving > 99 % two-qubit gate fidelities across all connected qubit pairs.

DOI
10.1103/6jdh-pm39

Related papers

Ring-Resonator-Based Coupling Architecture for Enhanced Connectivity in a Superconducting Multiqubit Network

Sumeru Hazra, Anirban Bhattacharjee, Madhavi Chand +4 more

Qubit coherence and gate fidelity are typically considered the two most useful metrics for characterizing a quantum processor. An equally useful metric is interqubit connectivity as it minimizes gate count and allows implementing algorithms efficiently with reduced error. However, interqubit connectivity in superconducting processors tends to be limited to nearest neighbor due to practical constraints in typical planar realizations. Here, we introduce a superconducting architecture that uses a ring resonator as a multiqubit coupling element to provide beyond nearest-neighbor connectivity without compromising on coupling uniformity or introducing fabrication complexities. We theoretically analyze the interqubit coupling as a function of frequency for a pair of qubits placed at different positions along the ring resonator and show that for carefully chosen operating frequency and angular spacing between the qubits, the variation of coupling can be minimized. For an operating frequency between the first two resonances of the ring resonator and a ${30}^{\ensuremath{\circ}}$ angular spacing for qubits, we compute interqubit coupling for a device capable of supporting up to 12 qubits with each qubit connected to nine other qubits. Using four qubits positioned strategically in the ring-resonator coupler, we experimentally verify the theoretical prediction for all possible angular spacings between the two qubits and demonstrate good agreement. Just like the standard bus resonator coupler, the coupling in the ring-resonator coupler is mediated via virtual photons since the operating frequency is far away from the resonant modes of the ring coupler. This ensures that any small internal loss in the ring resonator does not introduce decoherence during the coupling operation. We also compute extensions of this idea involving larger ring resonators and a multiring system and show the possibility of highly connected networks with larger number of qubits. Apart from being plug and play for existing superconducting architectures, our concept is scalable, adaptable to other platforms and has the potential to significantly accelerate progress in quantum computing, annealing, simulations, and error correction.

arXiv:2012.09463•Physical Review Applied•Aug 11, 2021•37 citations

Demonstration of two qubit entangling gates in a 2D ring resonator based coupler architecture

Anirban Bhattacharjee, Panya Jain, Jay Deshmukh +4 more

We report entangling two-qubit experiments implemented in a novel ring resonator architecture in 2D planar geometry. The ring resonator acts as a multi-path coupler between qubits and can provide beyond nearest neighbour interactions. We demonstrate pairwise coupling between three fixed-frequency transmon qubits connected to the ring resonator with measured coupling strengths (4.70 MHz, 2.80 MHz, and 2.65 MHz) in good agreement with those predicted from finite-element simulations. We implement an all-microwave controlled phase (CPHASE) gate between a pair of qubits with a gate time of 196 ns and demonstrate a two-qubit Bell state with a measured state fidelity of [Formula: see text]. Our results demonstrate the ability to entangle two qubits using the ring resonator and pave the way for creating highly connected multi-qubit networks in this architecture.

Scientific Reports•Feb 5, 2025•2 citations

Lattice patch structure for fixed-frequency transmon quantum computer with high-fidelity CNOT gates

Chanpyo Kim, Jeongsoo Kang, Younghun Kwon

Superconducting transmon processors represent a leading platform for large-scale quantum computing due to their high gate fidelities and scalability. However, conventional qubit-coupler-qubit (QCQ) architectures face critical physical and structural bottlenecks, notably frequency crowding [spectator qubit collisions] during system scaling and inefficient mapping onto the standard surface code.To overcome these limitations, we propose a novel lattice-patch architecture that couples four fixed-frequency transmons to a single fixed-frequency coupler.This design enhances qubit connectivity and maps directly onto the surface-code lattice unit [plaquette], thereby minimizing the compilation overhead associated with logical qubit implementation. Furthermore, utilizing an entirely fixed-frequency design intrinsically eliminates susceptibility to external flux noise, ensuring robust operational stability.Multi-level numerical simulations demonstrate CNOT gate fidelities exceeding 0.98 across all six connectivity directions within the patch. Nevertheless, the complex interaction network of the four-qubit architecture induces unintended residual phase accumulation during cross-resonance driving. This parasitic effect necessitates precise calibration, achievable via virtual $R_z$ gates [software phase updates]. Ultimately, our results establish the lattice-patch architecture as an efficient, robust building block for future fault-tolerant quantum computers.

quant-pharXiv:2606.27017•Jun 26, 2026•0 citations

Quantum Crosstalk Analysis for Simultaneous Gate Operations on Superconducting Qubits

Peng Zhao, Kehuan Linghu, Zhiyuan Li +5 more

Maintaining or even improving gate performance with growing numbers of parallel controlled qubits is a vital requirement for fault-tolerant quantum computing.For superconducting quantum processors, though isolated one-or two-qubit gates have been demonstrated with high fidelity, implementing these gates in parallel commonly shows worse performance.Generally, this degradation is attributed to various crosstalks between qubits, such as quantum crosstalk due to residual inter-qubit coupling.An understanding of the exact nature of these crosstalks is critical to figuring out respective mitigation schemes and improved qubit architecture designs with low crosstalk.Here we give a theoretical analysis of quantum crosstalk impact on simultaneous gate operations in a qubit architecture, where fixed-frequency transmon qubits are coupled via a tunable bus, and sub-100-ns controlled-Z (CZ) gates can be realized by applying a baseband flux pulse on the bus.Our analysis shows that for microwave-driven single-qubit gates, the dressing from the qubit-qubit coupling can cause non-negligible cross-driving errors when qubits operate near frequency collision regions.During CZ gate operations, although unwanted nearest-neighbor interactions are nominally turned off, sub-MHz parasitic next-nearest-neighbor interactions involving spectator qubits can still exist, causing considerable leakage or control error when one operates qubit systems around these parasitic resonance points.To ensure high-fidelity simultaneous operations, a request could be raised to figure out a better way to balance the gate error from target qubit systems themselves and the error from nonparticipating spectator qubits.Overall, our analysis suggests that towards useful quantum processors, the qubit architecture should be examined carefully in the context of high-fidelity simultaneous gate operations in a scalable qubit lattice.

arXiv:2110.12570•PRX Quantum•Apr 1, 2022•108 citations

Superconducting qubit-resonator quantum processor with effective all-to-all connectivity

Michael Renger, Jeroen Verjauw, Nicola Wurz +62 more

In this work, we introduce a superconducting quantum processor architecture that uses a transmission-line resonator to implement effective all-to-all connectivity between six transmon qubits. This architecture can be used as a test bed for algorithms that benefit from high connectivity. We show that the central resonator can be used as a computational element, which offers the flexibility to encode a qubit for quantum computation or to utilize its bosonic modes that further enables quantum simulation of bosonic systems. To operate the quantum processing unit (QPU), we develop and benchmark the qubit-resonator conditional Z gate and the qubit-resonator MOVE operation. The latter allows for transferring a quantum state between one of the peripheral qubits and the computational resonator. We benchmark the QPU performance and achieve a genuinely multiqubit entangled Greenberger-Horne-Zeilinger state over all six qubits with a readout-error mitigated fidelity of F | ψ 〉 GHZ = 0.86 .

arXiv:2503.10903•Physical Review Research•Dec 5, 2025•5 citations

Two-qubit-gate operation in a high-connectivity transmon lattice utilizing a tunable coupling to a shared mode

Tuure Orell, Hao Hsu, Joona Andersson +3 more

Increasing connectivity and decreasing qubit-state delocalization without compromising the speed and accuracy of elementary gate operations are topical challenges in the development of large-scale superconducting quantum computers. In this theoretical work, we study a special honeycomb qubit lattice where each qubit inside a unit cell is coupled to every other one via two dedicated tunable couplers and a common central element. This results in an effective multi-mode interaction enabling tunable, on-demand, all-to-all connectivity between each qubit pair within the unit cell. We provide a thorough analysis of the unit cell, including a proposal for a novel and efficient conditional-Z gate scheme which takes advantage of the effective multi-mode coupling. We develop an experimentally viable pulse protocol for a single-step gate implementation which considerably improves the gate speed compared to the previous two-qubit-gate realizations suggested for architectures utilizing a center mode. We also show numerical results on how the presence of spectator qubits affects the average two-qubit-gate fidelity, and analyse how the multi-mode coupling structure mitigates the delocalization-induced crosstalk during simultaneous single-qubit gates within the unit cell. We also provide analytical estimates for the errors caused by relaxation and dephasing during a two-qubit-gate operation, including noise terms for the multi-mode coupling structure. Our multi-mode coupling architecture results in a good balance between increased connectivity and available parallelism, especially when several interacting unit cells form a quantum processing unit. We anticipate that the obtained results pave the way towards high-connectivity quantum processors with efficient and low-overhead quantum algorithms.

quant-pharXiv:2603.10699•Mar 12, 2026•0 citations
Enhancing qubit connectivity in quantum processors is an important milestone for more efficient implementation of algorithms and novel error-correction protocols. While all-to-all connectivity has been demonstrated in quantum processors based on ion traps and neutral atoms, the exploration of enhanced connectivity in the superconducting qubit architecture has been a more recent endeavor. Here, we demonstrate the operation of a network comprising six superconducting transmon qubits with a novel, beyond-nearest-neighbor connectivity. We use a circular ring resonator with six qubits placed on the periphery at 60 ∘ spacing and capacitively coupled to it in a three-dimensional circuit-QED architecture. The ring resonator acts as a two-path coupler between each pair of qubits, and we present results demonstrating that each qubit has finite coupling to three other qubits with negligible coupling to the remaining two qubits. In this architecture, with fixed-frequency qubits and always-on coupling, we benchmark the single-qubit and two-qubit gates using randomized benchmarking. We find that our gate fidelities are strongly affected by the enhanced connectivity. We characterize the spread in qubit frequencies due to cross-Kerr terms and quantify its detrimental effect on the performance of single- and two-qubit gates. Finally, we analyze the effect of the spectator qubits using numerical simulations and propose a path toward optimizing this system and achieving > 99 % two-qubit gate fidelities across all connected qubit pairs.
Qubit coherence and gate fidelity are typically considered the two most useful metrics for characterizing a quantum processor. An equally useful metric is interqubit connectivity as it minimizes gate count and allows implementing algorithms efficiently with reduced error. However, interqubit connectivity in superconducting processors tends to be limited to nearest neighbor due to practical constraints in typical planar realizations. Here, we introduce a superconducting architecture that uses a ring resonator as a multiqubit coupling element to provide beyond nearest-neighbor connectivity without compromising on coupling uniformity or introducing fabrication complexities. We theoretically analyze the interqubit coupling as a function of frequency for a pair of qubits placed at different positions along the ring resonator and show that for carefully chosen operating frequency and angular spacing between the qubits, the variation of coupling can be minimized. For an operating frequency between the first two resonances of the ring resonator and a ${30}^{\ensuremath{\circ}}$ angular spacing for qubits, we compute interqubit coupling for a device capable of supporting up to 12 qubits with each qubit connected to nine other qubits. Using four qubits positioned strategically in the ring-resonator coupler, we experimentally verify the theoretical prediction for all possible angular spacings between the two qubits and demonstrate good agreement. Just like the standard bus resonator coupler, the coupling in the ring-resonator coupler is mediated via virtual photons since the operating frequency is far away from the resonant modes of the ring coupler. This ensures that any small internal loss in the ring resonator does not introduce decoherence during the coupling operation. We also compute extensions of this idea involving larger ring resonators and a multiring system and show the possibility of highly connected networks with larger number of qubits. Apart from being plug and play for existing superconducting architectures, our concept is scalable, adaptable to other platforms and has the potential to significantly accelerate progress in quantum computing, annealing, simulations, and error correction.
We report entangling two-qubit experiments implemented in a novel ring resonator architecture in 2D planar geometry. The ring resonator acts as a multi-path coupler between qubits and can provide beyond nearest neighbour interactions. We demonstrate pairwise coupling between three fixed-frequency transmon qubits connected to the ring resonator with measured coupling strengths (4.70 MHz, 2.80 MHz, and 2.65 MHz) in good agreement with those predicted from finite-element simulations. We implement an all-microwave controlled phase (CPHASE) gate between a pair of qubits with a gate time of 196 ns and demonstrate a two-qubit Bell state with a measured state fidelity of [Formula: see text]. Our results demonstrate the ability to entangle two qubits using the ring resonator and pave the way for creating highly connected multi-qubit networks in this architecture.
Superconducting transmon processors represent a leading platform for large-scale quantum computing due to their high gate fidelities and scalability. However, conventional qubit-coupler-qubit (QCQ) architectures face critical physical and structural bottlenecks, notably frequency crowding [spectator qubit collisions] during system scaling and inefficient mapping onto the standard surface code.To overcome these limitations, we propose a novel lattice-patch architecture that couples four fixed-frequency transmons to a single fixed-frequency coupler.This design enhances qubit connectivity and maps directly onto the surface-code lattice unit [plaquette], thereby minimizing the compilation overhead associated with logical qubit implementation. Furthermore, utilizing an entirely fixed-frequency design intrinsically eliminates susceptibility to external flux noise, ensuring robust operational stability.Multi-level numerical simulations demonstrate CNOT gate fidelities exceeding 0.98 across all six connectivity directions within the patch. Nevertheless, the complex interaction network of the four-qubit architecture induces unintended residual phase accumulation during cross-resonance driving. This parasitic effect necessitates precise calibration, achievable via virtual RzR_zRz​ gates [software phase updates]. Ultimately, our results establish the lattice-patch architecture as an efficient, robust building block for future fault-tolerant quantum computers.
Maintaining or even improving gate performance with growing numbers of parallel controlled qubits is a vital requirement for fault-tolerant quantum computing.For superconducting quantum processors, though isolated one-or two-qubit gates have been demonstrated with high fidelity, implementing these gates in parallel commonly shows worse performance.Generally, this degradation is attributed to various crosstalks between qubits, such as quantum crosstalk due to residual inter-qubit coupling.An understanding of the exact nature of these crosstalks is critical to figuring out respective mitigation schemes and improved qubit architecture designs with low crosstalk.Here we give a theoretical analysis of quantum crosstalk impact on simultaneous gate operations in a qubit architecture, where fixed-frequency transmon qubits are coupled via a tunable bus, and sub-100-ns controlled-Z (CZ) gates can be realized by applying a baseband flux pulse on the bus.Our analysis shows that for microwave-driven single-qubit gates, the dressing from the qubit-qubit coupling can cause non-negligible cross-driving errors when qubits operate near frequency collision regions.During CZ gate operations, although unwanted nearest-neighbor interactions are nominally turned off, sub-MHz parasitic next-nearest-neighbor interactions involving spectator qubits can still exist, causing considerable leakage or control error when one operates qubit systems around these parasitic resonance points.To ensure high-fidelity simultaneous operations, a request could be raised to figure out a better way to balance the gate error from target qubit systems themselves and the error from nonparticipating spectator qubits.Overall, our analysis suggests that towards useful quantum processors, the qubit architecture should be examined carefully in the context of high-fidelity simultaneous gate operations in a scalable qubit lattice.
In this work, we introduce a superconducting quantum processor architecture that uses a transmission-line resonator to implement effective all-to-all connectivity between six transmon qubits. This architecture can be used as a test bed for algorithms that benefit from high connectivity. We show that the central resonator can be used as a computational element, which offers the flexibility to encode a qubit for quantum computation or to utilize its bosonic modes that further enables quantum simulation of bosonic systems. To operate the quantum processing unit (QPU), we develop and benchmark the qubit-resonator conditional Z gate and the qubit-resonator MOVE operation. The latter allows for transferring a quantum state between one of the peripheral qubits and the computational resonator. We benchmark the QPU performance and achieve a genuinely multiqubit entangled Greenberger-Horne-Zeilinger state over all six qubits with a readout-error mitigated fidelity of F | ψ 〉 GHZ = 0.86 .
Increasing connectivity and decreasing qubit-state delocalization without compromising the speed and accuracy of elementary gate operations are topical challenges in the development of large-scale superconducting quantum computers. In this theoretical work, we study a special honeycomb qubit lattice where each qubit inside a unit cell is coupled to every other one via two dedicated tunable couplers and a common central element. This results in an effective multi-mode interaction enabling tunable, on-demand, all-to-all connectivity between each qubit pair within the unit cell. We provide a thorough analysis of the unit cell, including a proposal for a novel and efficient conditional-Z gate scheme which takes advantage of the effective multi-mode coupling. We develop an experimentally viable pulse protocol for a single-step gate implementation which considerably improves the gate speed compared to the previous two-qubit-gate realizations suggested for architectures utilizing a center mode. We also show numerical results on how the presence of spectator qubits affects the average two-qubit-gate fidelity, and analyse how the multi-mode coupling structure mitigates the delocalization-induced crosstalk during simultaneous single-qubit gates within the unit cell. We also provide analytical estimates for the errors caused by relaxation and dephasing during a two-qubit-gate operation, including noise terms for the multi-mode coupling structure. Our multi-mode coupling architecture results in a good balance between increased connectivity and available parallelism, especially when several interacting unit cells form a quantum processing unit. We anticipate that the obtained results pave the way towards high-connectivity quantum processors with efficient and low-overhead quantum algorithms.