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

npj Quantum Information

Gradiometric, fully tunable C-shunted flux qubits

Benedikt Berlitz, Alexander K. Händel, Etienne Daum, Alexey Vladimirovich Ustinov, Jürgen Lisenfeld

Published Jul 2, 2026• 0 citations
Publisher

Abstract

Abstract Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T 1 = 25 μ s and a 20-GHz wide theoretical frequency tunability range at the flux-insensitive sweet spot, of which a 3-GHz range was shown. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.

DOI
10.1038/s41534-026-01295-y

Related papers

Gradiometric, fully tunable C-shunted flux qubits

Benedikt Berlitz, Alexander K. Händel, Etienne Daum +2 more

Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T$_1$ = 25 μ s and a 20-GHz wide theoretical frequency tunability range at the flux-insensitive sweet spot, of which a 3-GHz range was shown. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.

•Jul 6, 2026•0 citations

A long-lived capacitively shunted flux qubit embedded in a 3D cavity

Leonid V. Abdurakhimov, Imran Mahboob, Hiraku Toida +2 more

We report the experimental realization of a 3D capacitively shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the range of 60–90 μs and a Hahn-echo coherence time of about 80 μs, which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling and unwanted two-level-system defects, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits, which is advantageous for applications such as quantum magnetometry and spin sensing.

arXiv:1911.04635•Applied Physics Letters•Dec 23, 2019•43 citations

Fast universal control of a flux qubit via exponentially tunable wave-function overlap

Svend Krøjer, Anders Enevold Dahl, Kasper Sangild Christensen +2 more

Fast, high-fidelity control and readout of protected superconducting qubits are fundamentally challenging due to their inherent insensitivity. We propose a flux qubit variation which enjoys a tunable level of protection against relaxation to resolve this outstanding issue. Our qubit design, the double-shunted flux qubit (DSFQ), realizes a generic double-well potential through its three-junction ring geometry. One of the junctions is tunable, making it possible to control the barrier height and thus the level of protection. We analyze single- and two-qubit gate operations that rely on lowering the barrier. We show that this is a viable method that results in high-fidelity gates as the noncomputational states are not occupied during operations. Further, we show how the effective coupling to a readout resonator can be controlled by adjusting the externally applied flux while the DSFQ is protected from decaying into the readout resonator. Finally, we also study a double-loop gradiometric version of the DSFQ which is exponentially insensitive to variations in the global magnetic field, even when the loop areas are nonidentical. Published by the American Physical Society 2024

arXiv:2303.01102•Physical Review Research•Apr 18, 2024•8 citations

Decoherence of a tunable capacitively shunted flux qubit

Robbyn Trappen, Xi Dai, Muhammet Ali Yurtalan +19 more

Quantum annealing is a method to solve optimization problems that leverages quantum tunneling in a coupled qubit system. We present a detailed study of the coherence of a tunable capacitively-shunted flux qubit, designed for coherent quantum annealing applications. We find that for high qubit frequencies, thermal noise in the bias line makes a significant contribution to the relaxation, arising from the design choice to experimentally explore both fast annealing and high-frequency control. The measured dephasing rate is primarily due to intrinsic low-frequency flux noise in the two qubit loops, with additional contribution from the low-frequency noise of control electronics used for fast annealing. Our results characterize decoherence in a realistic setup for quantum annealing and are relevant for ongoing efforts toward building superconducting quantum annealers with increased coherence. Quantum annealers hold promise of outperforming classical computers in solving hard optimization problems, but one main challenge is understanding the role of noise in quantum annealing. Here, the authors characterize the relevant noise sources in a tunable flux qubit, a building block for quantum annealers, and provide a benchmark for future work on highly-coherent quantum annealers.

arXiv:2307.13961•Communications Physics•Nov 19, 2025•3 citations

A Long-lived Tunable Qubit for Bosonic Quantum Computing

Leonid Victorovich Abdurakhimov, Imran Mahboob, Hiraku Toida +2 more

This article reviews a recent experiment that demonstrated a new hybrid type of tunable superconducting qubit consisting of a two-dimensional (2D) superconducting capacitively shunted flux qubit coupled to a 3D microwave cavity.Such a 3D hybrid flux qubit has extremely long relaxation times, which are comparable to or exceed the best values reported for 2D flux qubits.This new design of frequency-tunable qubits is uniquely suited for applications in bosonic quantum computing, which exploits multiphoton states of 3D microwave cavities.

NTT technical review•May 1, 2021•2 citations

Charge- and Flux-Insensitive Tunable Superconducting Qubit

Eyob Alebachew Sete, Matthew J. Reagor, Nicolas Didier +1 more

A central challenge in building a scalable quantum computer with superconducting qubits is simultaneously achieving long coherence times and fast two-qubit gates. Currently, fast (tunable) gates are obtained at the expense of coherence time. The authors propose a ``flatsonium'' qubit, based on the highly nonlinear fluxonium qubit, that yields both coherence and tunability---jackpot. Dephasing due to global flux noise is reduced by engineering flux-insensitive sweet spots in the qubit spectrum at the frequencies of interest, while leakage errors are eliminated by the typically large anharmonicity of its energy spectrum.

arXiv:1703.04613•Physical Review Applied•Aug 7, 2017•14 citations
Abstract Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T 1 = 25 μ s and a 20-GHz wide theoretical frequency tunability range at the flux-insensitive sweet spot, of which a 3-GHz range was shown. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.
Fully tunable flux qubits offer in-situ and independent controls of their energy potential asymmetry and tunnel barrier, making them versatile tools for quantum computation and the study of decoherence sources. However, only short coherence times have been demonstrated so far with this type of qubit. Here, we present a capacitively shunted flux qubit featuring improved relaxation times up to T1_11​ = 25 μ s and a 20-GHz wide theoretical frequency tunability range at the flux-insensitive sweet spot, of which a 3-GHz range was shown. As a model application, we demonstrate detection of two-level tunneling defects in a frequency range spanning one octave.
We report the experimental realization of a 3D capacitively shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the range of 60–90 μs and a Hahn-echo coherence time of about 80 μs, which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling and unwanted two-level-system defects, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits, which is advantageous for applications such as quantum magnetometry and spin sensing.
Fast, high-fidelity control and readout of protected superconducting qubits are fundamentally challenging due to their inherent insensitivity. We propose a flux qubit variation which enjoys a tunable level of protection against relaxation to resolve this outstanding issue. Our qubit design, the double-shunted flux qubit (DSFQ), realizes a generic double-well potential through its three-junction ring geometry. One of the junctions is tunable, making it possible to control the barrier height and thus the level of protection. We analyze single- and two-qubit gate operations that rely on lowering the barrier. We show that this is a viable method that results in high-fidelity gates as the noncomputational states are not occupied during operations. Further, we show how the effective coupling to a readout resonator can be controlled by adjusting the externally applied flux while the DSFQ is protected from decaying into the readout resonator. Finally, we also study a double-loop gradiometric version of the DSFQ which is exponentially insensitive to variations in the global magnetic field, even when the loop areas are nonidentical. Published by the American Physical Society 2024
Quantum annealing is a method to solve optimization problems that leverages quantum tunneling in a coupled qubit system. We present a detailed study of the coherence of a tunable capacitively-shunted flux qubit, designed for coherent quantum annealing applications. We find that for high qubit frequencies, thermal noise in the bias line makes a significant contribution to the relaxation, arising from the design choice to experimentally explore both fast annealing and high-frequency control. The measured dephasing rate is primarily due to intrinsic low-frequency flux noise in the two qubit loops, with additional contribution from the low-frequency noise of control electronics used for fast annealing. Our results characterize decoherence in a realistic setup for quantum annealing and are relevant for ongoing efforts toward building superconducting quantum annealers with increased coherence. Quantum annealers hold promise of outperforming classical computers in solving hard optimization problems, but one main challenge is understanding the role of noise in quantum annealing. Here, the authors characterize the relevant noise sources in a tunable flux qubit, a building block for quantum annealers, and provide a benchmark for future work on highly-coherent quantum annealers.
This article reviews a recent experiment that demonstrated a new hybrid type of tunable superconducting qubit consisting of a two-dimensional (2D) superconducting capacitively shunted flux qubit coupled to a 3D microwave cavity.Such a 3D hybrid flux qubit has extremely long relaxation times, which are comparable to or exceed the best values reported for 2D flux qubits.This new design of frequency-tunable qubits is uniquely suited for applications in bosonic quantum computing, which exploits multiphoton states of 3D microwave cavities.
A central challenge in building a scalable quantum computer with superconducting qubits is simultaneously achieving long coherence times and fast two-qubit gates. Currently, fast (tunable) gates are obtained at the expense of coherence time. The authors propose a ``flatsonium'' qubit, based on the highly nonlinear fluxonium qubit, that yields both coherence and tunability---jackpot. Dephasing due to global flux noise is reduced by engineering flux-insensitive sweet spots in the qubit spectrum at the frequencies of interest, while leakage errors are eliminated by the typically large anharmonicity of its energy spectrum.