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

arXiv:2603.23636 · Physical Review Research

Characterization and Comparison of Energy Relaxation in Fluxonium Qubits

Kate Azar, Lamia Ateshian, Mallika T. Randeria, Renée DePencier Piñero, Jeffrey M. Gertler, Junyoung An, Felipe Contipelli, Leon Ding, Michael Gingras, Kevin Grossklaus, Max Hays, Thomas M. Hazard, Junghyun Kim, Bethany M. Niedzielski, Hannah Stickler, Kunal L. Tiwari, Helin Zhang, Jeffrey A. Grover, Jonilyn L. Yoder, Mollie E. Schwartz, William D. Oliver, Kyle Serniak

quant-phcond-mat.mes-hallSubmitted Mar 24, 2026• Announced Mar 26, 2026• 0 citations
v1 · Mar 24, 2026v2 · Sep 8, 2026
arXivHTMLPDF

Abstract

Fluxonium superconducting qubits have demonstrated long coherence times and high single- and two-qubit gate fidelities, making them a favorable building block for superconducting quantum processors. We investigate the dominant limitations to fluxonium qubit energy relaxation time $T_1$ using a set of eight planar, aluminum-on-silicon qubits. We find that a circuit-based model for capacitive dielectric loss best captures the frequency dependence of $T_1$, which we analyze within both a two-level and a six-level energy relaxation model. We convert the measured $T_1$ into an effective capacitive quality factor $Q_\mathrm{C}^{\mathrm{eff}}$ to compare qubits on equal footing, accounting for independently estimated contributions from $1/f$ flux noise and radiative loss to the control and readout circuitry. We apply this methodology to compare qubits from two fabrication processes: a baseline process and one that applies a fluorine-based wet treatment prior to Josephson junction deposition. We resolve a small improvement of (13.8 $\pm$ 8.4$)\%$ in the process mean $Q_\mathrm{C}^{\mathrm{eff}}$, indicating that the fluorine treatment may have reduced loss from the metal-substrate interface, but did not address the primary source of loss in these fluxonium qubits.

Comments
25 pages, 20 figures

Related papers

Characterization of Loss Mechanisms in a Fluxonium Qubit

Hantao Sun, Feng Wu, Hsiang‐Sheng Ku +9 more

Using a fluxonium qubit with in situ tunability of its Josephson energy, we characterize its energy relaxation at different flux biases as well as different Josephson energy values. The relaxation rate at qubit energy values, ranging more than 1 order of magnitude around the thermal energy ${k}_{B}T$, can be quantitatively explained by a combination of dielectric loss and $1/f$ flux noise with a crossover point. The amplitude of the $1/f$ flux noise is consistent with that extracted from the qubit dephasing measurements at the flux sensitive points. In the dielectric loss dominant regime, the loss is consistent with that arising from the electric dipole interaction with two-level-system (TLS) defects. In particular, by increasing the Josephson energy, thus decreasing the qubit frequency at the flux insensitive spot, we find that the qubit exhibits increasingly weaker coupling to TLS defects, which is desirable for high-fidelity quantum operations. Our work establishes a generic noise model for the design of high coherence fluxonium and future generations of noise resilient superconducting qubits.

arXiv:2302.08110•Physical Review Applied•Sep 8, 2023•23 citations

High-Coherence Fluxonium Qubit

Long B. Nguyen, Yen‐Hsiang Lin, Aaron Somoroff +3 more

We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T 2 > 100 μs (T 2 > 400 μs in one device).Moreover, given the state-of-the-art values of the surface loss tangent and the 1=f flux-noise amplitude, the coherence time can be further improved beyond 1 ms.Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit-over 10 2 here-must be minimized for best qubit coherence.We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.

arXiv:1810.11006•Physical Review X•Nov 25, 2019•290 citations

Dissipation in superconducting quantum circuits

Karthik Srikanth Bharadwaj

In a world transitioning to the era of second quantum revolution, avoiding processes such as dissipation, which annihilates the quantum state unnecessarily, is crucial. Therefore, understanding the sources of loss responsible for the relaxation in a given quantum system is highly relevant. Since superconducting quantum circuits are the favorite contenders for quantum computing, two directions are followed in this work to study the sources of loss in such circuits.1.Superconducting high quality factor resonators have several applications in quantum technology. They are also the best test beds to study losses. In this work, the study of losses is carried out by realizing Aluminum resonators of different mean free path and capacitances. The behavior of the internal quality factor as a function of applied microwave power and temperature is investigated. The study focuses on the quasiparticle and dielectric losses in the resonators which have been argued to be the main cause of losses in superconducting quantum circuits. A comparative analysis of ground plane loss is also carried out in order to estimate the losses caused by the metallic ground plane. 2.Fluxonium qubit is one of the state of the art superconducting qubits. Their relaxation times are as high as 8-10 milliseconds in 3D architecture. To achieve large scalability, the qubits must be fabricated using 2D architecture. The change in architecture places new constraints on traditional sources of loss while also introducing new ones. The sources of loss causing qubit relaxation in 2D Fluxonium qubit are the area of emphasis in this work. Furthermore, the causes of noise in such a qubit are examined in order to understand the limitations on the coherence of the qubit.

•Oct 11, 2021•0 citations

Temperature and Magnetic-Field Dependence of Energy Relaxation in a Fluxonium Qubit

Lamia Ateshian, Max Hays, David A. Rower +14 more

Noise from material defects at device interfaces is known to limit the coherence of superconducting circuits, yet our understanding of the defect origins and noise mechanisms remains incomplete. Here we investigate the temperature and in-plane magnetic-field dependence of energy relaxation in a low-frequency fluxonium qubit, where the sensitivity to flux noise and charge noise arising from dielectric loss can be tuned by applied flux. We observe an approximately linear scaling of flux noise with temperature $T$ and a power-law dependence of dielectric loss $T^3$ up to 100 mK. Additionally, we find that the dielectric-loss-limited $T_1$ decreases with weak in-plane magnetic fields, suggesting a potential magnetic-field response of the underlying charge-coupled defects. We implement a multi-level decoherence model in our analysis, motivated by the widely tunable matrix elements and transition energies approaching the thermal energy scale in our system. These findings offer insight for fluxonium coherence modeling and should inform microscopic theories of intrinsic noise in superconducting circuits.

arXiv:2507.01175•Jul 1, 2025•0 citations

Integer Fluxonium Qubit

Raymond Mencia, Wei-Ju Lin, Hyunheung Cho +2 more

We describe a superconducting qubit derived from operating a properly designed fluxonium circuit in a zero magnetic field. The qubit has a frequency of about 4 GHz and an energy relaxation quality factor Q ≈ 0.7 × 10 7 , even though the dielectric loss quality factor of the circuit components is in the low- 10 5 range. The Ramsey coherence time exceeds 100 μ s , and the average fidelity of Clifford gates is benchmarked to F > 0.999 . These figures are expected to improve with optimized fabrication and measurement procedures. Our work establishes a ready-to-use “partially protected” superconducting qubit functioning in the frequency range of conventional transmons. Published by the American Physical Society 2024

arXiv:2403.16780•PRX Quantum•Nov 4, 2024•20 citations

Dissipation and Noise in Granular Aluminum Fluxonium Qubits

Patrick Paluch

The dream of universal quantum computing promises to revolutionize many fields due to its potential to drastically speed up certain calculations compared to classical computers. This has naturally attracted attention and investment. For example, in 2024, Google unveiled their Willow superconducting quantum processor, achieving a significant milestone by demonstrating quantum error correction below the surface code threshold. Despite these advances, the road to a fully fledged quantum computer is still uncharted, and current technology is not sufficient to get there. Therefore, unconventional materials hold great potential for addressing challenges faced by traditional approaches. This work focuses on fluxonium qubits made from the disordered superconductor granular aluminum (grAl), aiming to gain deeper insights into underlying loss mechanisms and noise sources. One of the main results of this work is the identification of inductive loss as the dominant decoherence mechanism in grAl fluxonium qubits with frequencies lower than 300 MHz at the half flux bias. The observed inductive loss tangent aligns with previously measured single-photon internal quality factors of grAl resonators, and the energy relaxation profiles are well described by a combination of inductive, dielectric, and Purcell loss. The study endeavors to deepen the understanding of decoherence by refining loss models via the quantum fluctuation-dissipation theorem and proposing an improved framework for Purcell loss. A comparison with other qubit materials underscores the universal nature of inductive loss in grAl fluxoniums, in contrast to well established Josephson junction array (JJA) based fluxoniums which show no such limitation at even lower qubit frequencies. The second outcome of this thesis is the validation of flexible striplines (`flexlines') for the use in future cryogenic microwave setups. This enables at least an order of magnitude increase in the density of microwave input circuitry without thermally overloading the cryostat, paving the way for increasingly complex superconducting detectors and quantum devices. The study found no significant differences in qubit performance between setups using flexlines and conventional coaxial cables. Passive heat load measurements indicated comparable photon shot noise-induced dephasing for both setups. The introduced heating pulse method demonstrated a faster thermalization time for flexlines compared to coaxial cables, and it can serve as a simple health check for other groups as well. An extended thermal model was developed to better understand the contributions of various attenuators in the input chains to the measured heat loads and to propose improved input chains. To further exploit the versatility of grAl for fluxonium qubits, the investigation of the microscopic origin of the limiting inductive loss is of interest to develop potential suppression methods. JJA-based fluxoniums could play a crucial role in this context, as particularly long arrays with increased plasma frequencies can emulate the properties of grAl while leveraging their better-understood behavior.

•Jan 1, 2026•0 citations
Fluxonium superconducting qubits have demonstrated long coherence times and high single- and two-qubit gate fidelities, making them a favorable building block for superconducting quantum processors. We investigate the dominant limitations to fluxonium qubit energy relaxation time T1T_1T1​ using a set of eight planar, aluminum-on-silicon qubits. We find that a circuit-based model for capacitive dielectric loss best captures the frequency dependence of T1T_1T1​, which we analyze within both a two-level and a six-level energy relaxation model. We convert the measured T1T_1T1​ into an effective capacitive quality factor QCeffQ_\mathrm{C}^{\mathrm{eff}}QCeff​ to compare qubits on equal footing, accounting for independently estimated contributions from 1/f1/f1/f flux noise and radiative loss to the control and readout circuitry. We apply this methodology to compare qubits from two fabrication processes: a baseline process and one that applies a fluorine-based wet treatment prior to Josephson junction deposition. We resolve a small improvement of (13.8 ±\pm± 8.4)%)\%)% in the process mean QCeffQ_\mathrm{C}^{\mathrm{eff}}QCeff​, indicating that the fluorine treatment may have reduced loss from the metal-substrate interface, but did not address the primary source of loss in these fluxonium qubits.
Using a fluxonium qubit with in situ tunability of its Josephson energy, we characterize its energy relaxation at different flux biases as well as different Josephson energy values. The relaxation rate at qubit energy values, ranging more than 1 order of magnitude around the thermal energy kBT{k}_{B}TkB​T, can be quantitatively explained by a combination of dielectric loss and 1/f1/f1/f flux noise with a crossover point. The amplitude of the 1/f1/f1/f flux noise is consistent with that extracted from the qubit dephasing measurements at the flux sensitive points. In the dielectric loss dominant regime, the loss is consistent with that arising from the electric dipole interaction with two-level-system (TLS) defects. In particular, by increasing the Josephson energy, thus decreasing the qubit frequency at the flux insensitive spot, we find that the qubit exhibits increasingly weaker coupling to TLS defects, which is desirable for high-fidelity quantum operations. Our work establishes a generic noise model for the design of high coherence fluxonium and future generations of noise resilient superconducting qubits.
We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T 2 > 100 μs (T 2 > 400 μs in one device).Moreover, given the state-of-the-art values of the surface loss tangent and the 1=f flux-noise amplitude, the coherence time can be further improved beyond 1 ms.Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit-over 10 2 here-must be minimized for best qubit coherence.We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.
In a world transitioning to the era of second quantum revolution, avoiding processes such as dissipation, which annihilates the quantum state unnecessarily, is crucial. Therefore, understanding the sources of loss responsible for the relaxation in a given quantum system is highly relevant. Since superconducting quantum circuits are the favorite contenders for quantum computing, two directions are followed in this work to study the sources of loss in such circuits.1.Superconducting high quality factor resonators have several applications in quantum technology. They are also the best test beds to study losses. In this work, the study of losses is carried out by realizing Aluminum resonators of different mean free path and capacitances. The behavior of the internal quality factor as a function of applied microwave power and temperature is investigated. The study focuses on the quasiparticle and dielectric losses in the resonators which have been argued to be the main cause of losses in superconducting quantum circuits. A comparative analysis of ground plane loss is also carried out in order to estimate the losses caused by the metallic ground plane. 2.Fluxonium qubit is one of the state of the art superconducting qubits. Their relaxation times are as high as 8-10 milliseconds in 3D architecture. To achieve large scalability, the qubits must be fabricated using 2D architecture. The change in architecture places new constraints on traditional sources of loss while also introducing new ones. The sources of loss causing qubit relaxation in 2D Fluxonium qubit are the area of emphasis in this work. Furthermore, the causes of noise in such a qubit are examined in order to understand the limitations on the coherence of the qubit.
Noise from material defects at device interfaces is known to limit the coherence of superconducting circuits, yet our understanding of the defect origins and noise mechanisms remains incomplete. Here we investigate the temperature and in-plane magnetic-field dependence of energy relaxation in a low-frequency fluxonium qubit, where the sensitivity to flux noise and charge noise arising from dielectric loss can be tuned by applied flux. We observe an approximately linear scaling of flux noise with temperature TTT and a power-law dependence of dielectric loss T3T^3T3 up to 100 mK. Additionally, we find that the dielectric-loss-limited T1T_1T1​ decreases with weak in-plane magnetic fields, suggesting a potential magnetic-field response of the underlying charge-coupled defects. We implement a multi-level decoherence model in our analysis, motivated by the widely tunable matrix elements and transition energies approaching the thermal energy scale in our system. These findings offer insight for fluxonium coherence modeling and should inform microscopic theories of intrinsic noise in superconducting circuits.
We describe a superconducting qubit derived from operating a properly designed fluxonium circuit in a zero magnetic field. The qubit has a frequency of about 4 GHz and an energy relaxation quality factor Q ≈ 0.7 × 10 7 , even though the dielectric loss quality factor of the circuit components is in the low- 10 5 range. The Ramsey coherence time exceeds 100 μ s , and the average fidelity of Clifford gates is benchmarked to F > 0.999 . These figures are expected to improve with optimized fabrication and measurement procedures. Our work establishes a ready-to-use “partially protected” superconducting qubit functioning in the frequency range of conventional transmons. Published by the American Physical Society 2024
The dream of universal quantum computing promises to revolutionize many fields due to its potential to drastically speed up certain calculations compared to classical computers. This has naturally attracted attention and investment. For example, in 2024, Google unveiled their Willow superconducting quantum processor, achieving a significant milestone by demonstrating quantum error correction below the surface code threshold. Despite these advances, the road to a fully fledged quantum computer is still uncharted, and current technology is not sufficient to get there. Therefore, unconventional materials hold great potential for addressing challenges faced by traditional approaches. This work focuses on fluxonium qubits made from the disordered superconductor granular aluminum (grAl), aiming to gain deeper insights into underlying loss mechanisms and noise sources. One of the main results of this work is the identification of inductive loss as the dominant decoherence mechanism in grAl fluxonium qubits with frequencies lower than 300 MHz at the half flux bias. The observed inductive loss tangent aligns with previously measured single-photon internal quality factors of grAl resonators, and the energy relaxation profiles are well described by a combination of inductive, dielectric, and Purcell loss. The study endeavors to deepen the understanding of decoherence by refining loss models via the quantum fluctuation-dissipation theorem and proposing an improved framework for Purcell loss. A comparison with other qubit materials underscores the universal nature of inductive loss in grAl fluxoniums, in contrast to well established Josephson junction array (JJA) based fluxoniums which show no such limitation at even lower qubit frequencies. The second outcome of this thesis is the validation of flexible striplines (`flexlines') for the use in future cryogenic microwave setups. This enables at least an order of magnitude increase in the density of microwave input circuitry without thermally overloading the cryostat, paving the way for increasingly complex superconducting detectors and quantum devices. The study found no significant differences in qubit performance between setups using flexlines and conventional coaxial cables. Passive heat load measurements indicated comparable photon shot noise-induced dephasing for both setups. The introduced heating pulse method demonstrated a faster thermalization time for flexlines compared to coaxial cables, and it can serve as a simple health check for other groups as well. An extended thermal model was developed to better understand the contributions of various attenuators in the input chains to the measured heat loads and to propose improved input chains. To further exploit the versatility of grAl for fluxonium qubits, the investigation of the microscopic origin of the limiting inductive loss is of interest to develop potential suppression methods. JJA-based fluxoniums could play a crucial role in this context, as particularly long arrays with increased plasma frequencies can emulate the properties of grAl while leveraging their better-understood behavior.