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Dissipation and Noise in Granular Aluminum Fluxonium Qubits

Patrick Paluch

Published Jan 1, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.5445/ir/1000193593

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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.
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.
With the demand for scalable cryogenic microwave circuitry continuously rising, recently developed flexible microwave striplines offer the tantalizing perspective of increasing the cabling density by an order of magnitude without thermally overloading the cryostat. We use a superconducting quantum circuit to test the thermalization of input flex cables with integrated 60 dB of attenuation distributed at various temperature stages. From the measured decoherence rate of a superconducting fluxonium qubit, we estimate a residual population of the readout resonator of (2.2±0.9)×10−3 photons and we measure a 0.28 ms thermalization time for the flexible stripline attenuators. Furthermore, we confirm that the qubit reaches an effective temperature of 26.4 mK, close to the base temperature of the cryostat, practically the same as when using a conventional semi-rigid coaxial cable setup.
Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (grAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/□ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from grAl with a room temperature resistivity of 4×10^{3} μΩ cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of Q_{i}=10^{5} in the single photon regime, and we demonstrate that nonequilibrium quasiparticles (QPs) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼20 s. The current level of coherence of grAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of nonequilibrium QPs.
With the demand for scalable cryogenic microwave circuitry continuously rising, recently developed flexible microwave striplines offer the tantalyzing perspective of increasing the cabling density by an order of magnitude without thermally overloading the cryostat. We use a superconducting quantum circuit to test the thermalization of input flex cables with integrated 60 60\,60dB of attenuation distributed at various temperature stages. From the measured decoherence rate of a superconducting fluxonium qubit, we estimate a residual population of the readout resonator below 3.5⋅10−33.5\cdot10^{-3}3.5⋅10−3 photons and we measure a 0.28 0.28\,0.28ms thermalization time for the flexible stripline attenuators. Furthermore, we confirm that the qubit reaches an effective temperature of 26.4 26.4\,26.4mK, close to the base temperature of the cryostat, practically the same as when using a conventional semi-rigid coaxial cable setup.
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.