Efforts to scale up superconducting processors that employ flux qubits face numerous challenges, among which is the crosstalk created by neighboring flux lines, which are necessary to bias the qubits at the zero-field and Φ $_0$ /2 sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux-locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux bias within Φ $_{eff}$ =−3 ×10 $^{−4}$ ⁢Φ $_0$ from the target, corresponding to only 15% degradation in 𝑇 $_{2,𝐸}$ when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing crosstalk and potentially eliminating the need for local flux bias.Efforts to scale up superconducting processors that employ flux qubits face numerous challenges, among which is the crosstalk created by neighboring flux lines, which are necessary to bias the qubits at the zero-field and Φ$_0$/2 sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux-locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux bias within Φ$_{eff}$ =−3 ×10$^{−4}$ ⁢Φ$_0$ from the target, corresponding to only 15% degradation in 𝑇$_{2,𝐸}$ when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing crosstalk and potentially eliminating the need for local flux bias.

Jan 1, 20250 citations

Flux-trapping fluxonium qubit

Kotaro Hida, Kohei Matsuura, Shu Watanabe +1 more

In pursuit of superconducting quantum computing, fluxonium qubits have recently garnered attention for their large anharmonicity and high coherence at the sweet spot. Towards the large-scale integration of fluxonium qubits, a major obstacle is the need for precise external magnetic flux bias: To achieve high performance at its sweet spot, each qubit requires a DC bias line. However, such lines inductively coupled to the qubits bring in additional wiring overhead, crosstalk, heating, and decoherence, necessitating measures for mitigating the problems. In this work, we propose a flux-trapping fluxonium qubit, which, by leveraging fluxoid quantization, enables the optimal phase biasing without using external magnetic flux control at the operating temperature. We introduce the design and working principle, and demonstrate the phase biasing achieved through fluxoid quantization.In pursuit of superconducting quantum computing, fluxonium qubits have recently garnered attention for their large anharmonicity and high coherence at the sweet spot. Towards the large-scale integration of fluxonium qubits, a major obstacle is the need for precise external magnetic flux bias: To achieve high performance at its sweet spot, each qubit requires a DC bias line. However, such lines inductively coupled to the qubits bring in additional wiring overhead, crosstalk, heating, and decoherence, necessitating measures for mitigating the problems. In this work, we propose a flux-trapping fluxonium qubit, which, by leveraging fluxoid quantization, enables the optimal phase biasing without using external magnetic flux control at the operating temperature. We introduce the design and working principle, and demonstrate the phase biasing achieved through fluxoid quantization.

arXiv:2505.02416Physical Review AppliedDec 9, 20251 citation

Operating in a deep underground facility improves the locking of gradiometric fluxonium qubits at the sweet spots

Daria Gusenkova, Francesco Valenti, Martin Spiecker +14 more

We demonstrate flux-bias locking and operation of a gradiometric fluxonium artificial atom using two symmetric granular aluminum (grAl) loops to implement the superinductor. The gradiometric fluxonium shows two orders of magnitude suppression of sensitivity to homogeneous magnetic fields, which can be an asset for hybrid quantum systems requiring strong magnetic field biasing. By cooling down the device in an external magnetic field while crossing the metal-to-superconductor transition, the gradiometric fluxonium can be locked either at 0 or Φ0/2 effective flux bias, corresponding to an even or odd number of trapped fluxons, respectively. At mK temperatures, the fluxon parity prepared during initialization survives to magnetic field bias exceeding 100 Φ0. However, even for states biased in the vicinity of 1 Φ0, we observe unexpectedly short fluxon lifetimes of a few hours, which cannot be explained by thermal or quantum phase slips. When operating in a deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes increase to days, indicating that ionizing events activate phase slips in the grAl superinductor.We demonstrate flux-bias locking and operation of a gradiometric fluxonium artificial atom using two symmetric granular aluminum (grAl) loops to implement the superinductor. The gradiometric fluxonium shows two orders of magnitude suppression of sensitivity to homogeneous magnetic fields, which can be an asset for hybrid quantum systems requiring strong magnetic field biasing. By cooling down the device in an external magnetic field while crossing the metal-to-superconductor transition, the gradiometric fluxonium can be locked either at 0 or Φ0/2 effective flux bias, corresponding to an even or odd number of trapped fluxons, respectively. At mK temperatures, the fluxon parity prepared during initialization survives to magnetic field bias exceeding 100 Φ0. However, even for states biased in the vicinity of 1 Φ0, we observe unexpectedly short fluxon lifetimes of a few hours, which cannot be explained by thermal or quantum phase slips. When operating in a deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes increase to days, indicating that ionizing events activate phase slips in the grAl superinductor.

arXiv:2201.09779Applied Physics LettersJan 31, 202223 citations

Fluxonium qubits with multiple degrees of freedom

Denis Bénâtre

According to Greek myths, Dionysus was born from the thigh of his father Zeus and promised to a great fate. He, the son of a mortal mother, was destined to become god of wine, festivities and frenzy. Physics too has a Dionysus, heir to the Nobel lineage of condensed matter, called to a bright quantum computing future and sometimes caught up in the media frenzy: the field of superconducting qubits. Like the god of madness and folly, qubits are unpredictable and difficult to control. Thus, generations of students and advisors have worked on designing, engineering and operating single-degree-of-freedom circuits, achieving high levels of accuracy in qubit manipulation and readout. Today, research builds upon these systems to add degrees of freedom, in particular in quantum processors and noise-protected qubits. This work fits within this line of research and aims to realize a protected qubit based on a proposed four-loop fluxonium circuit. In order to work as intended, the system first needs to trap and retain a persistent current equivalent to one flux quantum in the circuit, referred to as a fluxon. Second, it needs to be as insensitive as possible to the charge offset on its central island. These two requirements constitute the two challenges tackled in this thesis. The first challenge is to trap and retain a fluxon in the circuit. To do this, we propose to cool down the device in a magnetic field corresponding to one flux quantum in its most external loop. This flux-locking technique is demonstrated on gradiometric fluxoniums, in which an aluminum ring retains the trapped fluxon indefinitely. On average, four out of five qubits are locked simultaneously in a single waveguide. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux-bias within $\Phi_\mathrm{eff} = -3\times10^{-4} \,\Phi_0$ from the target, corresponding to only 15% degradation in coherence time $T_{2,\mathrm{E}}$ when operated in zero external field. Using such a gradiometric geometry for flux-tunable elements in quantum processors would reduce flux crosstalk and potentially eliminate the need for passive biasing. The second challenge is the charge-offset noise arising from the central island in the design. To address it, we engineer charge-tunable fluxoniums in which the main junction is replaced by two junctions in series, separated by an island. In such a system, quasiparticles tunneling through either junction change the charge offset by one elementary charge $e$ , a process called parity switching. By employing state-of-the-art techniques of infrared filtering and gap engineering, we increase the parity switching time to twenty seconds. We also identify other phenomena, such as a parity saturation by microwave pumping, allowing us to prepare the qubit in odd or even parity with above 80% fidelity. Finally, these devices can be used to uncover the coupling mechanism of qubits to long-lived two-level systems in their environment. Overall, this thesis paves the way to studying multiple degrees of freedom, be it in multi-qubit processors or in a single superconducting circuit based on the fluxonium architecture. It highlights the phenomena that need to be understood and mitigated in order to translate protected qubit concepts into experimental reality.According to Greek myths, Dionysus was born from the thigh of his father Zeus and promised to a great fate. He, the son of a mortal mother, was destined to become god of wine, festivities and frenzy. Physics too has a Dionysus, heir to the Nobel lineage of condensed matter, called to a bright quantum computing future and sometimes caught up in the media frenzy: the field of superconducting qubits. Like the god of madness and folly, qubits are unpredictable and difficult to control. Thus, generations of students and advisors have worked on designing, engineering and operating single-degree-of-freedom circuits, achieving high levels of accuracy in qubit manipulation and readout. Today, research builds upon these systems to add degrees of freedom, in particular in quantum processors and noise-protected qubits. This work fits within this line of research and aims to realize a protected qubit based on a proposed four-loop fluxonium circuit. In order to work as intended, the system first needs to trap and retain a persistent current equivalent to one flux quantum in the circuit, referred to as a fluxon. Second, it needs to be as insensitive as possible to the charge offset on its central island. These two requirements constitute the two challenges tackled in this thesis. The first challenge is to trap and retain a fluxon in the circuit. To do this, we propose to cool down the device in a magnetic field corresponding to one flux quantum in its most external loop. This flux-locking technique is demonstrated on gradiometric fluxoniums, in which an aluminum ring retains the trapped fluxon indefinitely. On average, four out of five qubits are locked simultaneously in a single waveguide. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux-bias within $\Phi_\mathrm{eff} = -3\times10^{-4} \,\Phi_0$ from the target, corresponding to only 15% degradation in coherence time $T_{2,\mathrm{E}}$ when operated in zero external field. Using such a gradiometric geometry for flux-tunable elements in quantum processors would reduce flux crosstalk and potentially eliminate the need for passive biasing. The second challenge is the charge-offset noise arising from the central island in the design. To address it, we engineer charge-tunable fluxoniums in which the main junction is replaced by two junctions in series, separated by an island. In such a system, quasiparticles tunneling through either junction change the charge offset by one elementary charge $e$, a process called parity switching. By employing state-of-the-art techniques of infrared filtering and gap engineering, we increase the parity switching time to twenty seconds. We also identify other phenomena, such as a parity saturation by microwave pumping, allowing us to prepare the qubit in odd or even parity with above 80% fidelity. Finally, these devices can be used to uncover the coupling mechanism of qubits to long-lived two-level systems in their environment. Overall, this thesis paves the way to studying multiple degrees of freedom, be it in multi-qubit processors or in a single superconducting circuit based on the fluxonium architecture. It highlights the phenomena that need to be understood and mitigated in order to translate protected qubit concepts into experimental reality.

Oct 21, 20260 citations

Dynamical Sweet Spot Engineering via Two-Tone Flux Modulation of Superconducting Qubits

Joseph A. Valery, Shoumik Chowdhury, Glenn Jones +1 more

Current superconducting quantum processors require strategies for coping with material defects and imperfect parameter targeting in order to scale up while maintaining high performance.To that end, in situ control of qubit frequencies with magnetic flux can be used to avoid spurious resonances.However, increased dephasing due to 1/f flux noise limits performance at all of these operating points except for noise-protected sweet spots, which are sparse under dc flux bias and monochromatic flux modulation.Here we experimentally demonstrate that two-tone flux modulation can be used to create a continuum of dynamical sweet spots, greatly expanding the range of qubit frequencies achievable while first-order insensitive to slow flux noise.To illustrate some advantages of this flexibility, we use bichromatic flux control to reduce the error rates and gate times of parametric entangling operations between transmons.Independent of the gate scheme, the ability to use flux control to freely select qubit frequencies while maintaining qubit coherence represents an important step forward in the robustness and scalability of near-term superconducting qubit devices.Current superconducting quantum processors require strategies for coping with material defects and imperfect parameter targeting in order to scale up while maintaining high performance.To that end, in situ control of qubit frequencies with magnetic flux can be used to avoid spurious resonances.However, increased dephasing due to 1/f flux noise limits performance at all of these operating points except for noise-protected sweet spots, which are sparse under dc flux bias and monochromatic flux modulation.Here we experimentally demonstrate that two-tone flux modulation can be used to create a continuum of dynamical sweet spots, greatly expanding the range of qubit frequencies achievable while first-order insensitive to slow flux noise.To illustrate some advantages of this flexibility, we use bichromatic flux control to reduce the error rates and gate times of parametric entangling operations between transmons.Independent of the gate scheme, the ability to use flux control to freely select qubit frequencies while maintaining qubit coherence represents an important step forward in the robustness and scalability of near-term superconducting qubit devices.

arXiv:2104.07835PRX QuantumMay 18, 202230 citations

Pareto Front Engineering of Dynamical Sweet Spots in Superconducting Qubits

Zhen Yang, Shan Jin, Yajie Hao +4 more

Operating superconducting qubits at dynamical sweet spots (DSSs) suppresses decoherence from low-frequency flux noise. A key open question is how long coherence can be extended under this strategy and what fundamental limits constrain it. Here we introduce a fully parameterized, multi-objective periodic-flux modulation framework that simultaneously optimizes energy relaxation $T_1$ and pure dephasing $T_φ$ , thereby quantifying the tradeoff between them. For fluxonium qubits with realistic noise spectra, our method enhances $T_φ$ by a factor of 3-5 compared with existing DSS strategies while maintaining $T_1$ in the hundred-microsecond range. We further prove that, although DSSs eliminate first-order sensitivity to low-frequency noise, relaxation rate cannot be reduced arbitrarily close to zero, establishing an upper bound on achievable $T_1$ . At the optimized working points, we identify double-DSS regions that are insensitive to both DC and AC flux, providing robust operating bands for experiments. As applications, we design single- and two-qubit control protocols at these operating points and numerically demonstrate high-fidelity gate operations. These results establish a general and useful framework for Pareto-front engineering of DSSs that substantially improves coherence and gate performance in superconducting qubits.Operating superconducting qubits at dynamical sweet spots (DSSs) suppresses decoherence from low-frequency flux noise. A key open question is how long coherence can be extended under this strategy and what fundamental limits constrain it. Here we introduce a fully parameterized, multi-objective periodic-flux modulation framework that simultaneously optimizes energy relaxation $T_1$ and pure dephasing $T_φ$, thereby quantifying the tradeoff between them. For fluxonium qubits with realistic noise spectra, our method enhances $T_φ$ by a factor of 3-5 compared with existing DSS strategies while maintaining $T_1$ in the hundred-microsecond range. We further prove that, although DSSs eliminate first-order sensitivity to low-frequency noise, relaxation rate cannot be reduced arbitrarily close to zero, establishing an upper bound on achievable $T_1$. At the optimized working points, we identify double-DSS regions that are insensitive to both DC and AC flux, providing robust operating bands for experiments. As applications, we design single- and two-qubit control protocols at these operating points and numerically demonstrate high-fidelity gate operations. These results establish a general and useful framework for Pareto-front engineering of DSSs that substantially improves coherence and gate performance in superconducting qubits.

quant-pharXiv:2601.19209Communications PhysicsJan 28, 20260 citations

_0

/2 sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux-locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux bias within Φ

_{eff}

=−3 ×10

^{−4}

⁢Φ

_0

from the target, corresponding to only 15% degradation in 𝑇

_{2,𝐸}

when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing crosstalk and potentially eliminating the need for local flux bias.

\Phi_\mathrm{eff} = -3\times10^{-4} \,\Phi_0

from the target, corresponding to only 15% degradation in coherence time

T_{2,\mathrm{E}}

when operated in zero external field. Using such a gradiometric geometry for flux-tunable elements in quantum processors would reduce flux crosstalk and potentially eliminate the need for passive biasing. The second challenge is the charge-offset noise arising from the central island in the design. To address it, we engineer charge-tunable fluxoniums in which the main junction is replaced by two junctions in series, separated by an island. In such a system, quasiparticles tunneling through either junction change the charge offset by one elementary charge

e

, a process called parity switching. By employing state-of-the-art techniques of infrared filtering and gap engineering, we increase the parity switching time to twenty seconds. We also identify other phenomena, such as a parity saturation by microwave pumping, allowing us to prepare the qubit in odd or even parity with above 80% fidelity. Finally, these devices can be used to uncover the coupling mechanism of qubits to long-lived two-level systems in their environment. Overall, this thesis paves the way to studying multiple degrees of freedom, be it in multi-qubit processors or in a single superconducting circuit based on the fluxonium architecture. It highlights the phenomena that need to be understood and mitigated in order to translate protected qubit concepts into experimental reality.

T_1

and pure dephasing

T_φ

, thereby quantifying the tradeoff between them. For fluxonium qubits with realistic noise spectra, our method enhances

T_φ

by a factor of 3-5 compared with existing DSS strategies while maintaining

T_1

in the hundred-microsecond range. We further prove that, although DSSs eliminate first-order sensitivity to low-frequency noise, relaxation rate cannot be reduced arbitrarily close to zero, establishing an upper bound on achievable

T_1

. At the optimized working points, we identify double-DSS regions that are insensitive to both DC and AC flux, providing robust operating bands for experiments. As applications, we design single- and two-qubit control protocols at these operating points and numerically demonstrate high-fidelity gate operations. These results establish a general and useful framework for Pareto-front engineering of DSSs that substantially improves coherence and gate performance in superconducting qubits.