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arXiv:2505.02416 · Physical Review Applied

Flux-trapping fluxonium qubit

Kotaro Hida, Kohei Matsuura, Shu Watanabe, Yasunobu Nakamura

Published Dec 9, 2025• 1 citation
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Abstract

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.

DOI
10.1103/b99c-y22b

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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.
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 Φ0_00​/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 Φeff_{eff}eff​ =−3 ×10−4^{−4}−4 ⁢Φ0_00​ from the target, corresponding to only 15% degradation in 𝑇2,𝐸_{2,𝐸}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.
In the pursuit of large-scale, fault-tolerant quantum computation, various superconducting architectures have been investigated as the potential building blocks for these systems. With the need for fast operations with low error rates, fluxonium qubits have recently gained an increasing amount of attention. The fluxonium qubit offers numerous advantages including a large anharmonicity, long coherence times, protection against flux noise, and resilience against dielectric loss. In an effort to maximize the potential of these devices, we employ flux-pulse-assisted readout to increase the signal to noise ratio and, thus, minimize the readout time required. This readout scheme capitalizes on the high dispersive shift afforded by avoided crossings between the readout resonator and higher order fluxonium transitions. We present experimental results in which we display an advantage of flux-pulse-assisted readout over static readout at the so-called "sweet-spot" on multiple fluxonium qubits.
Much attention has focused on the transmon architecture for large-scale quantum devices, however, the fluxonium qubit has emerged as a possible successor. With an additional shunting inductor in parallel, the fluxonium offers larger anharmonicity and stronger protection against dialectric loss, leading to higher coherence times. The extra inductive element of the fluxonium qubit leads to a rich dispersive shift landscape when tuning the external flux. Here we propose to exploit the features in the dispersive shift to improve qubit readout using fast flux pulsing. Specifically, we report on theoretical simulations showing improved readout times and error rates by performing the readout at a flux bias point with large dispersive shift. We suggest optimal energy parameters for the fluxonium architecture that will allow for the implementation of our proposed readout scheme.
Qubits based on the magnetic flux degree of freedom require a flux bias, whose stability and precision strongly affect the qubit performance, up to a point of forbidding the qubit operation. Moreover, in the perspective of multiqubit systems, it must be possible to flux-bias each qubit independently, hence avoiding the traditional use of externally generated magnetic fields in favour of on-chip techniques that minimize cross-couplings. The solution discussed in this paper exploits a persistent current, trapped in a superconducting circuit integrated on chip that can be inductively coupled with an individual qubit. The circuit does not make use of resistive elements that can be detrimental for the qubit coherence. The trapping procedure allows to control and change stepwise the amount of stored current; after that, the circuit can be completely disconnected from the external sources. We show in a practical case how this works and how to drive the bias circuit at the required value.
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.