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arXiv:2609.14230

An All-van-der-Waals Qubit

Sein Park, Sameia Zaman, Junghyun Kim, Junyoung An, Daniel Rodan-Legrain, Hung-Yu Tsao, Chia-Chin Tsai, Aranya Goswami, Réouven Assouly, William P. Banner, Gabriel D. Cutter, Kenji Watanabe, Takashi Taniguchi, Terry P. Orlando, Gil-Ho Lee, Kyle Serniak, Max Hays, Jeffrey A. Grover, Philip Kim, Pablo Jarillo-Herrero, Joel Î-j. Wang, William D. Oliver

quant-phcond-mat.mes-hallcond-mat.supr-conSubmitted Sep 13, 2026• Announced Sep 15, 2026• 0 citations• Revised Sep 16, 2026 (v2)
v1 · Sep 13, 2026v2 · Sep 16, 2026
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Abstract

Advances in solid-state physics, materials science, and device engineering have accelerated the development of superconducting qubits. Among emerging platforms, van der Waals (vdW) materials and their heterostructures are potentially attractive building blocks for quantum devices, yet their realization in qubit architectures remains largely underexplored. Here we report an all-vdW superconducting qubit based on a NbSe$_2$-hBN-NbSe$_2$ junction, in which a thin hBN layer simultaneously provides Josephson coupling and capacitive shunting between two NbSe$_2$ islands, forming a "merged-element" transmon. Temporal characterization using circuit quantum electrodynamics (cQED) techniques yields an average energy-relaxation time $T_{1,\mathrm{avg}} = 55 ~μs$, Hahn-echo coherence time $T_{2\mathrm{E},\mathrm{avg}} = 21 ~μs$, and Ramsey coherence time $T_{2\mathrm{R},\mathrm{avg}} = 1.9~μs$. The relatively low Ramsey time is primarily attributable to an enhanced sensitivity to charge noise consistent with the realized device parameters and not a fundamental limitation. These results show that lumped-element superconducting qubits based on vdW heterostructures can achieve coherence times comparable to those of conventional Al-AlO$_\mathrm{x}$-Al qubits, while offering a reduced device footprint and suppressed stray capacitive coupling.

Comments
18 pages, 4 figures;

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Advances in solid-state physics, materials science, and device engineering have accelerated the development of superconducting qubits. Among emerging platforms, van der Waals (vdW) materials and their heterostructures are potentially attractive building blocks for quantum devices, yet their realization in qubit architectures remains largely underexplored. Here we report an all-vdW superconducting qubit based on a NbSe2_22​-hBN-NbSe2_22​ junction, in which a thin hBN layer simultaneously provides Josephson coupling and capacitive shunting between two NbSe2_22​ islands, forming a "merged-element" transmon. Temporal characterization using circuit quantum electrodynamics (cQED) techniques yields an average energy-relaxation time T1,avg=55 μsT_{1,\mathrm{avg}} = 55 ~μsT1,avg​=55 μs, Hahn-echo coherence time T2E,avg=21 μsT_{2\mathrm{E},\mathrm{avg}} = 21 ~μsT2E,avg​=21 μs, and Ramsey coherence time T2R,avg=1.9 μsT_{2\mathrm{R},\mathrm{avg}} = 1.9~μsT2R,avg​=1.9 μs. The relatively low Ramsey time is primarily attributable to an enhanced sensitivity to charge noise consistent with the realized device parameters and not a fundamental limitation. These results show that lumped-element superconducting qubits based on vdW heterostructures can achieve coherence times comparable to those of conventional Al-AlOx_\mathrm{x}x​-Al qubits, while offering a reduced device footprint and suppressed stray capacitive coupling.
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Abstract Ultra low-loss microwave materials are crucial for enhancing quantum coherence and scalability of superconducting qubits. Van der Waals (vdW) heterostructure is an attractive platform for quantum devices due to the single-crystal structure of the constituent two-dimensional (2D) layered materials and the lack of dangling bonds at their atomically sharp interfaces. However, new fabrication and characterization techniques are required to determine whether these structures can achieve low loss in the microwave regime. Here we report the fabrication of superconducting microwave resonators using NbSe2 that achieve a quality factor Q > 105. This value sets an upper bound that corresponds to a resistance of ⩽ 192 μ Ω when considering the additional loss introduced by integrating NbSe2 into a standard transmon circuit. This work demonstrates the compatibility of 2D layered materials with high-quality microwave quantum devices.
Van-der-Waals assembly enables the fabrication of novel Josephson junctions featuring an atomically sharp interface between two exfoliated and relatively twisted Bi_{2}Sr_{2}CaCu_{2}O_{8+x} (Bi2212) flakes. In a range of twist angles around 45°, the junction provides a regime where the interlayer two-Cooper pair tunneling dominates the current-phase relation. Here we propose employing this novel junction to realize a capacitively shunted qubit that we call flowermon. The d-wave nature of the order parameter endows the flowermon with inherent protection against charge-noise-induced relaxation and quasiparticle-induced dissipation. This inherently protected qubit paves the way to a new class of high-coherence hybrid superconducting quantum devices based on unconventional superconductors.