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arXiv:2601.19581

Flux-tunable transmon incorporating a van der Waals superconductor via an Al/AlO$_x$/4Hb-TaS$_2$ Josephson junction

Eliya Blumenthal, Ilay Mangel, Amit Kanigel, Shay Hacohen-Gourgy

quant-phcond-mat.supr-conSubmitted Jan 27, 2026• Announced Jan 28, 2026• 0 citations
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Abstract

Incorporating van der Waals (vdW) superconductors into Josephson elements extends circuit-QED beyond conventional Al/AlO$_x$/Al tunnel junctions and enables microwave probes of unconventional condensates and subgap excitations. In this work, we realize a flux-tunable transmon whose nonlinear inductive element is an Al/AlO$_x$/4Hb-TaS$_2$ Josephson junction. The tunnel barrier is formed by sequential deposition and full in-situ oxidation of ultrathin Al layers on an exfoliated 4Hb-TaS$_2$ flake, followed by deposition of a top Al electrode, yielding a robust, repeatable hybrid junction process compatible with standard transmon fabrication. Embedding the device in a three-dimensional copper cavity, we observe a SQUID-like flux-dependent spectrum that is quantitatively reproduced by a standard dressed transmon--cavity Hamiltonian, from which we extract parameters in the transmon regime. Across measured devices we obtain sub-microsecond energy relaxation ($T_1$ from $0.08$ to $0.69~μ$s), while Ramsey measurements indicate dephasing faster than our $16$ ns time resolution. We also find a pronounced discrepancy between the Josephson energy inferred from spectroscopy and that expected from the Ambegaokar--Baratoff relation using room-temperature junction resistances, pointing to nontrivial junction physics in the hybrid Al/AlO$_x$/4Hb-TaS$_2$ system. Although we do not resolve material-specific subgap modes in the present geometry, this work establishes a practical route to integrating 4Hb-TaS$_2$ into coherent quantum circuits and provides a baseline for future edge-sensitive designs aimed at enhancing coupling to boundary and subgap degrees of freedom in vdW superconductors.

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Flux-tunable transmon with an Al/AlO x /4Hb-TaS2 Josephson junction

Eliya Blumenthal, Ilay Mangel, Amit Kanigel +1 more

Incorporating van der Waals (vdW) superconductors into Josephson elements extends circuit-QED beyond conventional Al/AlOx/Al junctions, enabling microwave probes of unconventional condensates and subgap excitations. We realize a flux-tunable transmon whose nonlinear inductive element is an Al/AlOx/4Hb-TaS2 Josephson junction, with a tunnel barrier formed by full in situ oxidation of ultrathin Al layers on an exfoliated 4Hb-TaS2 flake in a process compatible with standard transmon fabrication. In a three-dimensional copper cavity, the device shows a SQUID-like flux-dependent spectrum that is well reproduced by a dressed transmon–cavity Hamiltonian in the transmon regime. We measure sub-microsecond energy relaxation (T1=0.08–0.69 μs), while Ramsey measurements indicate dephasing faster than our 16 ns resolution. A naive Ambegaokar–Baratoff comparison using room-temperature junction resistances gives a suppressed effective energy scale; however, cross-sectional TEM reveals an amorphous region at the TaS2 interface, most likely produced by the pre-deposition ion-milling step. We therefore attribute the suppressed EJRn product primarily to a non-ideal hybrid interface. This work demonstrates the integration of 4Hb-TaS2 into a circuit-QED device and establishes an interface-optimization baseline for future edge-sensitive designs targeting boundary and subgap degrees of freedom in vdW superconductors.

Applied Physics Letters•Oct 12, 2026•0 citations

An All-van-der-Waals Qubit

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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 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.

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From Macroscopic Quantum Tunneling to Circuit QED: Coherence and Dissipation in 3D Transmon Qubits

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Abstract Superconducting quantum circuits are a leading platform for quantum-information processing, but their development requires expertise spanning device design, microfabrication, cryogenic operation, microwave control, and quantum characterization. In this work, we report the realization of a flux-tunable superconducting transmon platform and present an integrated workflow encompassing design, fabrication, measurement, and numerical modeling. The device consists of an $$Al/AlO_x/Al$$ transmon coupled to a three-dimensional microwave cavity operating in the circuit quantum electrodynamics (cQED) dispersive regime. Two-tone spectroscopy reveals a fundamental transition frequency of $$\omega _{01}=5.0122~\textrm{GHz}$$ and an anharmonicity of $$\alpha /2\pi =-202.4~\textrm{MHz}$$ , corresponding to $$E_J/E_C \approx 82.9$$ . Flux spectroscopy yields a cavity-qubit coupling strength of $$g/2\pi = 260~\textrm{MHz}$$ , while the cavity linewidth gives a loaded quality factor of $$Q_L=6127$$ and a Purcell limit of $$T_1^{\textrm{Purcell}}\approx 82~\mu \textrm{s}$$ . Time-domain measurements yield coherence times of $$T_1 = 17.4~\mu \textrm{s}$$ and $$T_2^* = 5.8~\mu \textrm{s}$$ . To investigate coherence and dissipation in the device, the experimental observations are analyzed within a Lindblad master-equation framework describing cavity photon loss and thermal excitation, qubit energy relaxation, and pure dephasing processes. Using a set of experimentally extracted parameters, the model reproduces the measured spectroscopic and time-domain observables, providing a quantitative description of the open-system dynamics of the transmon-cavity platform. The analysis shows that the measured coherence is primarily limited by pure dephasing and energy-relaxation processes, whereas thermal excitation plays a minor role under the experimental conditions. Inspired by the broader framework of quantum dissipation in macroscopic quantum systems pioneered by Caldeira and collaborators, this approach enables a quantitative analysis of relaxation and dephasing processes in superconducting quantum circuits. Beyond the characterization of a single device, the present work demonstrates the establishment of the expertise, infrastructure, and experimental workflow required for the design, fabrication, measurement, and modeling of superconducting quantum circuits, contributing to the development of quantum technologies in Brazil and related applications.

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State-of-the-art superconducting qubits rely on a limited set of thin-film materials. Expanding their materials palette can improve performance, extend operating regimes, and introduce new functionalities, but conventional thin-film fabrication hinders systematic exploration of new material combinations. Van der Waals (vdW) materials offer a highly modular crystalline platform that facilitates such exploration while enabling gate-tunability, higher-temperature operation, and compact qubit geometries. Yet it remains unknown whether a fully vdW superconducting qubit can support quantum coherence and what mechanisms dominate loss at both low and elevated temperatures in such a device. Here we demonstrate quantum-coherent merged-element transmons made entirely from vdW Josephson junctions. These first-generation, fully crystalline qubits achieve microsecond lifetimes in an ultra-compact footprint without external shunt capacitors. Energy relaxation measurements, together with microwave characterization of vdW capacitors, point to dielectric loss as the dominant relaxation channel up to hundreds of millikelvin. These results establish vdW materials as a viable platform for compact superconducting quantum devices.

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We demonstrate a superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon potentially offers a smaller footprint than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces the relative electric field participation from other interfaces. By combining micrometer-scale $\mathrm{Al}$/$\mathrm{Al}\mathrm{O}$${}_{x}$/$\mathrm{Al}$ junctions with long oxidations, we produce functional devices with ${E}_{J}/{E}_{C}$ in the low-transmon regime (${E}_{J}/{E}_{C}\ensuremath{\lesssim}30$). Cryogenic $I$-$V$ measurements show a sharp $dI/dV$ structure with low subgap conduction. Qubit spectroscopy of tunable versions shows a small number of avoided level crossings, suggesting the presence of two-level systems. We observe mean ${T}_{1}$ times typically in the range of $10--90\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$, with some annealed devices exhibiting ${T}_{1}>100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$ over several hours. The results suggest that energy relaxation in conventional small-junction transmons is not limited by junction loss.

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Josephson energy of superconducting junctions: amorphous versus crystalline tunnel barriers

Wanting Zhang, Aldilene Saraiva-Souza, Félix Beaudoin +3 more

The Josephson energy $E_J$ is a key parameter governing the properties of transmon superconducting qubits. In Al/AlO$_x$/Al junctions, $E_J$ is set by electron tunneling through an ultrathin oxide barrier and therefore depends exponentially on the atomic structure of the barrier. We compute $E_J$ by first-principles device modeling based on the NEGF-DFT quantum-transport method, comparing a junction with a crystalline Al$_2$O$_3$ barrier against ten junctions with melt--quenched amorphous Al$_2$O$_3$ barriers of the same thickness. From the Fermi-level transmission and the Ambegaokar--Baratoff relation, we obtain a mean $E_J/h$ of $2.78$ GHz for the amorphous ensemble, with a standard deviation of $4.67$ GHz, compared with $0.73$ GHz for the crystalline reference; individual amorphous values span nearly two orders of magnitude. Scattering-state analysis shows that transport is quantum tunneling and that the variability originates from stoichiometric inhomogeneity of the amorphous oxide: Al-rich, low-barrier regions can connect into percolation-like tunneling pathways that strongly enhance the conductance. A realistic $200\times200$ nm$^2$ junction self-averages over more than $2\times10^4$ such microscopic regions. These results establish a quantitative atomistic route from oxide microstructure to the superconducting-circuit energy scale $E_J$.

quant-pharXiv:2609.09426•Sep 10, 2026•0 citations
Flux-tunable transmon incorporating a van der Waals superconductor via an Al/AlOx_xx​/4Hb-TaS2_22​ Josephson junction
Incorporating van der Waals (vdW) superconductors into Josephson elements extends circuit-QED beyond conventional Al/AlOx_xx​/Al tunnel junctions and enables microwave probes of unconventional condensates and subgap excitations. In this work, we realize a flux-tunable transmon whose nonlinear inductive element is an Al/AlOx_xx​/4Hb-TaS2_22​ Josephson junction. The tunnel barrier is formed by sequential deposition and full in-situ oxidation of ultrathin Al layers on an exfoliated 4Hb-TaS2_22​ flake, followed by deposition of a top Al electrode, yielding a robust, repeatable hybrid junction process compatible with standard transmon fabrication. Embedding the device in a three-dimensional copper cavity, we observe a SQUID-like flux-dependent spectrum that is quantitatively reproduced by a standard dressed transmon--cavity Hamiltonian, from which we extract parameters in the transmon regime. Across measured devices we obtain sub-microsecond energy relaxation (T1T_1T1​ from 0.080.080.08 to 0.69 μ0.69~μ0.69 μs), while Ramsey measurements indicate dephasing faster than our 161616 ns time resolution. We also find a pronounced discrepancy between the Josephson energy inferred from spectroscopy and that expected from the Ambegaokar--Baratoff relation using room-temperature junction resistances, pointing to nontrivial junction physics in the hybrid Al/AlOx_xx​/4Hb-TaS2_22​ system. Although we do not resolve material-specific subgap modes in the present geometry, this work establishes a practical route to integrating 4Hb-TaS2_22​ into coherent quantum circuits and provides a baseline for future edge-sensitive designs aimed at enhancing coupling to boundary and subgap degrees of freedom in vdW superconductors.
Incorporating van der Waals (vdW) superconductors into Josephson elements extends circuit-QED beyond conventional Al/AlOx/Al junctions, enabling microwave probes of unconventional condensates and subgap excitations. We realize a flux-tunable transmon whose nonlinear inductive element is an Al/AlOx/4Hb-TaS2 Josephson junction, with a tunnel barrier formed by full in situ oxidation of ultrathin Al layers on an exfoliated 4Hb-TaS2 flake in a process compatible with standard transmon fabrication. In a three-dimensional copper cavity, the device shows a SQUID-like flux-dependent spectrum that is well reproduced by a dressed transmon–cavity Hamiltonian in the transmon regime. We measure sub-microsecond energy relaxation (T1=0.08–0.69 μs), while Ramsey measurements indicate dephasing faster than our 16 ns resolution. A naive Ambegaokar–Baratoff comparison using room-temperature junction resistances gives a suppressed effective energy scale; however, cross-sectional TEM reveals an amorphous region at the TaS2 interface, most likely produced by the pre-deposition ion-milling step. We therefore attribute the suppressed EJRn product primarily to a non-ideal hybrid interface. This work demonstrates the integration of 4Hb-TaS2 into a circuit-QED device and establishes an interface-optimization baseline for future edge-sensitive designs targeting boundary and subgap degrees of freedom in vdW superconductors.
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.
Abstract Superconducting quantum circuits are a leading platform for quantum-information processing, but their development requires expertise spanning device design, microfabrication, cryogenic operation, microwave control, and quantum characterization. In this work, we report the realization of a flux-tunable superconducting transmon platform and present an integrated workflow encompassing design, fabrication, measurement, and numerical modeling. The device consists of an Al/AlOx/AlAl/AlO_x/AlAl/AlOx​/Al transmon coupled to a three-dimensional microwave cavity operating in the circuit quantum electrodynamics (cQED) dispersive regime. Two-tone spectroscopy reveals a fundamental transition frequency of ω01=5.0122 GHz\omega _{01}=5.0122~\textrm{GHz}ω01​=5.0122 GHz and an anharmonicity of α/2π=−202.4 MHz\alpha /2\pi =-202.4~\textrm{MHz}α/2π=−202.4 MHz , corresponding to EJ/EC≈82.9E_J/E_C \approx 82.9EJ​/EC​≈82.9 . Flux spectroscopy yields a cavity-qubit coupling strength of g/2π=260 MHzg/2\pi = 260~\textrm{MHz}g/2π=260 MHz , while the cavity linewidth gives a loaded quality factor of QL=6127Q_L=6127QL​=6127 and a Purcell limit of T1Purcell≈82 μsT_1^{\textrm{Purcell}}\approx 82~\mu \textrm{s}T1Purcell​≈82 μs . Time-domain measurements yield coherence times of T1=17.4 μsT_1 = 17.4~\mu \textrm{s}T1​=17.4 μs and T2∗=5.8 μsT_2^* = 5.8~\mu \textrm{s}T2∗​=5.8 μs . To investigate coherence and dissipation in the device, the experimental observations are analyzed within a Lindblad master-equation framework describing cavity photon loss and thermal excitation, qubit energy relaxation, and pure dephasing processes. Using a set of experimentally extracted parameters, the model reproduces the measured spectroscopic and time-domain observables, providing a quantitative description of the open-system dynamics of the transmon-cavity platform. The analysis shows that the measured coherence is primarily limited by pure dephasing and energy-relaxation processes, whereas thermal excitation plays a minor role under the experimental conditions. Inspired by the broader framework of quantum dissipation in macroscopic quantum systems pioneered by Caldeira and collaborators, this approach enables a quantitative analysis of relaxation and dephasing processes in superconducting quantum circuits. Beyond the characterization of a single device, the present work demonstrates the establishment of the expertise, infrastructure, and experimental workflow required for the design, fabrication, measurement, and modeling of superconducting quantum circuits, contributing to the development of quantum technologies in Brazil and related applications.
State-of-the-art superconducting qubits rely on a limited set of thin-film materials. Expanding their materials palette can improve performance, extend operating regimes, and introduce new functionalities, but conventional thin-film fabrication hinders systematic exploration of new material combinations. Van der Waals (vdW) materials offer a highly modular crystalline platform that facilitates such exploration while enabling gate-tunability, higher-temperature operation, and compact qubit geometries. Yet it remains unknown whether a fully vdW superconducting qubit can support quantum coherence and what mechanisms dominate loss at both low and elevated temperatures in such a device. Here we demonstrate quantum-coherent merged-element transmons made entirely from vdW Josephson junctions. These first-generation, fully crystalline qubits achieve microsecond lifetimes in an ultra-compact footprint without external shunt capacitors. Energy relaxation measurements, together with microwave characterization of vdW capacitors, point to dielectric loss as the dominant relaxation channel up to hundreds of millikelvin. These results establish vdW materials as a viable platform for compact superconducting quantum devices.
We demonstrate a superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon potentially offers a smaller footprint than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces the relative electric field participation from other interfaces. By combining micrometer-scale Al\mathrm{Al}Al/AlO\mathrm{Al}\mathrm{O}AlOx{}_{x}x​/Al\mathrm{Al}Al junctions with long oxidations, we produce functional devices with EJ/EC{E}_{J}/{E}_{C}EJ​/EC​ in the low-transmon regime (${E}_{J}/{E}_{C}\ensuremath{\lesssim}30$). Cryogenic III-VVV measurements show a sharp dI/dVdI/dVdI/dV structure with low subgap conduction. Qubit spectroscopy of tunable versions shows a small number of avoided level crossings, suggesting the presence of two-level systems. We observe mean T1{T}_{1}T1​ times typically in the range of $10--90\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$, with some annealed devices exhibiting ${T}_{1}>100\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$ over several hours. The results suggest that energy relaxation in conventional small-junction transmons is not limited by junction loss.
The Josephson energy EJE_JEJ​ is a key parameter governing the properties of transmon superconducting qubits. In Al/AlOx_xx​/Al junctions, EJE_JEJ​ is set by electron tunneling through an ultrathin oxide barrier and therefore depends exponentially on the atomic structure of the barrier. We compute EJE_JEJ​ by first-principles device modeling based on the NEGF-DFT quantum-transport method, comparing a junction with a crystalline Al2_22​O3_33​ barrier against ten junctions with melt--quenched amorphous Al2_22​O3_33​ barriers of the same thickness. From the Fermi-level transmission and the Ambegaokar--Baratoff relation, we obtain a mean EJ/hE_J/hEJ​/h of 2.782.782.78 GHz for the amorphous ensemble, with a standard deviation of 4.674.674.67 GHz, compared with 0.730.730.73 GHz for the crystalline reference; individual amorphous values span nearly two orders of magnitude. Scattering-state analysis shows that transport is quantum tunneling and that the variability originates from stoichiometric inhomogeneity of the amorphous oxide: Al-rich, low-barrier regions can connect into percolation-like tunneling pathways that strongly enhance the conductance. A realistic 200×200200\times200200×200 nm2^22 junction self-averages over more than 2×1042\times10^42×104 such microscopic regions. These results establish a quantitative atomistic route from oxide microstructure to the superconducting-circuit energy scale EJE_JEJ​.