An All-van-der-Waals Qubit

AI summary

Overview

Research area: Superconducting quantum computing — materials and device physics of qubits built from two-dimensional van der Waals (vdW) heterostructures.

Technical level: Intermediate. The paper assumes familiarity with transmon qubits, circuit QED, and standard coherence measurements (T₁, Ramsey, Hahn echo), but the device concept and results are described in accessible terms.

Scope (one sentence): The paper reports the first all-vdW superconducting qubit — a NbSe₂–hBN–NbSe₂ "merged-element" transmon in which a single thin hBN barrier serves as both Josephson junction and shunt capacitor — and characterizes its spectroscopy, coherent control, and coherence times.

What This Paper Is About

Conventional superconducting qubits are made from aluminum and aluminum oxide, with large planar shunt capacitors roughly (300 µm)² in area; dielectric loss at surfaces and interfaces is a leading limit on their coherence. The authors ask whether an entirely van der Waals stack — superconducting NbSe₂ electrodes separated by a 2–3-layer hBN barrier — can replace both the junction and the capacitor in one compact overlap, and whether such a device can reach coherence comparable to standard Al–AlOₓ–Al transmons.

Key Contributions

1. Demonstration of an all-vdW superconducting qubit. A NbSe₂–hBN–NbSe₂ heterostructure operates as a transmon within a circuit-QED architecture, with qubit transition frequency f₀₁ = 2.00 GHz.
2. A "merged-element" device geometry. A single overlap area of a thin hBN layer simultaneously provides the Josephson coupling and the capacitive shunting, exploiting hBN's low microwave loss (tan δ_hBN < 10⁻⁵) and participation ratios exceeding 90% within the hBN.
3. Full temporal characterization of the qubit. Two-tone spectroscopy, Rabi oscillations, relaxation, Ramsey, and Hahn-echo measurements yield T₁,avg = 55 µs, T₂E,avg = 21 µs, and T₂R,avg = 1.9 µs, measured repeatedly over 10 hours.
4. Identification of the dephasing mechanism. Interleaved spectroscopy of the 0→3 transition and coherence measurements correlate short Ramsey times with offset-charge and quasiparticle-induced charge-parity sensitivity, traced to the modest E_J/E_C ≈ 33, rather than to a fundamental vdW limitation.

Main Findings

• Qubit parameters: Fitting the measured transitions to the Cooper-pair-box model gives E_J/h = 4.29 GHz and E_C/h = 0.13 GHz, so E_J/E_C ≈ 33. The calculated charge dispersion of the qubit transition is approximately 160 kHz.
• Spectroscopy: The fundamental transition f₀₁ appears at 2.00 GHz, the two-photon transition f₀₂/2 at 1.92 GHz, and the first-excited-state transition f₁₂ at 1.84 GHz.
• Circuit QED parameters: Resonator frequency f_res = 6.27 GHz, resonator linewidth κ/2π = 454 kHz, Lamb shift χ₀/2π = 155 kHz, and dispersive shift χ₀₁/2π = 34 kHz.
• Coherent control: Rabi chevron patterns are observed; at a drive amplitude of A_rabi = 0.3 V the on-resonance Rabi frequency is Ω_rabi/2π = 1.46 MHz, and the Rabi frequency increases linearly with drive amplitude.
• Relaxation: A representative inversion-recovery trace gives T₁ = 78 ± 3 µs. Over 50 consecutive measurements across 10 hours, the distribution averages T₁,avg = 55 µs with a standard deviation of 16 µs.
• Dephasing: Representative traces give T₂R,max = 3.9 ± 0.2 µs and T₂E,max = 28 ± 1 µs; averaged over the 10-hour run, T₂R,avg = 1.9 µs (standard deviation 0.7 µs) and T₂E,avg = 21 µs (standard deviation 3 µs).
• Pure dephasing: Using the Bloch-Redfield relation, the extracted pure dephasing times are T_φR,avg = 1.9 µs and T_φE,avg = 26 µs (propagated standard deviations 0.7 µs and 5 µs).
• Echo remains below the relaxation limit: T₂E stays well below 2T₁ ≈ 100 µs, indicating additional non-refocusable dephasing channels at higher frequency.
• Charge-parity sensitivity: The calculated 0→3 transition lies at f₀₃ = 5.515 GHz for the even-parity branch at n_g = 0, with a maximum parity splitting δf₀₃ = 44 MHz; near n_g ≈ 0.25 the branches approach a crossing at f₀₃ = 5.492 GHz. Repeated spectroscopy shows the transition frequency drifting in time, consistent with the calculated charge dispersion.
• Ramsey time depends on offset charge: T₂R improves to ∼6 µs near n_g ≈ 0.25 but remains ∼1–2 µs near n_g ≈ 0 and 0.5, consistent with quasiparticle tunneling limiting Ramsey coherence at parity-sensitive points.
• Echo time is charge-insensitive: T₂E shows no strong dependence on offset charge, so neither quasiparticle tunneling nor offset-charge fluctuations dominate the echo dephasing.
• Shot noise excluded: Explaining the observed echo dephasing via photon shot noise would require an effective resonator temperature of approximately 636 mK, far above the mixing chamber temperature or effective resonator temperatures seen in other experiments from the group.
• Cooldown variability: T₁, T₂R, and T₂E fluctuate from cooldown to cooldown, as also observed in conventionally fabricated superconducting qubits; the longest T₁ occurred in the fifth cooldown.
• Comparison to the state of the art: The authors state that vdW lumped-element qubits can reach coherence comparable to conventional Al–AlOₓ–Al qubits while offering a reduced device footprint and suppressed stray capacitive coupling.

Methodology in Plain English

The team built the qubit by stacking exfoliated flakes. A thin hBN flake was picked up at 160 K with a polydimethylsiloxane (PDMS) stamp and flipped so its surface faced upward; NbSe₂ flakes were picked up with an Elvacite polymer stamp, and the two were brought together so the hBN and NbSe₂ were retrieved sequentially, forming a NbSe₂–hBN–NbSe₂ sandwich. All exfoliation and transfer steps were done in an argon atmosphere to keep the interfaces clean. The finished stack was aligned and released onto an aluminum-based circuit-QED silicon chip; one chip carried three vdW qubits and one standard Al transmon.

In the circuit, the vdW qubit is capacitively coupled to a readout resonator that is inductively coupled to the feedline; readout and control tones are applied to the feedline and the transmitted signal is monitored. The team first performed two-tone spectroscopy to locate the transitions, fit them to the Cooper-pair-box Hamiltonian to extract E_J and E_C, then used Rabi oscillations to calibrate π and π/2 pulses. Coherence was measured with inversion recovery (T₁), Ramsey interferometry (T₂R), and Hahn-echo sequences (T₂E), repeated 50 times over 10 hours. Finally, they interleaved spectroscopy of the more charge-sensitive 0→3 transition with coherence measurements to correlate dephasing with offset charge and charge parity. A separate frequency-tunable device in a SQUID configuration was also fabricated.

Why This Matters

Impact on research: The work establishes vdW heterostructures as a viable materials platform for superconducting qubits, and shows that the observed charge-noise dephasing stems from a modest E_J/E_C ratio rather than from anything intrinsic to vdW materials. It also opens a route to using atomically clean, deterministically stacked interfaces as a laboratory for testing which material properties actually limit qubit coherence.

Real-world applications (potential):

• Compact, densely packable qubit arrays for quantum error correction, enabled by the reduced footprint of lumped-element vdW devices.
• Low-loss capacitors and circuit elements for microwave quantum hardware, exploiting hBN's loss tangent below 10⁻⁵.
• Materials-selectable superconducting circuits, where the junction barrier and electrodes can be chosen and stacked on demand.
• A testbed for studying two-level systems, quasiparticles, and charge noise in a controlled crystalline environment.

Industry relevance: The results target the central scaling bottleneck of superconducting quantum processors — footprint, stray coupling, and dielectric loss — and suggest a manufacturable materials route, though the authors explicitly flag that fabrication cleanliness and junction quality still need improvement.

Future Directions

• Increase E_J/E_C above 50, for example by enlarging the junction area, to suppress charge dispersion and the associated dephasing.
• Improve interface cleanliness and materials processing; the AFM image shows topographic features consistent with microscopic residues or transfer-related contamination.
• Identify the remaining non-refocusable dephasing channel behind T₂E, with critical-current fluctuations from microscopic defects in the tunnel barrier or nearby interfaces flagged as a possible source.
• Extend the platform to new superconducting circuit elements and scalable circuit-QED implementations based on atomically engineered materials, and further develop the SQUID-based frequency-tunable devices.

Target Audience

Graduate students and researchers in superconducting quantum computing, circuit QED, and two-dimensional materials; device physicists and fabrication engineers working on low-loss dielectrics and novel qubit architectures; and industry R&D teams evaluating materials platforms for scalable superconducting qubit arrays.

Authors’ 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.