Coherent and compact van der Waals transmon qubits

Abstract

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.

Figures (6)

• Figure 1: Van der Waals merged-element transmon. (a), (b) Optical micrographs of Device 1. A Josephson junction (light blue) is formed by the overlap of two $\text{NbSe}_2$ flakes (dark blue) sandwiching a flake of $\text{WSe}_2$ (green). The device is encapsulated from top and bottom by flakes of hBN, which are not outlined. Aluminum wires (orange) capacitively couple the MET to a $\lambda/2$ CPW readout resonator (res) and ground (gnd). A nearby wire is used to send control pulses (red). The readout resonator is capacitively coupled to a CPW feedline outside of the field of view. (c) Circuit diagram of the qubit and readout circuit. RF signals are sent through the feedline to measure the complex transmission coefficient. (d) Side-view schematic of a floating vdW MET, including the vdW Josephson junction stack and Al coupling electrodes (not to scale).
• Figure 2: MET spectroscopy and coherence in Device 1. (a) Two-tone spectroscopy showing the $0\rightarrow1$ qubit transition at $f_{01}$ and two-photon $0\rightarrow2$ transition at $f_{02}/2$. (b) Rabi oscillations around $f_{01}$. The black asterisk indicates the pulse length for a $\pi$ pulse. (c)--(e), Best-performing population inversion, Ramsey, and Hahn echo measurements, respectively. Black dashed lines are fits to the data, assuming exponential decay, from which qubit lifetimes are extracted (see main text). Schematic pulse sequences, including $\pi$ (dark grey), $\pi/2$ (light grey), and measurement $M$ (blue) pulses, are inset in each plot.
• Figure 3: Dielectric loss in van der Waals capacitors. (a) Circuit diagram of a CPW resonator (black rectangle) loaded with a vdW parallel plate capacitor consisting of $\text{NbSe}_2$ electrodes (dark blue) and $\text{WSe}_2$ dielectric (green). (b) Example of how a capacitive termination affects the resonance of a CPW resonator. (c) Optical micrograph of a vdW capacitor electrically connected to the resonator and ground via aluminum wires (orange). The overlap of the two $\text{NbSe}_2$ flakes defines the capacitor area (light blue). (d) Side-view schematic of the vdW capacitor and aluminum electrodes (not to scale). (e) Extracted dielectric loss tangents, plotted as $1/\tan{\delta}$. Solid green bars correspond to $\tan{\delta}$ extracted from mean $T_1$ times, except for Device 4 which is the best-$T_1$-derived value. Striped green bars are values extracted from resonator measurements.
• Figure 4: Temperature dependence of $T_1$ for Device 1. Mean $T_1$ vs. $T$ for Device 1. Error bars correspond to standard deviations of distributions taken at each temperature. The solid blue line is a fit to the spin-boson model leggett_dynamics_1987. Inset, dielectric loss vs. $T$ extracted from capacitor measurements of Device 5.
• Figure 5: Internal quality factors (top row) and relative resonance shifts (bottom row) of a reference resonator ($Q_\text{open}$, $f_\text{open}$) and a capacitor-terminated resonator ($Q_\text{term}$, $f_\text{term}$) as a function of microwave power (left column) and $T$ (right column) for Device 5. The relative resonance shifts here are defined as $\Delta f/f = (f_i - f_{i,0})/f_{i,0}$, where $i = \{\text{open, term}\}$ and the subscript 0 denotes the resonant frequency at the lowest measurement power (left column) or temperature (right column).