Perfect impedance matching unlocks sensitive radio-frequency reflectometry in 2D material quantum dots

TL;DR

The paper tackles the challenge of fast, sensitive RF readout for gate-defined quantum dots in 2D materials, where high contact resistances hinder impedance matching. It demonstrates impedance-matched RF reflectometry by integrating a tunable SrTiO3 varactor into the resonator for bilayer graphene and MoS2 devices, achieving deep S21 dips and Coulomb-oscillation signals in the demodulated output. The study also characterizes the varactor’s noise response, showing that the dielectric nonlinearity governs transduced fluctuations and that performance remains robust against perpendicular magnetic fields. The results establish SrTiO3-based varactors as effective, general-purpose matching components for high-speed qubit readout in 2D materials, enabling scalable, fast single-electron sensing.

Abstract

Two-dimensional (2D) materials are attractive platforms for realizing high-performance quantum bits (qubits). However, radio-frequency (RF) charge detection, which is a key technique for qubits readout, remains challenging in such systems. We demonstrate RF reflectometry with impedance matching for high-resistance quantum dot devices based on bilayer graphene and molybdenum disulfide. By integrating a tunable strontium titanate (SrTiO3) varactor into a resonant circuit, we achieve nearly perfect impedance matching, enabling sensitive charge detection. The demodulated RF signal clearly shows Coulomb oscillations, and the SrTiO3 varactor exhibits robustness against both magnetic fields and voltage noise on the varactor. Our results establish SrTiO3 varactors as effective tunable matching components for RF reflectometry in high-resistance 2D material quantum devices, providing a foundation for high-speed qubits readout.

Figures (5)

• Figure 1: (a) Schematic of the device structure. A quantum dot (QD) is defined electrostatically in a narrow channel beneath the finger-gate electrode. (b) Source–drain current $I_\mathrm{sd}$ as a function of finger-gate voltage $V_\mathrm{fg}$ at $V_\mathrm{sd}=1$ mV.
• Figure 2: (a) RF reflectometry setup with the RFSoC for impedance matching. (b) Transmission coefficient $S_{21}$ as a function of $V_\mathrm{STO}$, (c) $V_\mathrm{fg}$, and (d) perpendicular magnetic field $B$. The inset in (c) shows a schematic of $I_\mathrm{sd}$ corresponding to the measured $V_\mathrm{fg}$.
• Figure 3: (a) RF-detected Coulomb oscillations for various $V_\mathrm{STO}$. An offset is added to each trace. The black arrows indicate the positions of Coulomb peaks, which are clearly observed at $V_\mathrm{STO}=0$ and $10$ V. (b) Histograms of the Coulomb oscillations. Dashed lines indicate the maximum value of each histogram. Gray traces represent fitted curves obtained using Gaussian functions. (c) Close-up view of the Coulomb peak and $\frac{\mathrm{d}V_\mathrm{rf}}{\mathrm{d}V_\mathrm{fg}}$ at $V_\mathrm{STO}=10$ V. (d) Calculated readout error rate for single-electron transitions, evaluated for electrostatic couplings of $\mathrm{d}V_\mathrm{E}=1$ mV and $5$ mV.
• Figure 4: (a) Amplitude-modulation response of the measurement circuit to the SrTiO$_3$ varactor for $V_\mathrm{STO}=30$ V and (b) $26$ V, where the latter corresponds to a condition close to impedance matching. The SBP is defined as the height of the sidebands with respect to the noise background, as indicated. (c) $V_\mathrm{STO}$ dependence of the SBP at $f_\mathrm{m}=50$ kHz. (d) $f_\mathrm{m}$ dependence of the SBP for $V_\mathrm{STO}=30$ V and $26$ V.
• Figure 5: (a) Optical microscope image of the MoS$2$ device. (b) $I\mathrm{sd}$–$V_\mathrm{bg}$ characteristics at $V_\mathrm{sd}=0.65$ V. (c) $V_\mathrm{bg}$ dependence of $S_{21}$. The orange arrows indicate the vanishing points of the resonance. (d) RF-detected Coulomb diamond. The orange arrows correspond to those in (c), indicating the resonance vanishing points.