Radio-frequency charge detection on graphene electron-hole double quantum dots

High-fidelity detection of charge transitions in quantum dots (QDs) is a key ingredient in solid state quantum computation. We demonstrate high-bandwidth radio-frequency charge detection in bilayer graphene quantum dots (QDs) using a capacitively coupled quantum point contact (QPC). The device design suppresses screening effects and enables sensitive QPC-based charge readout. The QPC is arranged to maximize the readout contrast between two neighboring, coupled electron and hole QDs. We apply the readout scheme to a single-particle electron-hole double QD and demonstrate time-resolved detection of charge states as well as magnetic field dependent tunneling rates. This promises a high-fidelity readout scheme for individual spin and valley states, which is important for the operation of spin, valley or spin-valley qubits in bilayer graphene.

💡 Research Summary

In this work the authors demonstrate a high‑bandwidth radio‑frequency (RF) charge‑sensing technique for bilayer‑graphene (BLG) quantum‑dot (QD) devices. The device consists of a BLG flake encapsulated in hexagonal‑boron‑nitride (hBN) placed on a back‑gate. Split gates define a wide conducting channel and a narrow channel that hosts two coupled QDs: one electron‑type and one hole‑type. A quantum‑point‑contact (QPC) is formed in the wide channel directly at the T‑junction with the narrow channel and serves as a capacitively coupled charge detector. By arranging the QPC inline with the double‑dot, the authors maximize the detector‑dot capacitance and minimize the distance between detector and dots, which is crucial for high sensitivity.

The QPC is embedded in an LC resonator (L ≈ 3.3 µH, C ≈ 0.6 pF) that is impedance‑matched to a 50 Ω transmission line. An RF carrier near the resonance frequency (~115 MHz) is sent down the line, reflected from the resonator, and demodulated using homodyne detection. Both the amplitude (R_demod) and phase (φ) of the reflected signal are recorded. The authors quantify the detector response by the step height δR or δφ caused by a single‑electron or single‑hole transition in the double dot, and define the signal‑to‑noise ratio (SNR) as the step height divided by the RMS noise.

Key experimental findings include: (i) the phase response yields up to three times higher SNR than the amplitude response; (ii) SNR increases with RF excitation power up to a few dBm, after which power‑broadening reduces the phase SNR; (iii) the detector maintains SNR > 1 up to a bandwidth of ≈10 MHz, limited only by the sampling rate of the lock‑in amplifier (≈7 MHz). The authors also explore the influence of a middle gate (G1) that depletes carriers between the QPC and the dots, thereby reducing electrostatic screening. By sweeping V_G1 they show that the detector’s response to hole‑dot transitions grows as V_G1 becomes more negative, while the response to electron‑dot transitions is weaker due to larger lateral separation.

Self‑consistent Schrödinger‑Poisson simulations (using NGSolve) reproduce the measured charge‑carrier density profiles and reveal that thin hBN layers increase screening from the metallic gates, degrading detector sensitivity. The simulations also provide the expected potential shift at the QPC for a change in dot occupation, confirming the two mechanisms that govern sensitivity: (1) variation of the lateral dot‑detector distance and (2) modulation of carrier density in the intervening region.

Finally, the authors perform time‑resolved measurements of charge transitions at the inter‑dot transition of a weakly coupled single‑particle electron‑hole double dot. By applying a magnetic field they extract field‑dependent tunneling rates, observing non‑monotonic behavior that reflects the complex spin‑valley physics of BLG.

Overall, the paper establishes that RF reflectometry combined with an optimally placed QPC enables MHz‑bandwidth, high‑fidelity charge readout of electron‑hole double quantum dots in bilayer graphene. This capability is a crucial step toward single‑shot spin or valley qubit readout in graphene‑based quantum information platforms.