Single-Electron Double Quantum Dots in Bilayer Graphene

Abstract

We present transport measurements through an electrostatically defined bilayer graphene double quantum dot in the single electron regime. With the help of a back gate, two split gates and two finger gates we are able to control the number of charge carriers on two gate-defined quantum dot independently between zero and five. The high tunability of the device meets requirements to make such a device a suitable building block for spin-qubits. In the single electron regime, we determine interdot tunnel rates on the order of 2 GHz. Both, the interdot tunnel coupling, as well as the capacitive interdot coupling increase with dot occupation, leading to the transition to a single quantum dot. Finite bias magneto-spectroscopy measurements allow to resolve the excited state spectra of the first electrons in the double quantum dot; being in agreement with spin and valley conserving interdot tunneling processes.Electrostatically defined quantum dots (QDs) offer a compelling platform for spin-qubit-based quantum computation [1]. For that purpose, QDs in semiconductor heterostructures mainly based on GaAs [2, 3] and silicon [4,5] have been studied intensively. For example, high-fidelity single-qubit [6] and two-qubit [7-9] gate operations have been recently demonstrated for silicon qubit devices. Graphene has been early identified as an alternative attractive material platform for spin-qubits thanks to its low nuclear spin densities, weak hyperfine coupling and weak spinorbit interaction promising long spin decoherence times [10]. Physically etched graphene quantum devices including quantum dots [11,12] and double quantum dots (DQDs) [13,14] have been studied for about a decade. Major achievements include the implementation of charge detection [15,16], the observation of spin-states [12] and the measurement of charge relaxation times [17]. However, the influence of disorder, in particular edge disorder [18,19], prevented a precise control of the number of charge carriers on individual QDs making spin-qubit implementation impossible.The advancements in ultra-clean van der Waals heterostructures and in particular the use of local graphite gates allowed for the development of electrostatically defined bilayer graphene (BLG) quantum point contacts [20-23], quantum dots [24-26] and double quantum dots (DQDs) [27,28]. While single-electron and hole occupation has been demonstrated recently for individual QDs [24], the number of charge carriers in DQDs could not be controlled yet [27,28]. The precise control of the number of charge carriers is, however, a requirement for qubit operations in a semiconductor QD device.Here, we show single electron occupation of a bilayer graphene DQD. The electrostatically defined DQD allows for a high tunability of the electrochemical potential such that we can precisely control the number of electrons on * These authors contributed equally to this work. a Corresponding author: luca.banszerus@rwth-aachen.de each of the QDs independently down to zero. The gate voltages tune the interdot tunnel couplin...

Electrostatic Detection of Shubnikov–de Haas Oscillations in Bilayer Graphene by Coulomb Resonances in Gate‐Defined Quantum Dots

“…Sharp Coulomb resonances appear above V FG % 5.6 V. As observed in previous work, the resonances are grouped in quadruplets, representing the spin and valley degeneracy of BLG. [11,10,13,19] From finite bias spectroscopy measurements (not shown), we extract a charging energy of E c % 6 meV, a total capacitance of the QD of C tot % 27 aF, and a finger gate capacitance of C FG % 3.3 aF. Describing the QD using a model of a disk-shaped plate capacitor, we extract a QD diameter of d % 70 nm, which is in reasonable agreement with the lithographical dimensions of the gate electrodes.…”

“…The device studied in this work consists of a BLG flake encapsulated between two crystals of hexagonal boron nitride (hBN), placed on a graphite gate using the conventional van der Waals stacking technology. [6] Similar to previous work studying BLG gate-defined quantum point contacts [7][8][9] and QDs, [10][11][12][13][14][15] two layers of gold gates are evaporated on top: A pair of split gates (SGs) is used to form a 150 nm wide conducting channel connecting the source and drain reservoirs of the device (see Figure 1a). On top, separated by a 30 nm thick film of atomic layer deposited Al 2 O 3 , we place a gold finger gate (FG) with a width of 70 nm (see Figure 1a,b).…”

“…[16][17][18] Similar to recent works, we can form a QD underneath the FG by locally overcompensating the applied back gate voltage. [9][10][11]13,14] A small n-doped island is created underneath the FG, which is separated from the p-doped channel by the bandgap acting as a tunnel barrier (see schematic in Figure 1a). [13,11] We measure the current through the device as a function of V FG (Figure 1c).…”

“…[ 9–11,13,14 ] A small n‐doped island is created underneath the FG, which is separated from the p‐doped channel by the bandgap acting as a tunnel barrier (see schematic in Figure 1a). [ 13,11 ] We measure the current through the device as a function of (Figure 1c). At , the entire channel is p‐doped and well conductive.…”

Recent progresses of quantum confinement in graphene quantum dots

“…The novel confinement states, including the manipulation of Berry curvature and valley pseudospin in bilayer GQDs, inspire future works of using confinement states to study the properties of two-dimentional materials that have nontrivial Berry curvature, such as semiconducting transition metal dichalcogenides (TMDs) [167], topological insulators [168], and Weyl semimetals [169][170][171]. In addition, the technical advancements in bilayer GQDs works are also used to fabricate coupled double and multiple bilayer GQDs applied to realize the quantum bits based on valley and spin degrees of freedom [172,173].…”