Single to quadruple quantum dots with tunable tunnel couplings

Abstract

We prepare a gate-defined quadruple quantum dot to study the gate-tunability of single to quadruple quantum dots with finite inter-dot tunnel couplings. The measured charging energies of various double dots suggest that the dot size is governed by gate geometry. For the triple and quadruple dots we study gate-tunable inter-dot tunnel couplings. Particularly for the triple dot we find that the effective tunnel coupling between side dots significantly depends on the alignment of the center dot potential. These results imply that the present quadruple dot device has gate performance relevant for implementing spin-based four-qubit systems with controllable exchange couplings.Quantum dots (QDs) are artificial structures fabricated in semiconductors in which electrons are confined within the size of their de-Broglie wave length, typically tens of nanometers. Since QDs can trap single electrons isolated from the environment and these electron states are precisely controlled, they are attractive systems for both basic research of electron interaction and applications to quantum information processing. Recently several challenging experiments have demonstrated coherent manipulation of electron spins 1-4 following the proposal of electron-spin-based quantum computation 5 . We previously demonstrated two spin-1/2 qubits and exchange control with a double quantum dot (DQD) with a micro-magnet (MM) 4 and proposed a triple QD (TQD) with a MM suitable for implementing three spin-1/2 qubits 6 . Extending the number of qubits is an important step toward realization of quantum computation. Several types of few-electron TQDs have been demonstrated in recent years [7][8][9] . As for quadruple QDs (QQDs), some systems consisting simply of two capacitively coupled DQDs have been studied [10][11][12][13] . In these devices each DQD is used as a charge qubit 10,11 or a singlet-triplet spin qubit 12, 13 and the capacitive coupling between the two DQDs has been used to perform conditional operations between the qubits. However, no QQDs having finite tunnel couplings between all the neighboring dots have ever been fabricated. Furthermore, integration with a MM favors multiple QDs in a linear array. In this Letter we fabricated collinear QQDs with inter-dot tunnel coupling, which are designed to be fitted with a MM, and demonstrated gate-tunable formation of single, double, triple and quadruple QDs by adjusting gate voltages and observed the effect of inter-dot tunneling in the stability diagram.Figure 1(a) shows a scanning electron micrograph of our device. The geometry of the surface gate electrodes are designed by using the numerical simulation 14 of electrostatic potential to create four dots in a row. The gate-defined QQD is formed in a 100-nm deep two-dimensional electron gas (2DEG) at a GaAs/AlGaAs hetero-interface with a capping gate on top to effectively reduce the 2DEG density. All experiments to identify the charge states of the fabricated devices were performed at a bath temperature of 50 mK.Initially we applied appropriate gat...

Photon assisted long-range tunneling

“…Such a longrange charge transfer is relevant for chemical reactions 23 and has analogues in quantum optics 22 . The recent advances in the tunability of triple quantum dots [24][25][26][27][28][29][30] has remarkably achieved the observation of this effect [31][32][33] . It entails the transfer of charge or spin between the two ends of the triple dot via superpositions that do not include the center one 34 , opening the way to the transfer of quantum information between distant qubits with low decoherence.…”

Full control of quadruple quantum dot circuit charge states in the single electron regime

“…This particular configuration is however a priori less suitable for scalability than the series configuration discussed in this letter. Very recently, a tunnel coupled series-QQD device has been realized and studied in the multiple electrons regime 14 , showing a good control of the tunnel couplings and gate potentials to form the dots. In this letter, we show that by following the same architecture, we can reach the single electron regime for each QD, which is a mandatory condition for making four spin qubits.…”

Computer-automated tuning of semiconductor double quantum dots into the single-electron regime

“…[4][5][6][7][8][9][10] Enabled by advances in device technology, the number of quantum dots that can be accessed is quickly increasing from very few to many. 11,12 Up to date, all these quantum dots have been tuned by "hand." This is a slow process whereby gate voltages are tweaked carefully, first to reach a regime with one electron in each of the dots and then to adjust the strength of all the tunnel barriers.…”