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Silicon quantum dot spin qubits have great potential for application in large-scale quantum circuits as they share many similarities with conventional transistors that represent the prototypical example for scalable electronic platforms. However, for quantum dot formation and control, additional gates are required, which add to device complexity and, thus, hinder upscaling. Here, we meet this challenge by demonstrating the scalable integration of a multilayer gate stack in silicon quantum dot devices using self-alignment, which allows for ultra-small gate lengths and intrinsically perfect layer-to-layer alignment. We explore the prospects of these devices as hosts for hole spin qubits that benefit from electrically driven spin control via spin–orbit interaction. Therefore, we study hole transport through a double quantum dot and observe current rectification due to the Pauli spin blockade. The application of a small magnetic field leads to lifting of the spin blockade and reveals the presence of spin–orbit interaction. From the magnitude of a singlet-triplet anticrossing at a high magnetic field, we estimate a spin–orbit energy of ∼37μeV, which corresponds to a spin–orbit length of ∼48 nm. This work paves the way for scalable spin-based quantum circuits with fast, all-electrical qubit control.

A dual mode device behaving either as a field-effect transistor or a single electron transistor (SET) has been fabricated using silicon-on-insulator metal oxide semiconductor technology. Depending on the back gate polarisation, an electron island is accumulated under the front gate of the device (SET regime), or a field-effect transistor is obtained by pinching off a bottom channel with a negative front gate voltage. The gradual transition between these two cases is observed. This dual function uses both vertical and horizontal tunable potential gradients in non-overlapped silicon-on-insulator channel

We integrate ambipolar quantum dots in silicon fin field-effect transistors using exclusively standard complementary metal-oxide-semiconductor fabrication techniques. We realize ambipolarity by replacing conventional highly-doped source and drain electrodes by a metallic nickel silicide with Fermi level close to the silicon midgap position. Such devices operate in a dual mode, either as classical field-effect or single-electron transistor. We implement a classical logic NOT gate at low temperature by tuning two interconnected transistors into opposite polarities. In the quantum regime, we demonstrate stable quantum dot operation in the few charge carrier Coulomb blockade regime for both electrons and holes.Quantum information can be encoded in the spin state of a single electron or hole confined to a semiconductor quantum dot (QD) 1-3 . Several material systems have been explored in the search of a highly coherent spin quantum bit (qubit). Silicon (Si) is a particularly promising material platform for scalable spin-based quantum computing because of its fully developed, industrial manufacturing processes, which enable reliable and reproducible fabrication at the nanometer scale 4-6 . Furthermore, natural silicon consists of 95 % non-magnetic nuclei (92 % 28 Si, 3 % 30 Si), suppressing hyperfine-induced decoherence 7-9 . A nearly nuclear-spin-free environment can additionally be engineered by means of isotopic purification 10 . Electron spins in silicon are also subject to a weak spin-orbit interaction (SOI) and can thus be almost completely isolated from environmental noise 11 . As a result, an excellent dephasing time T * 2 of 120 µs has been demonstrated for the electron spin qubit in isotopically enriched silicon (≥ 99.9 % of 28 Si) 5 .For scalable quantum circuits, qubit control via electric rather than magnetic fields is more promising in terms of speed and hardware implementation. In this regard, the hole spin represents an attractive alternative to its electron counterpart [12][13][14] . The asymmetry of the silicon band structure with respect to the conduction (CB) and valence bands (VB) manifests itself in different characteristics for electrons and holes. While the electron Bloch function has s-wave symmetry, the hole has p-wave symmetry. Consequently, hole spins experience a weaker hyperfine, yet stronger SOI, which enables fast, all-electrical spin manipulation 15-17 . Despite these potential benefits, hole spin qubits in silicon are still largely unexplored. Recently, qubit functionality with fast, purely electrical, two-axis control was shown for a hole spin, yet with inferior coherence compared to the electron spin 6 .Usually, either electrons 18-21 or holes 6,17,22-24 are confined in silicon QDs. Ambipolar devices, by contrast, can be operated in both the electron and hole regime [25][26][27][28][29][30][31][32] . For planar silicon metal-oxide-semiconductor (MOS) QD structures, ambipolar behavior was demonstrated by integrating both n-and p-type reservoirs on the same device 33-37 . Ambipolar dev...

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