Ambipolar quantum dots
Use of the spin states of single electrons in coupled quantum dots were proposed in the 1990s for the implementation of one- and two-qubit gates. Specifically, the spin-up and the spin-down state of electrons could represent a quantum bit (qubit). Since then, several semiconductor platforms have been explored to isolate single electrons. Quantum information processing based on the spin of single-charge carriers requires long spin coherence times, and Si provides such an environment where spins can be controlled with minimal decoherence. In the SISQ (Silicon spin quantum bits) project, researchers proposed ambipolar quantum dots in Si. The aim was to exploit ambipolarity to operate quantum dots in either the electron or the hole regime and, thus, have the capability to compare the two charge carriers in the same crystalline environment. The starting point was a metal-oxide semiconductor field-effect transistor-based fabrication scheme that uses metal gates on top of a thin silicon dioxide (SiO2) layer to define quantum dots electrostatically. The researchers took this scheme, which combines micro- and nanofabrication, one step further. Regions heavily doped with both electrons and holes were incorporated into the ambipolar quantum dot device consisting of two gate layers. An electron or hole gas is formed at the Si/SiO2 interface by a lead gate. Nano-scale 'barrier' gates locally control the charge carrier density. The ambipolar design supports operation the SISQ device as an electron as well as a hole quantum dot. Moreover, the suitability of electron- and hole-spin qubits can be evaluated and the advantages of either qubit leveraged to improve future quantum complementary metal-oxide semiconductor technology.
Keywords
Ambipolar, quantum dots, quantum information processing, silicon, qubit, SISQ
