A tunable J-gate for spin qubits in donors coupled to an interface

A tunable J-gate for spin qubits in donors coupled to an interface
The spin qubit in a single donor in silicon has been proposed for quantum computing for over a decade. Extremely long spin coherence time has been demonstrated for both electron and nuclear spin in a silicon donor. However, tunable exchange interaction between electrons on nearby donors has not been demonstrated due to the lack of tunability of tunnel barrier. Here we show that the J-gate in the original Kane proposal can be effective as long as the donor depth is reduced, and an effective cotunneling and superexchange is mediated by coupling to an interface state. The quantum-dot-like confinement of interface state is provided by the potential of the two nearby donors, avoiding the fabrication complexity of a gate defined quantum dot. The tunability is achieved by using an applied E-field perpendicular to the interface to control the detuning between the donor and QD states, which is possible due to the different depth of the donor and QD state. By using tight-binding simulations, we show that the effective exchange interaction can be increased by three orders of magnitude from 2 MHz to 6 GHz for two donors separated by 20 nm and 5 nm below the interface.