Entanglement control and magic angles for acceptor qubits in Si

Abstract

Full electrical control of quantum bits could facilitate fast, low-power, scalable quantum computation. Although electric dipoles are highly attractive to couple spin qubits electrically over long distances, mechanisms identified to control two-qubit couplings do not permit single-qubit operations while two-qubit couplings are off. Here, we identify a mechanism to modulate electrical coupling of spin qubits which overcomes this drawback for hole spin qubits in acceptors which is based on the electrical tuning of the direction of the spin-dependent electric dipole by a gate. This allows the inter-qubit coupling to be turned off electrically by tuning to a “magic angle” of vanishing electric dipole-dipole interactions, while retaining the ability to manipulate the individual qubits. This effect stems from the interplay of the Td symmetry of the acceptor state in the Si lattice with the magnetic field orientation and the spin-3/2 characteristic of hole systems. The magnetic field direction also allows us to greatly suppress spin relaxation by phonons that limit single qubit performance, while retaining sweet spots where the qubits are insensitive to charge noise. We propose suitable protocols to practically achieve full electrical tunability of entanglement and the isolation of the qubit.