A Gate-driven Entanglement Switch, Magic Angles, and a Decoherence Free Subspace in Acceptor Spin Qubits in Si

Electrical control of quantum bits could pave the way for fast, low-power, scalable quantum computation. It was recently shown that an acceptor spin qubit in Si, based on holes rather than electrons, offers full electrical control, fast operations with long relaxation and dephasing times, and entanglement based on dipole-dipole interactions. The two-qubit operations are limited by the dipole-dipole interaction itself, since it can only be turned off by deactivating one of the qubits. The other limiting factor is the relaxation time T₁, which is difficult to enhance if fast operations are desired using purely electrical means. Here we show theoretically that an appropriate fixed magnetic field in-plane orientation allows to electrically switch two-qubit coupling on and off while enhancing the relaxation time T₁ near a decoherence free subspace. This magic angle stems from the interplay of the T_d symmetry of the acceptor in the Si lattice and the spin-3/2 characteristic of hole systems. Our findings can be directly applied to state-of-the-art acceptor based architectures, for which we propose suitable protocols to practically achieve full electrical tunability of entanglement and the realization of a decoherence-free subspace. arXiv:1706.08858