Decoherence of a donor-dot hybrid qubit in Si*

A recent proposal for a scalable donor-based quantum computer scheme promises excellent coherence properties and fast qubit couplings [1]. The system consists of two types of qubits per donor: a flip-flop qubit consisting of the electron and nuclear spins, and a charge qubit of the donor electron tunneling between the donor and an interface quantum dot. The proposal identifies a parameter regime where flip-flop qubit dephasing due to electrical noise is strongly suppressed.

We study the decoherence properties of the flip-flop qubit when positioned near this sweet spot. In particular, we study the effect of charge noise that is coupled to the flip-flop qubit via the dependence of the hyperfine interaction and the electron gyromagnetic ratio on the charge qubit state. We find that zero frequency noise is indeed suppressed at the sweet spot. In the meantime, finite-frequency contributions from the qubit coupling to the excited charge states come into play at shorter time scales, although their effects average out over longer times. We also explore the decoherence dependence on external control parameters such as the applied magnetic field and tunnel coupling between the donor and dot.

[1] G. Tosi et al., Nature Comms 8, 450 (2017).