Decoherence of Dipole Coupled Flip-Flop Qubits

A recent proposal for a scalable donor-based quantum computer scheme promises excellent coherence properties, fast qubit couplings and insensitivity to donor placement. The suggested system consists of two different types of qubits per donor: a flip-flop qubit consisting of the electron and nuclear spin states, and a charge qubit of the donor electron tunneling between the donor and an interface quantum dot. In this scheme, the qubits can be coupled to each other via the electric dipole interaction between their respective charge qubits. We study in detail this effective coupling, especially the effect of charge noise on two-qubit gates utilizing this coupling. We find that due to the proximity of the charge excited states to the flip-flop logical states, the presence of charge noise greatly reduces the fidelity of two-qubit operations. We calculate the qubit-noise interaction strengths, and identify leakage from the qubit Hilbert space as the main culprit of the reduced gate fidelity. We also explore different bias conditions to mitigate this decoherence channel.