Exchange coupling of spin qubits via a multielectron quantum dot*

We present a theoretical description for the effective exchange interaction between distant spin qubits mediated by a multielectron quantum dot. Our model is used to interpret experimentally observed exchange interactions between singlet-triplet qubits in such a configuration. We show that a non-zero spin correlation energy for the mediator dot is required to explain key features of the measured exchange profile. The competition between the standard singlet-favouring exchange and the triplet-favouring exchange results in a sweet-spot where the interaction is resistant to charge noise on gate voltages. We argue that multielectron quantum dots are a promising avenue for fast, noise-resistant coupling of spin qubits.