Mitigating noise for singlet-only always-on gapless exchange (SAGE) qubits: Two-qubit gates

The singlet-only always-on gapless exchange (SAGE) qubit is a novel type of exchange-only qubit that encodes a single qubit in the spins of four electrons while allowing universal baseband control. While conventional EO qubits are susceptible to local magnetic field gradients (caused, e.g., by local nuclear environments and g-factor variations), the SAGE qubit subspace is protected from magnetic-gradient-induced coherent errors, and leakage out of the computational subspace is suppressed energetically due to the exchange interactions being always-on. Here, we show that SAGE two-qubit gates can be performed with a single interqubit exchange pulse with times comparable in duration to conventional EO qubits without the need for a complicated multi-pulse sequence. We characterize the performance of SAGE two-qubit gates in the presence of 1/f charge noise and quasistatic magnetic noise, and find specific dynamical decoupling sequences to mitigate the charge noise while simultaneously suppressing leakage to noncomputational states.