  A new approach to single-qubit operations using exchange interactions of
single electrons in gate-defined quantum dots has recently been demonstrated:
the resonant exchange qubit. We show that two-qubit operations, specifically
the CPHASE gate, can be performed between resonant exchange qubits very
straightforwardly, using a single exchange pulse. This is in marked contrast to
the best known protocols for exchange qubits where such a gate requires many
pulses so that leakage processes arising from the exchange interaction can be
overcome. For resonant exchange qubits a simple two-qubit gate is possible
because in this mode of operation energy conservation suppresses leakage.
