  Spin qubits in silicon and germanium quantum dots are promising platforms for
quantum computing, but entangling spin qubits over micrometer distances remains
a critical challenge. Current prototypical architectures maximize transversal
interactions between qubits and microwave resonators, where the spin state is
flipped by nearly resonant photons. However, these interactions cause
back-action on the qubit, that yield unavoidable residual qubit-qubit couplings
and significantly affect the gate fidelity. Strikingly, residual couplings
vanish when spin-photon interactions are longitudinal and photons couple to the
phase of the qubit. We show that large longitudinal interactions emerge
naturally in state-of-the-art hole spin qubits. These interactions are fully
tunable and can be parametrically modulated by external oscillating electric
fields. We propose realistic protocols to measure these interactions and to
implement fast and high-fidelity two-qubit entangling gates. These protocols
work also at high temperatures, paving the way towards the implementation of
large-scale quantum processors.
