  Hole spin qubits are frontrunner platforms for scalable quantum computers
because of their large spin-orbit interaction which enables ultrafast
all-electric qubit control at low power. The fastest spin qubits to date are
defined in long quantum dots with two tight confinement directions, when the
driving field is aligned to the smooth direction. However, in these systems the
lifetime of the qubit is strongly limited by charge noise, a major issue in
hole qubits. We propose here a different, scalable qubit design, compatible
with planar CMOS technology, where the hole is confined in a curved germanium
quantum well surrounded by silicon. This design takes full advantage of the
strong spin-orbit interaction of holes, and at the same time suppresses charge
noise in a wide range of configurations, enabling highly coherent, ultrafast
qubit gates. While here we focus on a Si/Ge/Si curved quantum well, our design
is also applicable to different semiconductors. Strikingly, these devices allow
for ultrafast operations even in short quantum dots by a transversal electric
field. This additional driving mechanism relaxes the demanding design
constraints, and opens up a new way to reliably interface spin qubits in a
single quantum dot to microwave photons. By considering state-of-the-art
high-impedance resonators and realistic qubit designs, we estimate interaction
strengths of a few hundreds of MHz, largely exceeding the decay rate of spins
and photons. Reaching such a strong coupling regime in hole spin qubits will be
a significant step towards high-fidelity entangling operations between distant
qubits, as well as fast single-shot readout, and will pave the way towards the
implementation of a large-scale semiconducting quantum processor.
