  The spin states of electrons confined in semiconductor quantum dots form a
promising platform for quantum computation. Recent studies of silicon CMOS
qubits have shown coherent manipulation of electron spin states with extremely
high fidelity. However, manipulation of single electron spins requires large
oscillatory magnetic fields to be generated on-chip, making it difficult to
address individual qubits when scaling up to multi-qubit devices. The
spin-orbit interaction allows spin states to be controlled with electric
fields, which act locally and are easier to generate. While the spin-orbit
interaction is weak for electrons in silicon, valence band holes have an
inherently strong spin-orbit interaction. However, creating silicon quantum
dots in which a single hole can be localised, in an architecture that is
suitable for scale-up to a large number of qubits, is a challenge. Here we
report a silicon quantum dot, with an integrated charge sensor, that can be
operated down to the last hole. We map the spin states and orbital structure of
the first six holes, and show they follow the Fock-Darwin spectrum. We also
find that hole-hole interactions are extremely strong, reducing the two-hole
singlet-triplet splitting by 90% compared to the single particle level spacing
of 3.5 meV. These results provide a route to single hole spin quantum bits in a
planar silicon CMOS architecture.
