  A single hole spin in a semiconductor quantum dot has emerged as a quantum
bit that is potentially superior to an electron spin. A key feature of holes is
that they have a greatly reduced hyperfine interaction with nuclear spins,
which is one of the biggest difficulties in working with an electron spin. It
is now essential to show that holes are viable for quantum information
processing by demonstrating fast quantum gates and scalability. To this end we
have developed InAs/GaAs quantum dots coupled through coherent tunneling and
charged with controlled numbers of holes. We report fast, single qubit gates
using a sequence of short laser pulses. We then take the important next step
toward scalability of quantum information by optically controlling two
interacting hole spins in separate dots.
