  The ability to control and exploit quantum coherence and entanglement drives
research across many fields ranging from ultra-cold quantum gases to spin
systems in condensed matter. Transcending different physical systems, optical
approaches have proven themselves to be particularly powerful, since they
profit from the established toolbox of quantum optical techniques, are
state-selective, contact-less and can be extremely fast. Here, we demonstrate
how a precisely timed sequence of monochromatic ultrafast (~2-5 ps) optical
pulses, with a well defined polarisation can be used to prepare arbitrary
superpositions of exciton spin states in a semiconductor quantum dot, achieve
ultrafast control of the spin-wavefunction without an applied magnetic field
and make high fidelity read-out the quantum state in an arbitrary basis simply
by detecting a strong (~2-10$ pA) electric current flowing in an external
circuit. The results obtained show that the combined quantum state preparation,
control and read-out can be performed with a near-unity (>97%) fidelity. Our
methods are fully applicable to other quantum systems and have strong potential
for scaling to more complex systems such as molecules and spin-chains.
