"Initialization, Readout and Control of the Spin State of Single Charge Carriers' Pairs Using Single Picosecond Laser Pulses."

The ability to coherently control matter qubits is essential for realizations of quantum information processing. In general, quantum information processing requires in addition to the ability to write and read a qubit state, also to "rotate" the state from any point on its Bloch sphere to any other desired point on it. This ability is referred to as single qubit gates. It goes without saying that these operations should be performed with high fidelity, much faster than the qubit's life- and decoherence-times. Controlling the spins of charge carriers in semiconductor quantum dots is particularly attractive, since quantum dots form an excellent interface between "flying" qubits (photons) and "anchored" ones (spins). Hence, significant world-wide efforts are devoted for achieving this goal. We demonstrate, for the first time, initialization, readout [1] and complete control over the spin of single correlated electron-hole pairs (excitons) confined in single semiconductor quantum dots. Each one of these operations is performed using a single, picoseconds long, polarized optical pulse tuned to a specific optical resonance of these nanostructures. Our novel approach requires no external magnetic field and it does not depend on the coherent evolution of the pairs' spin as in the case of quantum dot confined electron spins [2, 3]. We achieved three degrees of freedom in rotating the exciton spin, establishing thus an arbitrary rotation of the spin-qubit,  a complete set of single qubit gates. Two degrees of freedom are readily achieved by choosing the polarization of the optical control pulse. The pulse polarization defines the direction in space of the axis around which the exciton spin is forced to rotate. The third degree of freedom, the angle of rotation, is achieved by detuning the control pulse from resonance.

1. Benny, Y. et al. Coherent optical writing and reading of the exciton spin state in single quantum dots. Phys. Rev. Lett. 106, 040504 (2011).

2. Berezovsky, J. et al. Picosecond coherent optical manipulation of a single electron spin in a quantum dot. Science 320, 349 (2008).

3. Press, D et al. Complete quantum control of a single quantum dot spin using ultrafast optical pulses. Nature 456, 218 (2008).

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