The ability to coherently control matter qubits is essential for realizations of quantum information processing (QIP). In general, QIP requires "rotation" of the qubit state from any point on its Bloch sphere to any other, desired point on it. Such rotations should be performed with high fidelity, much faster than the qubit's life- and decoherence times.Controlling spins of charge carriers in semiconductor quantum dots (QDs) are particularly important in this respect, since they form an excellent interface between "flying" qubits (photons) and "anchored" ones (spins). Therefore, significant efforts are devoted towards achieving these goals. The control of the spin of a single QD confined electron was recently demonstrated using stimulated Raman scattering [1, 2]. Another suggested approach is to use the accumulation of a geometrical phase through a 2π optical pulse [3]. Implementation of this approach was demonstrated on ensembles of QD confined electrons using frequency locking [4], and more recently, in a limited way also on a single QD electron spin [5].  In these works, however, the preparation and/or the control are achieved by a set of sequential optical pulses and the coherent evolution of the spins between the pulses is an essential part of the control scheme. These limit the methods' applicability.

We demonstrate, for the first time, complete control over the spin of a single correlated electron-hole pair (exciton) confined in a single QD.  We do this by a single picosecond long, polarized optical pulse tuned to a specific two-exciton (biexciton) resonance. Our approach requires no external magnetic field and it does not depend on the coherent evolution of the excitonic spin. We achieved three degrees of freedom in rotating the exciton spin, which permit an arbitrary rotation of the spin-qubit. Two degrees of freedom are readily achieved by choosing the polarization of the optical control pulse. The pulse polarization defines the direction of the axis around which the exciton spin is forced to rotate. A third degree of freedom is achieved by detuning the pulse from resonance. While a resonant 2π pulse results in a π rotation about the pulse-polarization axis, detuning results in spin rotation, at any other desired angle.

We performed a sequence of experiments using three optical pulses to demonstrate this control. The first, initiating pulse, is a left hand circularly polarized (L) π-pulse tuned into an excitonic resonance of a semiconductor QD embedded in a one wavelength planar microcavity. It photogenerates a coherent superposition of the two non-degenerate eigenstates of the exciton, |H> and |V>, which give rise to horizontally (H) and vertically (V) linearly polarized emission lines [6]. The spin of this coherent state |L>, precesses in time along the equator of the spin's Bloch sphere, as we have recently demonstrated [6]. A third, L-polarized π- pulse, is tuned to a biexciton resonance. This pulse is used to probe the spin state of the precessing exciton since the excitonic population that it transfers to the biexciton depends on the exciton's spin direction at that time [6]. The second, control pulse, rotates the exciton's spin. It is a variably polarized 2π-pulse tuned (or slightly detuned) from the biexciton resonance and exactly one precession period (T=122 psec) before the probe pulse.

 References

[1] Berezovsky J. et al., Science 320, 349, (2008).

[2] Press D. et al., Nature, 456, 218, (2008).

[3] Economou S. et al, Phys. Rev. Lett. 99, 217401 (2007).

[4] Greilich A. et al., Nat. Phys. 5, 262 (2009).

[5] Kim E. D. et al., Phys. Rev. Lett. 104, 167401, (2010).

[6] Benny Y, et al., Phys. Rev. Lett. 106, 040504, (2011).