Most schemes for quantum information processing require fast single qubit operations. For spin-based qubits, this involves performing arbitrary coherent rotations of the spin state on timescales much faster than the spin coherence time. Through the optical, or AC, Stark effect an intense, non-resonant optical pulse creates a large effective magnetic field (here, BStark ~ 10 tesla) along the direction of light for the duration of the pulse (~ 30 ps).

A single electron spin in a GaAs quantum dot (QD) is manipulated using this optical Stark effect and measured using time-resolved Kerr rotation. To perform this sequential initialization, manipulation, and read-out scheme, we use three synchronized, independently tunable optical pulse trains that are focused onto the sample: the pump (initializes the spin), the tipping pulse (manipulates the spin), and the probe (measures the spin).

Figure 1 shows the coherent precession of a single spin in a transverse magnetic field Bz = 715 G without (top panel) and with (bottom panel) an applied tipping pulse. The detuning and intensity of the tipping pulse is chosen such as to induce ~ π rotation about the y-axis. The diagrams on the right schematically show the evolution of the spin on the Bloch sphere.

Further details of this spin manipulation can be investigated by varying the tipping pulse intensity and detuning from the transition energy, as shown in fig. 2. The arrival of the tipping- and probe pulse is fixed, as illustrated in fig. 2b. It is observed that the single electron spin state can be coherently rotated though arbitrary angles up to π radians. The gray lines are one-parameter fits to our model, which includes the effects of nuclear polarization. It is seen that when the tipping pulse is further detuned from the transition energy it induces a smaller rotation of the spin for the same intensity. In fig. 2c the intensity required for a π-rotation is plotted for different detunings. A linear dependence on detuning is observed as also expected from theory.

To learn more about our studies, please refer to: "Picosecond Coherent Optical Manipulation of a Single Electron Spin in a Quantum Dot", J. Berezovsky, M. H. Mikkelsen, N. G. Stoltz, L. A. Coldren, D. D. Awschalom, Science 320, 349 (2008).