Fast Optical Preparation, Control, and Readout of a Single Quantum Dot Spin

We propose and demonstrate the sequential initialization, optical control, and readout of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged-exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent readout of the hole spin state.

Figure 1

Illustration of operating principle. Preparation: (a) Resonant excitation of the $0-X^0$ transition creates a spin-polarized electron-hole pair. Filled (open) arrows are electron (hole), respectively. (b) Under applied electric field the electron tunnels from the dot, leaving a spin-polarized hole. Readout: (c) A circularly polarized $\pi$ pulse creates a charged exciton only if the hole is in the target spin state. (d) Carriers tunnel from the dot, the creation of a charged exciton resulting in a change in the photocurrent proportional to the occupation of the target hole spin state. Inset: Control (i) Energy levels of heavy-hole or charged-exciton system which acts as two decoupled 2-level atoms: $h_{\pm }-X_{\pm }^{+}$. (ii) Driving a Rabi rotation with ${\sigma }_{+}$ circular polarization addresses the $h_{-}-X_{-}^{+}$ transition only.

Figure 2

Photocurrent versus laser detuning. (Lower part) Single circular polarized $\pi$ pulse, with $0-X^0$ peak. (Middle and upper parts) Two pulse spectra, with preparation pulse resonant with $0-X^0$ transition, with short (long) time delay. The upper trace is offset for clarity. Note the emergence of the $h-X^{+}$ line at longer time delays for cross-polarized excitation.

Figure 3

(a) Time-resolved measurements of heavy-hole population. Photocurrent versus time delay between prepulse and a control pulse resonant with the $h-X^{+}$ transition. (b) Rabi rotation of the $h-X^{+}$ transition, where creation of the charged exciton is conditional on the heavy-hole spin state. The $\pi$ pulse provides a readout of the hole spin state. (c) Polarization dependence $({\Omega }_{-},{\Omega }_{+})\equiv \Omega [\cos (\alpha ),\sin (\alpha )]$ of the $h-X^{+}$ Rabi rotation. (▾) Inverse period of $h-X^{+}$ Rabi rotation versus polarization angle $\alpha$ of the control pulse, for a ${\sigma }_{-}$ preparation pulse. The line shows the $|\sin (\alpha )|$ dependence confirming the independence of the $h_{\pm }-X_{\pm }^{+}$ transitions. (▴) Amplitude of Rabi rotation for ${\sigma }_{+}$-polarized control pulse versus the polarization angle of the preparation pulse.