Resonant optical control of the spin of a single Cr atom in a quantum dot

A Cr atom in a semiconductor host carries a localized spin with an intrinsic large spin to strain coupling, which is particularly promising for the development of hybrid spin-mechanical systems and coherent mechanical spin driving. We demonstrate here that the spin of an individual Cr atom inserted in a semiconductor quantum dot can be controlled optically. We first show that a Cr spin can be prepared by resonant optical pumping. Monitoring the time dependence of the intensity of the resonant fluorescence of the quantum dot during this process permits us to probe the dynamics of the optical initialization of the Cr spin. Using this initialization and readout technique we measured a Cr spin relaxation time at $T=5$ K in the microsecond range. We finally demonstrate that, under a resonant single-mode laser field, the energy of any spin state of an individual Cr atom can be independently tuned by using the optical Stark effect.

Figure 1

(a) PL spectra of the exciton in a Cr-doped QD (X-Cr) for co- and cross-circularly polarized excitation/detection. (b) Linear polarization intensity map of X-Cr. (c) Energy levels in a Cr-doped QD and configuration of excitation/detection for resonant optical pumping experiments. The Cr states $S_z=0,\pm 1$ are split by the magnetic anisotropy $D_0{S}_z^2$. In X-Cr, the exchange interaction with the bright exciton ($|\pm 1\rangle$) split the states $S_z=\pm 1$. The higher energy Cr states, $S_z=\pm 2$, and the dark exciton states are not displayed.

Figure 2

(a) PL transients recorded in circular polarization on lines 4 and 3 under the resonant (pump on 1) and quasiresonant (probe at $E_{\mathrm{exc}.}\approx 2068$ meV) optical excitation sequences displayed at the bottom. Inset: PL of X-Cr and configuration of the resonant excitation and detection. (b) Excitation power dependence of the optical pumping time. (c, d) Energy detuning dependence of resonant PL intensity ($I_1$ at the beginning and $I_2$ at the end of the pump pulse) and of the corresponding normalized amplitude of pumping transient ($I_1-I_2)/I_2$.

Figure 3

(a) Time evolution of the PL intensity of line 4 of X-Cr under resonant excitation on line (1) with a circularly polarized excitation pulse. (b) Evolution of the amplitude of the pumping transient $(I_1-I_2)/I_2$ as a function of the dark time between the excitation pulses. The black line is an exponential evolution with a characteristic time ${\tau }_{\mathrm{Cr}}=1.7\mu \mathrm{s}$.

Figure 4

(a) PL of X-Cr and configuration of excitation in the resonant optical control experiments. The inset illustrates the laser-induced splittings in the ground and excited states for a $\sigma +$ excitation on $S_z=-1$. (b) PL intensity maps of lines 5 and 4 for an excitation on 1 as a function of the detuning (top) and of the excitation intensity (bottom). The PL is produced by a second nonresonant laser. The corresponding emission line shapes are presented in panel (c) for line 5. The insets in panel (c) show the splitting of the PL doublet as a function of the excitation intensity (bottom) and laser detuning (top). The fit is obtained with $\hslash {\mathrm{\Omega }}_r=100\mu \text{eV}$. (d) PL of lines 1 and 2 for a laser on resonance with the dark exciton state (5). Inset: PL intensity map of line 1 as a function of the laser detuning around (5).