Single-spin optical read-out in CdTe/ZnTe quantum dot studied by photon correlation spectroscopy

Spin dynamics of a single electron and an exciton confined in CdTe/ZnTe quantum dot is investigated by polarization-resolved correlation spectroscopy. Spin memory effects extending over at least a few tens of nanoseconds have been directly observed in magnetic field and described quantitatively in terms of a simple rate equation model. We demonstrate an effective (68%) all-optical read-out of the single carrier spin state through probing the degree of circular polarization of exciton emission after the capture of an oppositely charged carrier. The perturbation introduced by the pulsed optical excitation serving to study the spin dynamics has been found to be the main source of the polarization loss in the read-out process. In the limit of low laser power, the read-out efficiency extrapolates to a value close to 100%. The measurements allowed us as well to determine neutral exciton spin relaxation time ranging from $3.4\pm 0.1\text{ns}$ at $B=0\text{T}$ to $16\pm 3\text{ns}$ at $B=5\text{T}$.

Figure 1

(Color online) Polarization-resolved emission spectra of the selected quantum dot at (a) $B=0\text{T}$ and (b) $B=5\text{T}$ detected in linear $H\text{−}V$ and circular $\sigma +/\sigma -$ polarization bases, respectively. Excitation at the energy of 3.1 eV with the average power $\sim 0.8P_{\text{SAT}}$ (the saturation power of the $X$ emission $P_{\text{SAT}}=1.2\mu \text{W}$ at $B=0\text{T}$).

Figure 2

(Color online) Emission energies of (a) $X$, (b) $CX$, and (c) $XX$ versus angle of detected linear polarization (points). Solid line represents a sinusoidal fit (a) and (c) or a linear fit (b).

Figure 3

(Color online) Energy of $X$, $CX$, and $XX$ emissions relative to zero-field $X$ energy, measured in both circular polarizations as a function of magnetic field in Faraday configuration. Solid lines represent the calculation of the Zeeman splitting with Landé factor $g=-3.4$ the same for each line.

Figure 4

(Color online) Polarized cross correlation of the $XX$ and $X$ emission. Each panel contains histograms for four possible polarization combinations of the photon pair. Polarizations of correlated transitions are indicated in $XX/X$ order. Points represent integrated counts in a peak, normalized to the average value for large delays. Solid lines are guides to the eye. Magnitudes of zero delay peaks in respective histograms are given in each panel. Polarization correlation of photons from $XX\text{−}X$ cascade is evidenced in (a) linear $H/V$ polarization basis at $B=0\text{T}$ and in (c) circular $\sigma +/\sigma -$ polarization basis at $B=5\text{T}$. There is no polarization correlation in (b) circular $\sigma +/\sigma -$ polarization basis at $B=0\text{T}$. Panel (a) shows additionally a raw coincidence counts histogram (left axis).

Figure 5

(Color online) Histograms of $CX\text{−}X$ cross correlation measured in the circular $\sigma +/\sigma -$ polarization basis for four possible combinations of the polarization of the $CX\text{−}X$ photon pair (a) at $B=0\text{T}$ and at (b) $B=5\text{T}$. Polarizations of correlated transitions are indicated in $CX/X$ order. Points represent integrated and normalized (to the average at large delays) number of counts in a peak. Solid lines are guides to the eye. Values given in panels represent normalized number of counts in $m=1$ peak of a respective histogram. Excitation power at $I_{XX}/I_X=0.30$ (see Sec. 4). Enhanced $m=1$ peaks in the case of correlation between orthogonal $CX$ and $X$ polarizations indicate transfer of spin orientation from $CX$ to $X$ over time of the repetition period (b).

Figure 6

(Color online) Polarization correlation versus pulse number at $B=5\text{T}$. Experimental values (squares) are compared with calculation (dots). Model parameters $\alpha =0.71$, $\beta =0.66$, $\xi =0.22$, and $\kappa =0.69$ (see text). Excitation power at $I_{XX}/I_X=0.26$. Lines added to guide the eye.

Figure 7

(a) Polarization correlation at $B=5\text{T}$ for the first pulse after $CX$ emission $P_{\text{exp}}^{\left(1\right)}$ versus excitation power represented by the ratio $I_{XX}/I_X$. Solid line calculation $\left(\kappa =1\right)$ according to Eq. (2). (b) Coefficient $\kappa$ of the carrier spin conservation at $B=5\text{T}$ determined from fitting of the $P_{\text{calc}}^{\left(1\right)}$ to $P_{\text{exp}}^{\left(1\right)}$ for different excitation powers.