Figure 1

(a) Schematic energy diagram of the device, containing two layers of self-assembled InGaAs QDs, separated by a 9 nm GaAs tunnel barrier and embedded in a GaAs Schottky diode. The voltage $V$ applied between the Si-doped $n^{+}$-GaAs back contact and the semitransparent top gate (2 nm of Ti plus 8 nm of Au) controls the CQD charging state and allows both QDs of a pair to be filled with a single electron. (b) Spin-singlet ($S$) and -triplet ($T$) states in the $(1,1)$ charging regime. (c) Energy diagram showing the different ground states (bottom panel) and optically excited states (top panel) versus $V$. State $(1,1)S$ is coupled via electron tunneling to $(2,0)S$ and $(0,2)S$, in which both electrons reside in QD-B or QD-R, respectively (as illustrated in the gray boxes, where filled circles depict electrons and open circles holes). The coupling gives rise to two anticrossings between the $S$ states that split $(1,1)S$ from $(1,1)T$, since the latter does not experience tunnel coupling to any of the $S$ states [17]. Dashed lines indicate energy levels not used in the experiment. Inset: Optical selection rules for transitions from the $(1,1)S$ and $T$ states to the fourfold degenerate optically excited states $X$.Reuse & Permissions

Figure 2

PL (in color scale) as a function of gate voltage. (a) PL from QD-B. Dotted vertical lines separate regions with a different total number of electrons in the CQD; the inferred ground state charge distribution for each region is indicated below the panel. In the $(1,1)$ charging region (highlighted by orange boxes), PL involving the $S$ state is identified by its characteristic curvature and by its $\sim 3$ times weaker intensity compared to PL involving the threefold degenerate $T$ states. $X_B^0$ ($X_B^{1-}$) indicates emission from the neutral exciton (negative trion) in QD-B. Inset: Schematic energy diagram illustrating $X_B^{1-}$ emission in the $(1,1)$ regime. Because holes can tunnel from QD-B to QD-R before recombination, PL from QD-B is weaker than that from QD-R. (b) PL from QD-R. $X_R^0$ ($X_R^{1-}$) indicates emission from the neutral exciton (negative trion) in QD-R. Inset: Schematic energy diagram illustrating emission from the optically excited states $X$ to states $S$ or $T$ in the $(1,1)$ regime.Reuse & Permissions

Figure 3

(a) Resonance fluorescence detected with a spectrometer, when resonantly driving the $S\mathrm{\text{−}}X$ transition (upper trace) or the $T\mathrm{\text{−}}X$ transition (lower trace) close to saturation (Rabi frequencies ${\Omega }_{S,T}\sim 1\mu \mathrm{eV}$), at $V=-121\mathrm{mV}$. Orange arrows indicate the excitation energy. The reflected linearly polarized excitation laser is suppressed by using a polarizer. Traces are offset vertically for clarity. (b) Differential transmission $dT/T$ (in color scale) of a laser (with ${\Omega }_S=0.5\mu \mathrm{eV}$) scanned across the $S\mathrm{\text{−}}X$ transition versus $V$ throughout the $(1,1)$ regime. Inset: Schematic energy diagram of the lambda system driven by a laser on the $S\mathrm{\text{−}}X$ transition. (c) Differential transmission $dT/T$ (in color scale) on the $T\mathrm{\text{−}}X$ transition, with ${\Omega }_T=1.0\mu \mathrm{eV}$; the contrast of the resonance is only $-0.011\%$. Inset: Schematic energy diagram of the lambda system driven by a laser on the $T\mathrm{\text{−}}X$ transition.Reuse & Permissions

Figure 4

(a) Differential reflection $dR/R$ (in color scale) of a weak probe laser (Rabi frequency ${\Omega }_T=0.6\mu \mathrm{eV}$) scanned across the $T$ transition, in the presence of a strong but undetected pump laser stepped across the $S$ transition, at $V=-94\mathrm{mV}$. The two lasers have orthogonal linear polarizations, allowing the reflected pump light to be extinguished by using a polarizer. The strong pump leads to small fluctuations in the exact resonance condition due to the creation of charges around the CQD [20]. The size of the Autler-Townes splitting in the bottom panel allows a calibration of ${\Omega }_S$ in terms of the pump laser power on the $S$ transition, $P_S$. (b) Schematic energy level diagram of the lambda system formed by states $S$, $T$, and $X$. ${\Omega }_S$ and ${\Omega }_T$ indicate the laser Rabi frequencies; the effective spontaneous emission rate from $X$ to the combined triplet states is about 3 times faster than to the singlet state (${\Gamma }_T\approx 3{\Gamma }_S$). We observe fast relaxation (with rate $\gamma$) from $T$ to $S$. (c) $dR/R$ (in color scale) of a weak probe laser (${\Omega }_S=0.5\mu \mathrm{eV}$) scanned across the $S$ transition, in the presence of a strong but undetected pump laser (${\Omega }_T=10.4\mu \mathrm{eV}$) resonant with the $T$ transition, versus $V$. Inset: ${\Omega }_T$ as a function of the square root of the pump laser power $\sqrt{P_T}$. ${\Omega }_T$ is determined from the Rabi splitting in measurements as shown in the main panel.Reuse & Permissions