Coherent driving of direct and indirect excitons in a quantum dot molecule

Quantum dot molecules (QDMs) are one of the few quantum light sources that promise deterministic generation of one- and two-dimensional photonic graph states. The proposed protocols rely on coherent excitation of the tunnel-coupled and spatially indirect exciton states. Here, we demonstrate power-dependent Rabi oscillations of direct excitons, spatially indirect excitons, and excitons with a hybridized electron wave function. An off-resonant detection technique based on phonon-mediated state transfer allows for spectrally filtered detection under resonant excitation. Applying a gate voltage to the QDM device enables a continuous transition between direct and indirect excitons and, thereby, control of the overlap of the electron and hole wave function. This does not only vary the Rabi frequency of the investigated transition by a factor of $\approx 3$, but also allows to optimize graph state generation in terms of optical pulse power and reduction of radiative lifetimes.

Figure 1

Rabi oscillations of the neutral exciton in a QDM. (a) Schematic band structure of a QDM represented by a double-well potential. An AlGaAs barrier below the molecule prolongs tunneling times for electrons while not affecting tunneling for holes. One hole (empty circle) is located in the upper QD, while electrons (filled circles) occur in both QDs. As a consequence, direct (red ellipse) and indirect (blue ellipse) excitons arise. A gate voltage U applied to the sample facilitates tuning of the direct and indirect exciton energies relative to each other. (b) Voltage-dependent photoluminescence of the neutral exciton. The red and blue dashed lines indicate the energies of the direct and indirect excitons. Tunnel coupling between the two QDs leads to an avoided crossing with a symmetric (pink) and an antisymmetric (green) electron eigenstate. The upper (lower) energy transition is called UP (LOW). Triangles indicate the excitation energy and voltage applied in Fig. 2. (c) Neutral exciton state diagram illustrating the excitation and detection scheme for monitoring Rabi oscillations. While a resonant light field (green) is driving UP, a phonon-mediated state transfer with rate ${\gamma }_P$ (black arrow) is enabling emission from both UP and the energetically detuned LOW. (d) Power-dependent Rabi oscillations when exciting UP and detecting UP (green) or LOW (pink) at 0.1 V.

Figure 2

Rabi oscillations of the UP and LOW branch at 0.1 V (left) and 0.22 V (right) by phonon-mediated state transfers. The red data points correspond to a direct, the blue to an indirect, driven transition.

Figure 3

Measured voltage-dependent Rabi frequency of UP (green) and LOW (pink), plotted on the left axis. The right axis visualizes the calculated overlap of the electron and hole wave functions as a function of the voltage, where the pink/green dashed line corresponds to the lowest/second-lowest electron eigenenergy. The red/blue shaded background indicates the direct/indirect character of the transition.