Coupled Quantum Dots

Fig. 1: Transmission electron microscope (TEM) image of a pair of vertically stacked, self assembled quantum dots.

The realization of robust and scalable hardware for quantum information processing is one of the most challenging goals of solid-state physics. Single charge excitations (excitons) in semiconductors quantum dots represent a particularly attractive quantum bit because they can be coherently manipulated within their decoherence time using ultrafast laser pulses. In order to explore the potential these systems provide for scalability and demonstrate quantum conditional logic, a fundamental prerequisite is the controllable coupling of individual vertically stacked pairs of self assembled quantum dots form a quantum dot molecule.

  Fig. 2: Schematic of the type of device.

Quantum coupling in individual QD-molecules and its manipulation using static electric fields along the growth direction can be observed using low temperature confocal laser microscopy. Photoluminescence (PL) spectroscopy on a single pair of stacked, self assembled InGaAs/GaAs QDs which were embedded in a n-i Schottky junction is shown in figure 2. This device enables us to tune the electric field (F) oriented along the stacking axis of the molecule via a gate voltage applied between the back n-contact and the top Schottky-gate. A clear anti-crossing of spatially direct (electron & hole in same dot) and indirect (electron & hole in different QDs) excitonic states is observed as the electric field along the QD-molecule axis is tuned.

Fig. 3: PL of anti-crossing of electron tunnel coupled states in a single QDM .

The two excitonic species have differing dipole moments, leading to different shift rates due to the DC-Stark effect, given by , which is weak and quadratic for direct excitons and strong and linear for indirect excitons, respectively. The states can, therefore, be tuning into resonance by varying the applied field. At the anticrossing the electron wavefunction hybridizes into bonding and antibonding states which are split by the tunnel coupling energy.

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We employ ultrafast pump-probe spectroscopy to directly monitor electron tunneling between discrete orbital states in a pair of spatially separated quantum dots. Immediately after excitation, several peaks are observed in the pump-probe spectrum due to Coulomb interactions between the photo-generated charge carriers. By tuning the relative energy of the orbital states in the two dots and monitoring the temporal evolution of the pump-probe spectra the electron and hole tunneling times are separately measured and resonant tunneling between the two dots is shown to be mediated both by elastic and inelastic processes. Ultrafast (<5 ps) inter-dot tunneling is shown to occur over a surprisingly wide bandwidth, up to ~8meV, reflecting the spectrum of exciton-acoustic phonon coupling in the system.

Funding

We gratefully acknowledge funding from:

• the German Science Foundation via SFB631 - Project B5