Excited state quantum couplings and optical switching of an artificial molecule

Abstract

We optically probe the spectrum of ground and excited state transitions of an individual, electrically tunable self-assembled quantum dot molecule. Photocurrent absorption measurements show that the spatially direct neutral exciton transitions in the upper and lower dots are energetically separated by only ∼ 2 meV. Excited state transitions ∼ 8 − 16 meV to higher energy exhibit pronounced anticrossings as the electric field is tuned due to the formation of hybridized electron states. We show that the observed excited state transitions occur between these hybridized electronic states and different hole states in the upper dot. By simultaneously pumping two different excited states with two laser fields we demonstrate a strong (88% on-off contrast) laser induced switching of the optical response. The results represent an electrically tunable, discrete coupled quantum system with a conditional optical response.Quantum dot (QD) nanostructures formed by strain driven self-assembly are ideal for solid state quantum optics experiments due to their discrete optical spectrum, strong interaction with light and robust quantum coherence for both interband polarization 1,2 and spin 3 . The ease with which such nanostructures can be embedded into electrically active devices allows for tuning of the transition frequency and control of charge occupancy 4 . Self-assembly provides a natural way to realize few dot systems via vertical stacking to produce more sophisticated nanostructures with coherent inter-dot coupling due to carrier tunneling 5-12 . When combined with the potential to coherently manipulate excitons over ultrafast timescales using precisely timed laser and electrical control pulses 13-15 such systems raise exciting prospects for the operation of small scale few qubit systems in a solid-state device. Very recently, conditional quantum dynamics for a single resonantly driven QD-molecule (QDM) 16 and spin dependent quantum jumps have been observed 8,17 .In this paper we employ photocurrent (PC) absorption, photoluminescence (PL) emission and PLexcitation (PLE) spectroscopy to trace the spectrum of ground and excited state transitions of an individual selfassembled QD-molecule as their character is electrically tuned from spatially direct to indirect. PC absorption allows us to identify the spatially direct neutral exciton transitions in both the upper (X ud ) and lower (X ld ) dots in the molecule. A number of excited state transitions are identified in PLE ∼ 8 − 16 meV above X ud . These excited states exhibit pronounced anticrossings (energy splitting ∆E ∼ 3.2 − 3.5 meV) as the electric field F is tuned. Excited state transitions are identified from voltage dependent PLE measurements to correspond to transitions between these hybridized electronic states and different hole orbitals in the upper dot. By performing a multi-color experiment where the QDM is simultaneously excited with different frequency lasers, we demonstrate how the resonant excitation of indirect excitons or exci-X ld X ud X ind FIG. 1. (Color onli...

Excitons in quantum dot molecules: Coulomb coupling, spin-orbit effects, and phonon-induced line broadening

“…The most pronounced features emerging in the QDM spectra are resonances (anticrossings) that appear at certain intersections between spatially direct and indirect exciton states (that is, states with the electron and the hole residing in the same QD or in different QDs, respectively). 14,19 Recent experiments investigated also the phonon-assisted interdot tunneling between the direct and indirect configurations. 32 Additionally, it was shown, that the phonon-induced relaxation time of an exciton at a tunnel-resonance (anticrossing) exhibits an oscillatory behavior due to an interplay between the phonon wavelength and the distance between the dots.…”

“…Up to the second order in the phonon coupling and neglecting phonon-induced energy shifts G n,m (ω) can be approximated by (19) with the imaginary part of the self-energỹ…”

“…4,13,14 This coupling causes the formation of bonding and anti-bonding states in the double dot systems, hence they are often referred to as quantum dot molecules (QDMs). External fields can be used to effectively manipulate carrier localizations 2,15 and to characterize different exciton states, [16][17][18][19][20] which is essential for many applications.…”

Coulomb Mediated Hybridization of Excitons in Coupled Quantum Dots

“…The importance of Coulomb interactions has been pointed out for the form and position of resonances in QD-molecule systems [19][20][21]. However, single particle tunneling that either hybridizes single electrons or holes remains the dominant coupling mechanism in these cases [11][12][13][22][23][24][25]. Most recently another resonant coupling mechanism with an inherently two particle nature has been predicted theoretically that entirely relies on the Coulomb mediated interaction between two different exciton states [26].…”

Coherent control with optical pulses for deterministic spin-photon entanglement

“…[21][22][23][24][25][26] Several groups demonstrated new spin readout methods, [27][28][29] addressing one of DiVincenzo's criteria for quantum computing. 30 Impressive progress has been made recently in characterizing and manipulating InAs quantum dot molecules, 28,[31][32][33][34][35][36] including demonstration of two-qubit gates. 37 A necessary next step for quantum dot based quantum computing is to extend such results to many-qubit systems.…”