QD-based Single Photon Emission

Single photons (fock states) are one of the examples of non-classical light. It can be used in Quantum Information Technology (QIT) to transmit qubits over long distances. Single photons can be generated in isolated quantum systems such as a single atom, a trapped ion, or a quantum dot  (QD) - an "artificial atom".  For the realization of an effective QD-based single photon emitter (SPE), it is necessary to localize QD,  effectively excite it (optically or electrically), and effectively collect an emission.  All the above-mentioned principles can be solved by modern semiconductor technology allowing an integration of a single QD into a micro-resonator.  In recent years,  a lot of efforts have been focused on the choice of semiconductor materials for a desired wavelength emission,  the choice of optical technology for QD fabrication and positioning, the development of effective QD excitation schemes, and, also, on the development of resonator design providing maximum internal quantum efficiency.

Figure: 

Resonant excitation scheme of a single QD to create a single photon.

QD-based Entangled Photon Emission

Among possible entangled photon sources, semiconductor QDs are considered ideal, being able to generate polarization-entangled photon pairs on demand via the biexciton (XX) – exciton (X) recombination cascade.  QDs suffer from different entanglement degrading effects such as recapture processes, valence band mixing or spin-flip processes. However, the main requirement for QDs to be an ideal entangled photon emitter is a low fine structure splitting (FSS) of X state, induced generally by a QD asymmetry  (shape, composition etc.).  A high value of the FSS makes unpractical the use of QDs for entangled photon pairs generation. However, by employing different filtering schemes (spectral or temporal), it is possible to increase the range of an acceptable FSS value and still generate a substantially strong entanglement. But, of course, lowering the FSS is the main goal of QD fabrication for entangled photon generation.

Figure: 

XX-X recombibation cascade. The XX state decays under the emission of a right circular (R) (left (L)) polarized photon to a single X state, which subsequently decays to the ground (G) state under the emission of an orthogonal-circular polarized photon. The polarization of the emitted photons is governed by the electron and hole spin configuration of the recombining electron-hole pair. 

Our approach

We grow our QDs for single and entangled photon applications using Droplet Epitaxy (DE) and Local Droplet Etching (LDE) techniques in the MBE chamber.  It offers better control over the QD self-assembly dynamics and guarantees a fine tuning of the shape, size, and density. 

We work with two systems: GaAs/AlGaAs QDs emitting at 780-795 nm compatible with Rb-based quantum memories and InAs/InGa(Al)As QDs emitting at telecommunication wavelengths for long-distance quantum communication.

Also, our solution for the development of highly symmetric QDs with low FSS is to self-assemble them on (111)-oriented substrates due to the natural C3v symmetry of the surface but problematic for the growth via Stranski-Krastanov (SK) growth mode, since on (111) the relaxation of a strained III-V semiconductor epilayer immediately proceeds through the nucleation of misfit dislocation at the interface rather than through the formation of coherent 3D islands. Additionally, (111)A surface permits the crystallization under the As supply of the Ga nanodroplets into QDs even at a relatively high temperature  (compared to (001)-oriented substates) which improves the crystallinity of the QDs by reducing the concentration of point defects.

Figure: 

Top Panel:  (a) AFM scan of a single DE GaAs/AlGaAs(111)A QD; (b) height profiles taken along [11−2] and equivalent crystallographic directions; (c) histogram of the FSS values from GaAs/AlGaAs(111)A QDs emitting above 770 nm.

Bottom Panel:  (a) AFM scan of a DE InAs/InAlAs(111)A QDs; (b) FSS statistical distribution of DE InAs/InAlAs(111)A QDs.