Tailored quantum dots for entangled photon pair creation

Abstract

We compare the asymmetry-induced exchange splitting δ1 of the bright-exciton ground-state doublet in self-assembled (In,Ga)As/GaAs quantum dots, determined by Faraday rotation, with its homogeneous linewidth γ, obtained from the radiative decay in time-resolved photoluminescence. Post-growth thermal annealing of the dot structures leads to a considerable increase of the homogeneous linewidth, while a strong reduction of the exchange splitting is simultaneously observed. The annealing can be tailored such that δ1 and γ become comparable, whereupon the carriers are still well confined. This opens the possibility to observe polarization entangled photon pairs through the biexciton decay cascade.PACS numbers: 71.36.+c, 73.20.Dx, 78.47.+p, Entangled photon pairs are a key requirement for the implementation of quantum teleportation schemes.[1] Typically, such photon pairs are created by parametric down conversion of a strongly attenuated laser beam in a non-linear optical crystal, with limited efficiency. Recently, the decay of a biexciton complex confined in a quantum dot (QD) has been suggested as an efficient source for polarization entangled photon pairs.[2] This concept was based on the assumption of an idealistic QD structure for which the valence band ground state has pure heavy hole character with angular momentum projections J h,z = ±3/2 along the heterostructure growth direction. When an electron-hole pair is injected, the momenta of the carriers become coupled by the exchange interaction. If the dot has perfect D 2d -symmetry, angular momentum is a good quantum number: the optically active states with momenta M = ±1 are degenerate, and their decay leads to emission of σ ± -circularly polarized photons.If the dot ground states are occupied by two electrons and two holes, each with opposite spin orientations, a spin singlet biexciton X 2 is formed, for whose decay two channels exist, as shown in Fig. 1 (upper panel left). The first photon is emitted with either σ + or σ − -polarization, and then the second photon with opposite polarization, as long as no spin flip occurs after the first process. Unless a polarization measurement is performed, the two photon polarization state is therefore described by2, forming an entangled state. A key requirement is that the photons emitted at each stage of the cascade are quasi-degenerate within their homogeneous linewidth, such that they cannot be distinguished by an energy measurement.Experiments have failed up to now to demonstrate such an entanglement, as only classical correlations were observed.[3] While some of the idealizations of the original proposal are well fulfilled, for example, for strongly confined self-assembled (In,Ga)As/GaAs quantum dots (such as the long exciton spin relaxation time as com- pared to the radiative lifetime [4], or the almost pure heavy-hole character of the valence band ground state [6]), a fundamental problem arises from the broken D 2d symmetry, which is reduced to at least C 2v or even lower symmetry in realistic dot structures. [7,8,...

Formation and optical characterization of single InAs quantum dots grown on GaAs nanoholes

“…15,16 On the other hand, introducing an annealing step during the growth or afterwards, their height decreases to 2 -3 nm shifting up the ground state energy well above 1.3 eV. 17 Therefore, we conclude that the height values found for type-A and type-B QDs are compatible with the fundamental transitions observed in their PL spectra.…”

Energy dependence of the electron‐hole in‐plane anisotropy in InAs/GaAs quantum dots

“…We have observed that the anisotropic splitting con-tinuously decreases with the interband energy emission of the QDs. Similar trends have already been observed in thermally annealed samples [4][5][6][7], and have been interpreted as a symmetrizing of the inplane e-h confinement potential induced by the In diffusion. The anisotropic splitting is a fundamental property of interest: for instance, its cancellation would allow the creation of entangled photon pairs with QDs [8].…”

Directed self‐assembly of single quantum dots for telecommunication wavelength optical devices

“…Control of the dot dimensions allows tuning of the electronic structure of the dot, and thus the emission energies and level spacings [49]. This is of particular relevance to entangled photon pair generation [50][51][52][53], where the dot size and shape will determine whether the photon polarizations are entangled, or simply correlated [47].…”

“…In anisotropic quantum dots the electron-hole exchange interaction splits the two excitonic states by ¡ AES (see in the ¡ AES through the application of external magnetic [70] and electric [43,44] fields, or through spectral filtering [71], applied uniaxial stress [72], and quantum size and composition engineering [50][51][52][53]. An alternative scheme [73,74] involves manipulation of the biexciton binding energy.…”