Polarization entangled photons from quantum dots embedded in nanowires

We present a first measurement of photon polarization entanglement from the biexciton to ground state cascade of a single InAsP quantum dot embedded in an InP nanowire. We observe a fidelity of 0.76(2) to a reference maximally entangled state as well as a concurrence of 0.57(6)

Light-emitting diode spherical packages: an equation for the light transmission efficiency

Virtually all light-emitting diodes (LEDs) are encapsulated with a transparent epoxy or silicone-gel. In this paper we analyze the optical efficiency of spherical encapsulants. We develop a quasi-radiometric equation for the light transmission efficiency, which incorporates some ideas of Monte-Carlo ray tracing into the context of radiometry. The approach includes the extended source nature of the LED chip, and the chip radiance distribution. The equation is an explicit function of the size and the refractive index of the package, and also of several chip parameters such as shape, size, radiance, and location inside the package. To illustrate the use of this equation, we analyze several packaging configurations of practical interest; for example, a hemispherical dome with multiple chips, a flat encapsulation as a special case of the spherical package, and approximate calculations of an encapsulant with a photonic crystal LED or with a photonic quasi crystal LED. These calculations are compared with Monte-Carlo ray-tracing, giving almost identical values.Comment: 30 pages, 7 figures, 1 tabl

Exciton-photon interaction in a quantum dot embedded in a photonic microcavity

We present a detailed analysis of exciton-photon interaction in a microcavity made out of a photonic crystal slab. Here we have analyzed a disk-like quantum dot where an exciton is formed. Excitonic eigen-functions in addition to their eigen-energies are found through direct matrix diagonalization, while wave functions corresponding to unbound electron and hole are chosen as the basis set for this procedure. In order to evaluate these wave functions precisely, we have used Luttinger Hamiltonian in the case of hole while ignoring bands adjacent to conduction band for electron states. After analyzing Excitonic states, a photonic crystal based microcavity with a relatively high quality factor mode has been proposed and its lattice constant has been adjusted to obtain the prescribed resonant frequency. We use finite-difference time-domain method in order to simulate our cavity with sufficient precision. Finally, we formulate the coupling constants for exciton-photon interaction both where intra-band and inter-band transitions occur. By evaluating a sample coupling constant, it has been shown that the system can be in strong coupling regime and Rabi oscillations can occur for Excitonic state population.Comment: Journal of Physics B: Atomic and Molecular Physics (to appear

Nanowire Quantum Dots as Sources of Single and Entangled Photons

Realization of linear quantum computation and establishing secure quantum communication among interacting parties demand for triggered quantum sources delivering genuine single and entangled photons. However, the intrinsic energy level spectrum of nanostructures made by the nature or developed under a random growth process energetically lacks the expected figures of merit to produce such quantized states of photons. Here, I present the semi-empirical modeling and experimental investigation on the spin fine structure of strongly confining quantum dots embedded in III-V nanowires. To this end, the quantum dot is numerically modeled via the Configuration Interaction method at two different levels: 1) single-particle level, where its pure energy level structure is resolved in the presence of strain and spin-orbit interaction. 2) Few-particle level, at which the few-body interactions appear as perturbative energy corrections and orbital correlations. I demonstrate the influence of quantum confinement on the binding energies and spin fine structure of excitons in the absence of hyperfine interaction. Importantly, the high-symmetry character of excitonic orbitals in nanowire quantum dots restore the degeneracy of optically-active ground-state excitons, offering an ideal spectrum for the entangled photon pair generation. To experimentally verify the idea, we design and fabricate defect-free nanowire quantum dots with ultra-clean excitonic spectrum, and construct the time correlation function of emitted photons through performing a series of low-temperature statistical quantum optics measurements. We observe a decent performance in terms of single photon generation under low excitation powers. Moreover, photon pairs emitted from the biexciton-exciton cascade of nanowire quantum dots exhibit color indistinguishability and polarization entanglement owing to the trivial fine structure splitting of the ground-state excitons. We further extend the idea by proposing the hybridized states of a nanowire-based quantum dot molecule as the potential source of higher-order entangled states. Tracing the field-dependent spectrum suggests the appearance of dominant features under the weak localization of electrons and coherent tunneling of holes. In addition to their Coulomb correlation, excitons also remain spatially correlated, opening new transition channels normally forbidden in the ground state of a single dot. The proposed structure can be exploited to create tripartite hybrid, GHZ and W-entangled states.4 month