# Waveguide chirality for spin state demultiplexing

Chiral points with right or left circular light polarization arise in completely symmetric waveguides made of non-chiral material. These points are located where the transverse and longitudinal electric field components of the fundamental guided mode have equal amplitudes and a phase difference of .

This task presents numerical simulations from the work of [Coles, R., Price, D., Dixon, J. et al. Chirality of nanophotonic waveguide with embedded quantum emitter for unidirectional spin transfer. Nat Commun 7, 11183 (2016) (opens new window)] where it was demonstrated that a quantum dot positioned at chiral waveguide points would emit in-plane circularly polarized single photons unidirectionally, with a high correlation between the spin state of the quantum dot and the which-path information of the photon. This provides a promising platform for quantum technology implementations.

The photonic structure consists of a suspended GaAs nanobeam waveguide with out-coupler gratings in its opposite ends. Two PointDipole (opens new window) objects are used to create the circularly polarized light which arises from recombinations of up and down exciton spin states from the quantum dot. A Parameter Sweep simulation object is then employed to switch the dipole position and phase, and to include/exclude the grating couplers in the simulation.

When the grating couplers are excluded from the simulation and the quantum dot is positioned at the center of the waveguide, the results of Figure 1(f) are reproduced. This allows the user to visualize light coupling in both waveguide directions. By displacing the dipole positions to 90 nm in the y-direction, spin-dependent directional emission is created for left- and right-circularly polarized light, thus reproducing the results from Figures 1(g) and 1(h), respectively. The effects of grating coupler inclusion in the simulation can also be observed, and the far-field patterns from left- and right-propagating outcoupled photons visualized.

To view the full example in our web GUI, please click here (opens new window).