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Chiral coupling occurs when a semiconductor quantum dot (QD) is positioned close to a C-Point (chiral point) of a nano-photonic waveguide. In these conditions, there is a strong dependence of the propagation direction on the circular polarization of the optical mode, with spin-up and spin-down exciton spins coupling to opposite propagation directions. This provides a mechanism by which a QD can impart a $\pi$ phase shift depending on the spin of a charge carrier within the dot, forming the building block for chip-based spin quantum networks [1].
Early work at Sheffield focused on InAs/GaAs QDs in GaAs nanobeam waveguides, where spin-dependent directional photoluminescence and initialization were observed [2, 3]. The spin-dependent phase shift deduced from analysis of the directional transmission data was around $0.1 \pi$ [4]. The magnitude of the phase shift is related to the coupling to the waveguide (i.e. the $\beta$-factor), and can be increased by using more sophisticated waveguide designs that combine high chirality with strong Purcell enhancement. Examples include glide-plane [5,6] and topological [7] photonic crystals designs. At Sheffield we have observed strong chirality in valley-Hall topological photonic crystals [8], and both strong chirality and Purcell enhancement in glide-plane designs [9]. Our detailed statistical analysis and simulations suggest that the glide-plane designs offer the best route towards obtaining both high chirality and strong Purcell enhancement, as required to achieve the full $\pi$ phase shift [10].
This work was supported by EPSRC Programme Grants EP/V026496/1 and the EPSRC Quantum Communications Hub Grant EP/T001011/1.
References
1. Lodahl, P. et al., Nature 541, 473 (2017).
2. Coles, R.J. et al., Nature Communications, 7, 11183 (2016).
3. Coles, R.J. et al., Phys. Rev. B, 95, 121401(R) (2017).
4. Hurst, D.L. et al., Nano Letters, 18, 5475 (2018).
5. Sollner, I. et al., Nature Nanotechnology 10, 775 (2015).
6. Mahmoodian, S. et al., Opt. Mater. Express 7, 43 (2017).
7. Barik, S. et al., Science 359, 666 (2018).
8. Jalali Mehrabad, M., et al, Optica, 7, 1690 (2020).
9. Siampour, H. et al., NPJ Quantum Information, 9, 15 (2023).
10. Martin, N. et al., Phys. Rev. Research 6, L022065 (2024).
