Quadrupole electric field for erasing the fine structure splitting in a single quantum dot M. Zeeshan, 1, a) N. Sherlekar, 1 A. Ahmadi, 2 R.L. Williams, 3 and M.E. Reimer 1 1) Institute for Quantum Computing and Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 2) Institute for Quantum Computing and Department of Physics and Astronomy, University of Waterloo, Waterloo, N2L 3G1, Canada 3) National Research Council of Canada, Ottawa, Ontario, Canada, K1A 0R6 (Dated: 10 September 2018) Entangled photon sources are crucial for quantum optics, quantum sensing and quantum communication. Semiconductor quantum dots generate on-demand entan- gled photon pairs via the biexciton-exciton cascade. However, the pair of photons are emitted isotropically in all directions, thus limiting the collection efficiency to a fraction of a percent. Moreover, strain and structural asymmetry in quantum dots lift the degeneracy of the intermediate exciton states in the cascade, thus degrading the measured entanglement fidelity. Here, we propose an approach for generating a pair of entangled photons from a semiconductor quantum dot by application of a quadrupole electrostatic potential. We show that the quadrupole electric field corrects for the spatial asymmetry of the excitonic wavefunction for any quantum dot dipole orientation and fully erases the fine-structure splitting without compro- mising the spatial overlap between electrons and holes. Our approach is compatible with nanophotonic structures such as microcavities and nanowires, thus paving the way towards a deterministic source of entangled photons with high fidelity and collection efficiency. Quantum dots generate polarization entangled photons on-demand via the biexciton- exciton cascade 1–4 . However, an energy splitting of the intermediate exciton states, known as the fine structure splitting (FSS), introduces a which-path information within the biexciton-exciton cascade and reduces the measured polarization entanglement 5,6 . This energy splitting can be caused by an asymmetric quantum dot shape 7,8 , piezoelectric field 9 , stress 10 and inhomogeneous alloying 11 , which reduces the symmetry of the excitonic confinement potential. Several quantum dot growth techniques have been developed 12–14 to minimize the FSS, but only a limited number of quantum dots on the sample will have an FSS close to zero. Therefore, post-growth perturbation techniques are introduced to further reduce the FSS such as application of electric fields 15 , strain fields 16 , magnetic fields 17,18 , annealing 19 and by a combination of strain and an electric field 4,20 . Using strain was shown to be the most versatile approach in addressing the challenge of minimizing the FSS towards zero 21,22 . However, such post-growth tuning techniques have not been implemented on quantum dots in photonic nanostructures such as nanowires 23,24 or micropillar cavities 25 for enhanced photon collection efficiency with near-unity single mode fiber coupling 26 . In this letter, we propose to remove the FSS by applying a quadrupole electric field to a single quantum dot while maintaining a high light extraction efficiency. A schematic view of the device design is shown in Fig. 1(a) with four gates positioned around a single quantum dot in a tapered nanowire. The tapered nanowire geometry allows for a high light extraction efficiency, whereas the four gates surrounding the quantum dot removes the FSS by an applied quadrupole electric field. Using this quadrupole electric field we show that the FSS can be removed for any in-plane quantum dot dipole orientation without compromising a) Electronic mail: m5zeeshan@uwaterloo.ca arXiv:1809.02538v1 [quant-ph] 7 Sep 2018