Executive Summary : | Quantum protocols like quantum communication, boson sampling, and controlled-NOT (CNOT) rely on single photons for their optical realization. A CNOT gate flips the target qubit state based on the control qubit, enabling the construction of any quantum computing circuit when combined with single qubit gates. The optical realization of a CNOT gate requires an on-demand source emitting one and only one photon at a time with unity efficiency and indistinguishability. Neutral quantum dots show the highest efficiency and near unity indistinguishability, but their maximum efficiency is only 33%. The primary reason for limited efficiency is the requirement for polarization and spectral filtering of single photons to distinguish them from the pump laser pulse. To overcome these issues, researchers plan to harness and investigate a single negatively charged quantum dot (NCQD) embedded in an appropriately designed micropillar cavity. The NCQD's favorable energy level structure allows the pump laser pulse and emitted single photons to be in orthogonal polarizations, mitigating the need for polarization and spectral filtering.
However, there are fundamental issues that may hinder the realization of unity efficiency and indistinguishability, such as an electron spin confined in a NCQD interacting with the nuclear spins of the host material. An externally applied magnetic field can be used to freeze the fluctuations of the electron spin state. The proposed proposal will involve designing a suitable micropillar cavity using 3D finite difference time domain method based Ansys Lumerical software, modeling and simulating a NCQD-cavity system, and demonstrating a near-unity fidelity CNOT gate using superior single photons. |