This study demonstrates giant optical polarization rotations induced by a single quantum dot (QD) spin in a pillar-based cavity. The research focuses on achieving deterministic spin-dependent photon polarization states, crucial for optical quantum computing and communications. The team used an electrically contacted pillar cavity with a single InGaAs QD to achieve polarization rotations of π/2, π, and -π/2 in the Poincaré sphere, with fidelities of 97±1%, 84±7%, and 90±8%, respectively. The results show that enhanced light-matter coupling, limited cavity birefringence, and reduced spectral fluctuations enable precise control over polarization states. The study introduces a complete tomography approach to extrapolate the output polarization Stokes vector, conditioned by specific spin states, despite spin and charge fluctuations. The device allows for conditional polarization rotations in the Poincaré sphere, with control both in longitude and latitude. This polarization control is essential for adapting spin-photon interfaces to various quantum information protocols. The research highlights the importance of reducing spectral fluctuations and achieving high Purcell enhancement to enable large polarization rotations. The findings suggest that the device can be used to generate maximally entangled spin-photon states and improve quantum information processing. The study also discusses the challenges of achieving deterministic spin-photon interfaces, including the need for high-quality cavity-QED devices with moderate birefringence and efficient spin control. The results demonstrate the potential of using QD-based spin-photon interfaces for quantum communication and computing, with applications in photonic quantum gates, quantum memories, and quantum non-demolition detectors. The study emphasizes the importance of polarization tomography and the need for further improvements in device efficiency and spin initialization to achieve high-fidelity operations.This study demonstrates giant optical polarization rotations induced by a single quantum dot (QD) spin in a pillar-based cavity. The research focuses on achieving deterministic spin-dependent photon polarization states, crucial for optical quantum computing and communications. The team used an electrically contacted pillar cavity with a single InGaAs QD to achieve polarization rotations of π/2, π, and -π/2 in the Poincaré sphere, with fidelities of 97±1%, 84±7%, and 90±8%, respectively. The results show that enhanced light-matter coupling, limited cavity birefringence, and reduced spectral fluctuations enable precise control over polarization states. The study introduces a complete tomography approach to extrapolate the output polarization Stokes vector, conditioned by specific spin states, despite spin and charge fluctuations. The device allows for conditional polarization rotations in the Poincaré sphere, with control both in longitude and latitude. This polarization control is essential for adapting spin-photon interfaces to various quantum information protocols. The research highlights the importance of reducing spectral fluctuations and achieving high Purcell enhancement to enable large polarization rotations. The findings suggest that the device can be used to generate maximally entangled spin-photon states and improve quantum information processing. The study also discusses the challenges of achieving deterministic spin-photon interfaces, including the need for high-quality cavity-QED devices with moderate birefringence and efficient spin control. The results demonstrate the potential of using QD-based spin-photon interfaces for quantum communication and computing, with applications in photonic quantum gates, quantum memories, and quantum non-demolition detectors. The study emphasizes the importance of polarization tomography and the need for further improvements in device efficiency and spin initialization to achieve high-fidelity operations.