Resonant Two-Laser Spin-State Spectroscopy of a Negatively Charged Quantum-Dot–Microcavity System with a Cold Permanent Magnet

Abstract

A high-efficiency spin-photon interface is an essential piece of quantum hardware necessary for various quantum technologies. Self-assembled (In,Ga)As quantum dots have excellent optical properties: if embedded in an optical microcavity, they can show near-deterministic spin-photon entanglement and spin readout. To address the individual spin states, an external magnetic field is required, and a superconducting magnet is usually used. Here we show a compact cryogenically compatible Sm-Co magnet design that delivers a 475-mT in-plane (Voigt-geometry) magnetic field at 5 K, which is suitable for lifting the energy degeneracy of the electron spin states and trion transitions of a single (In,Ga)As quantum dot. This quantum dot is embedded in a birefringent high-finesse optical microcavity that enables efficient collection of single photons emitted by the quantum dot. We demonstrate spin-state manipulation by addressing the trion transitions with a single laser field and two laser fields. The experimental data agree well with our model, which covers single-and two-laser cross-polarized resonance fluorescence, Purcell enhancement in a birefringent cavity, and variation of the laser powers.