Partial-Wave Resolved Spin-Orbit Dynamics.

A novel projective measurement technique utilizing conventional photoelectron velocity-map imaging to probe the spatial part of the wave function by projecting it into momentum space is demonstrated. Oscillations between the states |m_{l}=0,m_{s}=±1/2⟩ and |m_{l}=±1,m_{s}=∓1/2⟩ are observed with no detectable decoherence over 170 cycles with a speed of around 1.7 THz driven purely by spin-orbit interaction. Our measurements achieve a quantum operation fidelity of approximately 90%, limited by the photoionization detection scheme. This Letter establishes a new platform for high-fidelity quantum operations driven by the intrinsic spin-orbit coupling.