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 $|m_l=0,m_s=\pm 1/2⟩$ and $|m_l=\pm 1,m_s=\mp 1/2⟩$ 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.

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• Multiphoton or tunneling ionization & excitation
• Spin-orbit coupling
• Ultrafast phenomena