A new method to track the motion of a single particle in the field of a high-finesse optical resonator is described. It exploits near-degenerate higher-order Gaussian cavity modes, whose symmetry is broken by the phase shift on the light induced by the particle. Observation of the spatial intensity distribution behind the cavity allows direct determination of the particles position with approximately wavelength accuracy. This is demonstrated by generating a realistic atomic trajectory using a semiclassical simulation including friction and diffusion and comparing it to the reconstructed path. The path reconstruction itself requires no knowledge about the forces on the particle.
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