Stochastic motion of particles in a highly unstable potential generates a number of diverging trajectories leading to undefined statistical moments of the particle position. This makes experiments challenging and breaks down a standard statistical analysis of unstable mechanical processes and their applications. A newly proposed approach takes advantage of the local characteristics of the most probable particle motion instead of the divergent averages. We experimentally verify its theoretical predictions for a Brownian particle moving near an inflection in a highly unstable cubic optical potential. The most likely position of the particle atypically shifts against the force, despite the trajectories diverging in the opposite direction. The local uncertainty around the most likely position saturates even for strong diffusion and enables well-resolved position detection. Remarkably, the measured particle distribution quickly converges to a quasistationary one with the same atypical shift for different initial particle positions. The demonstrated experimental confirmation of the theoretical predictions approves the utility of local characteristics for highly unstable systems which can be exploited in thermodynamic processes to uncover energetics of unstable systems.

Charles University Faculty of Mathematics and Physics Department of Macromolecular Physics 5 Holešovičkách 2 180 00 Praha 8 Czech Republic

Department of Optics Palacký University 17 listopadu 1192 12 771 46 Olomouc Czech Republic

Institute of Scientific Instruments of the Czech Academy of Sciences Královopolská 147 612 64 Brno Czech Republic

Universität Leipzig Institut für Theoretische Physik Postfach 100 920 D 04009 Leipzig Germany

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