A droplet bouncing on a vertically vibrated bath can become coupled to the surface wave it generates. It thus becomes a "walker" moving at constant velocity on the interface. Here the motion of these walkers is investigated when they pass through one or two slits limiting the transverse extent of their wave. In both cases a given single walker seems randomly scattered. However, diffraction or interference patterns are recovered in the histogram of the deviations of many successive walkers. The

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... imilarities and differences of these results with those obtained with single particles at the quantum scale are discussed. As shown recently [1], a droplet can bounce indefinitely on a vertically vibrated bath of the same fluid. Near the Faraday instability threshold [2], this bouncing becomes subharmonic and the drop emits a localized Faraday wave packet. A bifurcation occurs by which the drop becomes spontaneously self-propelled and moves on the liquid surface at constant velocity. This occurs when there is a lockin phenomenon so that the drop falls systematically on the forward front of the wave generated by its previous bouncings [3,4]. We called "walker" the moving droplet dressed with the wave-packet it emits. A walker is a "symbiotic" structure: if the droplet disappears (by coalescence with the substrate), the wave vanishes. In reverse, if the wave is damped, the droplet stops moving. Two interacting walkers can have discrete stable orbits [3,4] demonstrating that they have both an inertia due to their mass and nonlocal interactions due to the interference of their waves. In order to obtain a better characterization of this double particle-wave behavior of walkers we undertook the present experiments on their trajectories when they pass through apertures limiting the transverse extent of their wave. They are inspired by the well-known experiments on diffraction and interference performed at a low flux of particles. In Young's two slits configuration it was shown with photons [5], then with electrons [6 -8] that interference patterns could be obtained even when only a single particle at a time was present in the system. These patterns were then observed through the accumulation of successive individual events. Such results are usually thought to be possible only in quantum physics [9]. The experiments are performed in square cells [Figs. 1(a) and 1(b)] filled with silicon oil and submitted to a vertical oscillating acceleration m cos2f o t.