quantum mechanics]. In 1974, the Italian physicists Pier Giorgio Merli, Gian Franco Missiroli, and Giulio Pozzi performed the first experiment of single particle interferometry using electrons and a biprism (instead of slits), showing that each electron interferes with itself as predicted by quantum theory. In 2018, single particle interference has been first demonstrated for antimatter in the Positron Laboratory (L-NESS) of Rafael Ferragut in Como (Italy), by a group led by Marco Giammarchi. Heisenberg's uncertainty principle In his work on formulating quantum mechanics, Werner Heisenberg postulated his uncertainty principle, which states: where here indicates standard deviation, a measure of spread or uncertainty; and are a particle's position and linear momentum respectively. is the reduced Planck's constant (Planck's constant divided by 2). Heisenberg originally explained this as a consequence of the process of measuring: Measuring position accurately would disturb momentum and vice versa, offering an example (the "gamma-ray microscope") that depended crucially on the de Broglie hypothesis. The thought is now, however, that this only partly explains the phenomenon, but that the uncertainty also exists in the particle itself, even before the measurement is made. In fact, the modern explanation of the uncertainty principle, extending the Copenhagen interpretation first put forward by Bohr and Heisenberg, depends even more centrally on the wave nature of a particle. Just as it is nonsensical to discuss the precise location of a wave on a string, particles do not have perfectly precise positions; likewise, just as it is nonsensical to discuss the wavelength of a "pulse" wave traveling down a string, particles do not have perfectly precise momenta that correspond to the inverse of wavelength. Moreover, when position is relatively well defined, the wave is pulse-like and has a very ill-defined wavelength, and thus momentum. And conversely, when momentum, and thus wavelength, is relatively well defined, the wave looks long and sinusoidal, and therefore it has a very ill-defined position. de Broglie–Bohm theory De Broglie himself had proposed a pilot wave construct to explain the observed wave–particle duality. In this view, each particle has a well-defined position and momentum, but is guided by a wave function derived from Schrödinger's equation. The pilot wave theory was initially rejected because it generated non-local effects when applied to systems involving more than one particle. Non-locality, however, soon became established as an integral feature of quantum theory and David Bohm extended de Broglie's model to explicitly include it. In