Double-slit diffraction is a corner stone of quantum mechanics. It illustrates key features of quantum mechanics: interference and the particle-wave duality of matter. In 1965, Richard Feynman presented a thought experiment to show these features. Here we demonstrate the full realization of his famous thought experiment. By placing a movable mask in front of a double-slit to control the transmission through the individual slits, probability distributions for single-and double-slit arrangements were observed. Also, by recording single electron detection events diffracting through a double-slit, a diffraction pattern was built up from individual events.

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Basic explanations of the double slit diffraction phenomenon include a description of waves that emanate from two slits and interfere. The locations of the interference minima and maxima are determined by the phase difference of the waves. An optical wave, which has a wavelength l and propagates a distance L, accumulates a phase of L 2 . p l / A matter wave, also having wavelength l that propagates the same distance L, accumulates a phase of L , p l / which is a factor of two different from the optical case. Nevertheless, in most situations, the phase difference, , j D for interfering matter waves that propagate distances that differ by L, D is approximately L 2 , p l D / which is the same value computed in the optical case. The difference between the matter and optical case hinders conceptual explanations of diffraction from two slits based on the matter-optics analogy. In the following article we provide a path integral description for matter waves with a focus on conceptual explanation. A thought experiment is provided to illustrate the validity range of the approximation

We provide support for the claim that momentum is conserved for individual events in the electron double slit experiment. The natural consequence is that a physical mechanism is responsible for this momentum exchange, but that even if the fundamental mechanism is known for electron crystal diffraction and the Kapitza-Dirac effect, it is unknown for electron diffraction from nano-fabricated double slits. Work towards a proposed explanation in terms of particle trajectories affected by a vacuum field is discussed. The contentious use of trajectories is discussed within the context of oil droplet analogues of double slit diffraction. 1. Introduction. Recently we performed the electron double slit experiment, and the pattern was recorded one electron at-a-time [1]. The electron detection rate was about one electron per second. This made it possible to manually turn off the electron source after the first electron was recorded. This electron can, by chance, land in a first diffraction order (see Fig.1). This can be considered a completed single-event experiment. Often single events experiments are only considered in a probabilistic way as the best theory available to compare with, that is Quantum Mechanics, is probabilistic. Nevertheless, a quantum description also includes the correct prediction that the individual, in this case position, outcomes are eigenvalues of operators. Even more is known about single events. This becomes clear upon asking the question: "Is momentum conserved for this experiment?" We will provide support for the claim that the generally accepted answer is yes. The natural follow-up question that is the central theme of this paper is: "By what mechanism do the electron and the slit exchange momentum?" We claim that the answer is not known and that the question is a valid one. Some discussion on possible mechanism is given. In particular, the role of image charge interaction between the electron in double slit walls and the vacuum field is discussed. The proposed explanation that the double slit provides a boundary condition for the vacuum field, which in turn provides a means by which the electron trajectory exchanges momentum[2-6] with the slit is discussed within the context of the theory Stochastic Electrodynamics (SED) [2,7]. The provocative possibility of any trajectory explanation is considered in view of the well-known oil-droplet double slit analogue. The validity range of SED and the relation with the Heisenberg uncertainty relation are discussed for the Harmonic oscillator. The intent of this paper is to raise questions and discuss ongoing work that is unfinished and as of yet inconclusive.

A computer vision system is presented that can be used to acquire position data for an experiment where human observations have traditionally been used. A simple algorithm based on color discrimination is used to automatically locate an object of interest. Experiments can be monitored in real time using inexpensive cameras. Examples are given to illustrate the applicability of this system for autonomous data acquisition.