One of the most popular experiments in quantum physics, whose experimental realizations are still perfected nowdays, is Young-Feynman experiment. Originally set up for light in 1801 by Young, so called the double-slit or gedanken experiment, was described by Feynman as containing all of the mysteries of quantum mechanics. 

The wave behavior of material particles is, in the first part of this experiment, demonstrated in the way single electrons are behaving. Imagine a ‘gun’ firing electrons, one at the time, in random directions on the wall with two holes (of the size comparable to that of the electron) in it. Going through the hole is the only way for electron to make it pass the wall to the ‘target’ positioned behind the wall. Target is here a detector, i.e. the way for us to see where electron ‘landed’. If electrons were behaving like classical particles, i.e. small bullets, the highest number of electrons would be lending on the target in two groups behind the slit, and the intensity (or in this case number) of them arriving at each spot would be just sum of intensities observed when the first slit is opened and when the second slit is opened. (illustrations and detailed explanations can be found in famous Feynman Lectures). 

Instead, what is observed is interference pattern typical for waves (see image at the top of the page). When wave comes to the wall it is diffracted at the slit, and new circular waves spread out from each slit. If both slits are opened interference happens (and intensity in each spot is definitely different from sum of intensities corresponding to passing through each individual hole). So this part of the experiment shows wave-like nature of electron. But still, this is not the wave in the classical sense: when monitoring arrival of electrons at the target, we observe that they arrive at a single position each, while interference pattern is formed when accumulating many discrete arrivals (see the video below).

As if not already strange enough, the real mystery comes in the second part of the Young-Feynman experiment. What happens if we close one of the slits and keep on firing electrons? If one compares electron distributions recorded at the target before and after one of the slits is closed the remarkable fact comes to the light: each time electrons are passing through just one slit, they behave as classical particles. Various experimental approaches in which one of the two slits is partially or totally obstructed in a controllable way and the change in the interference pattern is monitored. It is then possible to observe the transition of the diffraction pattern from the two- to the one-slit configuration, highlighting the wave-particle duality of the electrons(see recent work of Tavabi et al). 

Each time that the path of electron is determined (either by closing one slit or ‘monitoring’ which slit it goes through) electron unmistakably behaves as a classical particle and interference pattern do not form. This is closely related to unsertanity principle and also interpreted as interaction of the measurement process with the electron leading to decocherence of electron wave and transformation from quantum to classical bechaviour. How much of interaction is need to observe the transition from a quantum interference pattern to a classical particle-like intensity distribution? As Acoury and al. demonstrated, a system of two electrons is already sufficient to observe such transition for an individual electron. In their experiment however the quantum coherence is not destroyed, but remains in the entangled two-electron system. This is also very interesting topic for the realization of quantum computers, and we’ll look more into it in a future posts. 

This third part of the Young-Feynman experiment is also known as which-path experiment, and many ideas were realized in order to trick the electron and obtain the information of its path in the very moment or even after the pass through slit happened. However, the only thing demonstrated is that interference phenomena (or wave-like bechaviour of electron) is in a collision with the information about which slit the electron passes through (particle-like information).

video contribution of Alf Eaton [CC BY 3.0], via Wikimedia Commons

"In quantum phenomena there is something all-embracing in space and time, which is in a conceptual collision with locality we are used to. Locality demands that any event in some point in space-time influence only nearby points, and that this influence propagates with a finite speed." (text loosely translated by post author)

What is here described for electrons goes for other quantum systems, such as protons, atoms, even large molecules such as fullerene (60 carbon atoms). Part of the mystery of these quantum systems is our struggle to look beyond our everyday experience in classical world and comprehend this unique behavior whose mathematical description overcomes our 'local' reasoning.