Experiment of Afshar

The Expérience of Afshar is an experiment of quantum Mécanique making it possible to exceed the Principe of complementarity of Niels Bohr, i.e. making it possible to observe, simultaneously, the corpuscular and undulatory aspect of the light.

This experiment, was developed by the physicist américano-Iranian Shahriar Afshar. Initially created in 2001, was altered on several occasions with an aim of bringing a demonstration as irreproachable as possible. Despite everything, it is the subject currently (2007) of many polemics as for its interpretation. Many criticisms, as of alternative interpretations of the phenomenon are regularly presented on the Web sites, in the conferences of physics, the academic conferences and the scientific files ArXiv.

Preliminary recalls

See also: Slits of Young, Duality wave-corpuscle

The Principe of complementarity stipulates that it is not possible to measure two quantum sizes simultaneously. It is the case for example when one wants simultaneously to observe the two possible demonstrations of the light: wave and particle. Consequently, it is not possible to observe and measure simultaneously the undulatory and corpuscular character of a Photon.

The way simplest to highlight the undulatory character of the light is the experiment of the Fentes of Young, the interference rings which are created prove the existence of waves (fig 1). More surprising is that if one uses a source of light able to emit a photon at the same time, the figure of interference remains visible (fig 2), it then consists of a fog of luminous points of which the density varies while following the intensity of the interference rings.

Then arises the question of knowing by which of the two holes of the device the photon with good which been able to pass. Traditional interpretation consists in supposing that the photon passed by the two holes at the same time and ended up interfering with itself. Any attempt to know by which hole the photon passed (what would make it possible to show the corpuscular character of the light) involves the disappearance of the figure of interference.

The goal of the experiment of Afshar is thus to manage to know the way borrowed by the photon (corpuscular character) while not disturbing the interference rings (undulatory character).

Description of the experiment

The experiment is based on the slits of Young. In the alternative of Afshar, they are holes and not slits. After the holes, a lens focuses the light in such way that the light resulting from each hole is focused on a photo independent detector (fig 3). In this way, a photon passing by the hole number 1 will be exclusively received on photo detector 1 and vice versa. Configured thus, the assembly makes it possible to make sure of the corpuscular nature of the photons.

In parallel, the interference rings continue to be materialized between the holes and the lens. The fact of observing them directly made that no photon arrives more at the photo detectors. The idea of Ashfar is to place a grid at the site where the interference rings are formed. This very fine metal wire grid, made up is laid out of such way that each wire is located at the site of a dark band of the figure of interference. To arrive to that, the figure of interference is first of all measured in order to determine the position of each wire.

If a hole is open (Fig 4), the figure of interference cannot be formed any more, this one being replaced by a spot of light. Part of the photons is then stopped by metal wire, which causes to degrade the quality of the image more especially as a phenomenon of Diffraction appears because of the presence of the metal wire network. These effects were really observed during the experiment.

Now, one lets pass the light by the two holes (Fig 5). The degradation of the image by metal wire then is strongly attenuated, the result is comparable so that one obtains when the wire grid is not installed. Consequently, one can suppose that the photons avoid metal wire, which would mean that they pass preferentially where the figure of interference present of the maxima of intensity.

In addition, it will be noted that this experiment gives the same results that one uses a source of individual photons (laser impulse) or a source with continuous flow (laser traditional).

Interpretation

Afshar says that the lumère presents an undulatory character when it passes between metal wire, since the probability of presence of the photons seems to be in correlation with the figure of interference, but also presents a corpuscular character after being focused by the lens on the photo detector.

In conclusion, this behavior contradicts the principle of complementarity since the undulatory and corpuscular characters are simultaneously highlighted.

See Too

Duality wave-corpuscle
Slits of Young

External bonds

Présentation of the experiment
Discussion on the subject
Point of view criticizes on the experiment