When either the vertical or the horizontal filter covers both slits, the double-slit interference pattern is preserved, albeit at a reduced intensity compared to no filter. The which-path marker consists of two, mutually perpendicular, polarizing filters. The blue LED, linear polarizer, and 5 micron entrance slit are all mounted in standard lens holders and the entire assembly is attached to the end-cap of the PVC pipe. The light source and entrance slit are at one end of the PVC pipe and the Luca camera is attached to the other end.
A graph of the intensity distribution as well as the pixel counts is included in the display. The PC displays the interference pattern as it accumulates and integrates over a period of two minutes. The image data are downloaded to a PC every 1/2 second and integrated over time. The detector is a thermoelectrically cooled EMCCD camera (Andor Luca DL-658M), providing single photon detection sensitivity with a QE of 50%. The double-slit (two, 200 micron wide, slits separated 1.0 mm) is located in the middle. Light from a blue LED enters through a 5 micron slit. How it works: The layout of the apparatus is illustrated below.Ī 4-meter long PVC pipe supports all the optics and acts as a light shield. Even if not actually measured, the mere possibility that an observer could determine which slit the photon passed through causes the interference pattern to switch to non-interference. Thus, the act of measurement and the design of the experiment affect what is being measured. This can be followed by a quantum eraser (to erase the which-path information) to recover interference. In this so-called which-path case, the Young's double-slit interference pattern does not manifest itself. The experiment is designed to also make it possible (in principle) to know which of the two slits the photons are passing through. This addresses the question of how can single photons interfere with photons that have already gone through the apparatus in the past, or with those that will go through in the future, or with themselves. In this demonstration we perform the double-slit interference experiment with extremely dim light and show that even when the light intensity is reduced down to several photons/sec, the audience can see the familiar Young's double-slit interference pattern build up over a period of time. Wave/particle duality observed in Young's double slit experiment with camera sensitive to individual photons.
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