Double Slit Hide and Seek

1. Double Slit Hide and Seek by Robert J. Nemiroff  Michigan Tech

2. Physics X: About This Course • Officially "Extraordinary Concepts in Physics" • Being taught for credit at Michigan Tech • Light on math, heavy on concepts • Anyone anywhere is welcome • No textbook required • Wikipedia, web links, and lectures only • Find all the lectures with Google at: • "Starship Asterisk" then "Physics X"  • http://bb.nightskylive.net/asterisk/viewforum.php?f=39

3. Afshar Experiment We will build up to the full Afshar experiment with this simple start: A classic double-slit experiment is preformed and the location of the dark interference minima are recorded.  Now wires are placed at these minima and the image screen is moved back.  What pattern now appears on the image screen? 1.  An interference pattern. 2.  No interference pattern. 3.  An image of Albert Einstein asking you to "stop this foolishness."

4. Afshar Experiment 1.  An interference pattern. The wires will have little effect.  They would have no affect if the flux at interference minima were really zero.  (This can't be, however, because the minima closer to one pinhole should receive a higher amplitude quantum wave than from the other pinhole.)

5. Afshar Experiment Next, Afshar and others have tried to come up with a double pinhole experiment that determines "which pinhole" path information for photons AFTER they pass the wires placed at the interference minima.   The idea is that determining "which path" information should destroy the interference pattern.  But what happens if you do this AFTER the very existence of the interference pattern was established by the wires?

6. Afshar Experiment Next, the above double pinhole experiment is repeated, except now we will attempt to determine "which pinhole" information for photons. For now, the opaque wires are removed.  A lens is placed after the slits that (would) create an image of the pinhole screen on an image screen.   Now, however, two mirrors are added so that light from the north pinhole is directed only to the north image screen, while light going through the south pinhole is directed only to the south image screen.  What patterns are seen on the north and south image screens? 1.  Interference patterns. 2.  No-interference patterns.

7. Afshar Experiment The experimental setup:

8. Afshar Experiment 2.  No-interference patterns. Each detector sees only the image from its slit.  So the north detector sees only an image of the north slit, while the south detector sees only an image of the south slit.  Since there is no overlap, no interference is expected.  This makes sense also because which-pinhole information is available.  This can be seen when visualizing that only one photon at a time is released by the source.

9. Afshar Experiment The double pinhole experiment is now repeated, but this time the wires that were placed at the interference minima are replaced.   As before, a lens is placed after the wires that refocuses the light.  As before, two mirrors are added so that light from the north pinhole is directed only to the north image screen, while light going through the south pinhole is directed only to the south image screen.  What patterns are seen on the north and south image screens? • Interference patterns. • No-interference patterns. • Bar codes that, when scanned, charge $ 6.66.

10. Afshar Experiment The experimental setup:

11. Afshar Experiment 2.  No-interference patterns. Adding the wires does not cause the photons to revert to the interference pattern.

12. Afshar Experiment Now if the photons were not interfering, one would expect the wires to reflect and diffract a significant amount of source flux.   Therefore, the experiment is now repeated except that the total flux at the image screens is measured.  Do the wires significantly affect the flux measurement by the detectors, as they will scatter photons, or do the wires NOT affect the flux measurement since they occur at flux minima of the now-gone interference pattern? • Interference flux. • No-interference flux. • Left out details determine everything.

13. Afshar Experiment 1. Interference flux. The wires scatter very little light, as if they were at the minima of the interference pattern.  Yet the image screens show images of the slits as if there were no interference patterns.   Does this violate complementary?  Has this experiment both shown an interference pattern AND "which-path" information for its photons? 1.  Yes. 2.  No. 3.  Maybe so.

14. Afshar Experiment Afshar and others think "yes". (Many) others claim "no", and the consensus appears to be "no" although the exact reason is still debated.  Interpretations of the experiment are controversial, and the debate becomes quite technical.  One answer I find somewhat satisfying is that the specific photons for which "which path" information has been determined cannot themselves wholly create an interference pattern.