A delayed post for a delayed (choice) experiment.

(It’s only fitting, after all.)

One of the defining features of Quantum Mechanics is its weirdness. And although, it is easy to dismiss the “spookiness” of quantum mechanics as something which can only be observed in only state-of-the-art laboratories, the truth is you can witness this spookiness at home using readily available materials.

In this post, I’ll tell you how to conduct your very own “delayed-choice experiment”. These were first proposed by John A. Wheeler in 1978 as thought experiments, that seemed to imply that light can somehow “sense” our experimental apparatus and then “decide” whether to act like a particle or a wave. (This tied into one of the most hotly debated issues at the time, wave-particle duality.)

What we’re going to do, is build a “Quantum Eraser”. If you’ve been reading this blog for a while, you’ll know all about the love I have for the  double-slit experiment. The watered down version of the experiment is this: We allow single photons to pass through two, narrow slits. If we watch the screen and pay no attention to the slits, we’ll observe an interference pattern. (A series of dark and bright fringes.) If we instead place some sort of detector on the slits themselves, the interference pattern disappears. The reason behind this is that we can only extract so much information from our system. If we have the “which-path” information (i.e, we detect which slit the photon passed through), the photon will behave as a particle and hence won’t show interference. If we don’t have this information, the photon will behave as a wave and give us the interference fringes.


The Quantum Eraser is a modified version of the double-slit experiment. It uses detectors (placed after the slits) to gather the which-path information. The eraser is another device that helps us to destroy (or erase) this “which-path” information. Basically, what it all boils down to is this: We can choose to observe or not observe the photon as it passes through the slits long after it has actually passed through the slits.(Hence, the delayed choice.) And this choice would then determine whether or not we see an interference pattern. With me?

Okay. So here is how you build your own quantum eraser:



  1. Darkness: The elixir of all evil scientists.
  2. Polarising film: This is probably the only thing which might be a little hard to find (depending on where you live). You can order it online. For best results, use experimental grade film  and avoid any which is coloured.
  3. A laser: If the laser is polarised, align it 45 degrees to the verticle.  If it’s unpolarised, place a polarizer in front of it at an angle of 45 degrees. (You can easily check whether your laser is polarised using your film.)
  4. A very thin wire. (Strands of hair work too but they are infinitely harder to handle.)
  5. Some aluminium/tin foil.
  6. A screen: can be a plain white wall or a piece of paper.
  7. Stands to mount everything on. You can use anything from paper clips to cardboard boxes to thermocol.

The How:

  1. Cover the tip of your laser with the tin foil and poke a small pinhole in the centre.
  2. Mount the laser on one of the stands about five-six feet away from the screen. On turning it on you should see a bright spot on the screen.
  3. Position the wire vertically between the laser and the screen. You’d have to play around with this for a bit to determine the position that gives you an interference pattern on the screen.
  4. Next, cut up the polarising film into six-inch squares. Take two such squares and tape them side by side (no gaps or overlaps) such that their polarising axes are perpendicular. You can check this by placing one square on top of the other. If no light passes through, you’re good to go. This will be the detector that gives us the “which-path” information.
  5. Either position this detector just behind the wire or tape the wire at the joint of the two polarizers.
  6. On switching on the laser, you’ll find that the interference pattern has now disappeared and all you see is a spot on the screen. This is because now we can effectively determine the “which-path” information (by measuring the polarisation of light).
  7. Use a third square of the polarising film as the eraser. Place it in between the detector-wire arrangement and the screen. When this polarizer is aligned at a 45-degree angle, you’ll observe that the interference pattern reappears!
  8. You can rotate this eraser and watch the pattern appearing and disappearing in real time!

P.S. The last two months were a very unexpected blogging hiatus. Here’s to hoping that that doesn’t happen again.

Until next time!





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