DOUBLE SLIT IV: QUANTUM ERASER: HOME VERSION by Robert Nemiroff Michigan Tech Physics X: About This Course • Officially "Extraordinary Concepts in Physics" • Being taught for credit at Michigan Tech o Light on math, heavy on concepts o Anyone anywhere is welcome • No textbook required o Wikipedia, web links, and lectures only o Find all the lectures with Google at: "Starship Asterisk" then "Physics X" o http://bb.nightskylive.net/asterisk/viewforum.php?f=39 DOUBLE SLIT @ HOME Based on: http://www.scientificamerican.com/slideshow.cfm? id=a-do-it-yourself-quantum-eraser by Rachel Hillmer and Paul Kwiat Take an ordinary laser and point it at a wall. A spot should appear. If not, find a laser that works. DOUBLE SLIT @ HOME Next, find a pin. Place the pin between the laser and the wall. What happens? 1. An interference pattern appears. 2. No interference pattern appears. 3. You find that you now fully understand quantum mechanics. DOUBLE SLIT @ HOME 1. An interference pattern appears. This is sort of like the double slit experiment but now the slits are infinitely wide. Still, the laser beam is likely not very wide, and so the laser light on either side of the pin acts like two effective slits. DOUBLE SLIT @ HOME: TWO POLARIZERS Next, find a plastic sheet that polarizes light. Cut this sheet and re-paste it so that half the sheet polarizes light vertically, while the other half polarizes light horizontally. Put the pin right in the middle of this sheet dividing the two sections. Call this sheet the "slit screen". Now, repeat the previous experiment and shine the laser at slit screen in front of a wall (called the image screen). What will the image screen show? DOUBLE SLIT @ HOME: TWO POLARIZERS 1. An interference pattern. 2. No interference pattern. 3. An illuminated pin. 4. Caspar the Friendly Ghost (tm). DOUBLE SLIT @ HOME: TWO POLARIZERS 2. No interference pattern. Comment: The polarizers have allowed you to distinguish which photons went to one side of the pin and were hence polarized vertically, and which photons went to the other side of the pin and were hence polarized (e.g. labeled) horizontally. Since "which way" information was available, no interference pattern will be visible. DOUBLE SLIT @ HOME: A THIRD POLARIZER: PARALLEL Next, find another large plastic sheet that polarizes light. Orient this whole sheet so that it polarizes vertically. Hold the whole sheet between the slit screen and the image screen. making sure that all of the laser light hits it. What will the image screen show? DOUBLE SLIT @ HOME: A THIRD POLARIZER: PARALLEL 1. An interference pattern. 2. No interference pattern. 3. An illuminated pin. 4. The neighbors are wondering what is going on. DOUBLE SLIT @ HOME: A THIRD POLARIZER: PARALLEL 2. No interference pattern. Now only light that passed on one side of the pin can strike the image screen. That side will now appear brighter. The other side will not appear completely dark, however, since the pin will diffract some light in that direction. Note that if the third polarizer was rotated by 90 degrees, the polarization would pass only photons passing on the other side of the slit and another "no-interference" pattern would appear. DOUBLE SLIT @ HOME: A THIRD POLARIZER: DIAGONAL Take the third polarizer and now rotate it 45 degrees. It will now rotate the light coming from both sides of the slit screen by 45 degrees and make the resulting photons from each side again identical. What will the image screen show? DOUBLE SLIT @ HOME: A THIRD POLARIZER: DIAGONAL 1. An interference pattern. 2. No interference pattern. 3. An illuminated pin. 4. The neighbors have called the ghostbusters. DOUBLE SLIT @ HOME: A THIRD POLARIZER: DIAGONAL 2. An interference pattern. This is the home version of the quantum eraser. Since photons that strike the screen could have come from either side of the pin, the "which way" is erased and so the interference pattern is restored. DOUBLE SLIT @ HOME: DELAYED CHOICE QUANTUM ERASER This version of the experiment cannot (easily) be done at home. Say that photons are released one at a time, and that you get to choose between aligning the polarizer with the photons arriving from one side of the pin, or align the polarizer 45 degrees in between, AFTER each photon has passed the slit screen. What happens is that it doesn't matter that the photons have already passed the slit screen when made your delayed choice -- how the third polarizer is aligned will completely determine the pattern on the image screen.