Free How to Teach Physics to Your Dog by CHAD ORZEL Page B

filter. When we talk about light in terms of photons, though, the filter is an all-or-nothing proposition. Any given photon either makes it through, or gets absorbed by the filter. There are no “parts” of photons.
    We handle the interaction between photons and polarizing filters by saying that each photon has a probability of passing through the filter that is equal to the fraction of the total wave that makes it through in the classical model. If a beam of light with a polarization at 60° from vertical encounters a vertical polarizing filter, the beam on the far side will be one-fourth as bright, meaning that it has one-fourth as many photons. That means that each individual photon has only one chance in four of making it through the polarizing filter.
    Each photon making it through the filter will also have its polarization determined by the filter. Only one photon in four may make it through a vertically oriented polarizing filter, but every one of those photons will pass through a second vertical filter, and none of them will pass through a horizontal filter.“Vertical” and “horizontal” are then the allowed states of the single photon’s polarization—when we measure the polarization using a filter, we will find the photon in one of those two states (either passing through the vertical filter, or being absorbed by it), and not anywhere in between.
    Polarized photons thus provide an excellent system for looking at the core principles of quantum mechanics. Each individual photon can be described in terms of a wavefunction , with two parts corresponding to the two allowed states , horizontal and vertical polarization. That wavefunction gives you the probability of the photon passing through a polarizing filter, and after you make a measurement of the polarization with a filter, the photons are in only one of the allowed states. A single photon passing through a polarizing filter demonstrates all the essential features of quantum physics. As a result, polarized photons have been used in many experiments demonstrating quantum phenomena.
    “So, let me get this straight. A photon at an angle between horizontal and vertical is in a superposition state? And sending it through a polarizing filter is the same as measuring it?”
    “Yes. You get all the features of quantum superposition and measurement—wavefunctions, allowed states, probability, and measurement—using single polarized photons.”
    “But I thought you said all this stuff worked the same way when you talked about light as a classical wave?”
    “Well, yeah. The end result is the same as the classical polarized wave description.”
    “What’s the big deal, then? I mean, your big example of quantum weirdness is something that just reproduces classical results?”
    “Well, no. I mean, that’s not my big example. The big example of quantum weirdness is in the next section.”
    “Oh. Well, carry on, then.”

(UN)MEASURING A PHOTON: THE QUANTUM ERASER
    One of the best demonstrations of the weirdness of quantum superpositions is an experiment called a quantum eraser. The quantum eraser encapsulates everything that’s strange about single-particle quantum physics in a single experiment: particle-wave duality, superposition states, and the active nature of measurement. If you can understand the quantum eraser, you’ve understood the essential elements of quantum physics.
    Many different variants of quantum-eraser experiments have been done over the years, * but the simplest starts with a variant of Young’s double-slit experiment (page 18). If we send a beam of photons at a pair of narrow slits, we will see an interference pattern on the far side of the slits, built up out of single photons detected at particular points (as shown in the figure on the next page). We can see the pattern only because light passes through both slits at the same time. If we block one slit, the interference pattern will disappear, and we’ll see only a broad scattering