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  • In order for us to see something, it either needs to emit light directly (like the sun

  • or a lightbulb filament or firefly) or else have photons of light bounce off of it and

  • into our eyes.

  • But how do we see light itself? You can't bounce light off of light (just like you can't

  • bounce slinky waves or ripples in the water off of each other - they just pass right through!),

  • plus, if you "look" at a photon of light in the normal everyday way, that means your eye

  • or camera or photodetector will absorb it - and then it's gone. Destroyed! Annihilated!

  • It's like if you want to test how much weight a bridge can support before it falls down

  • once you've done your measurement, you have the information you wanted but you no longer

  • have a bridge.

  • So in order to "see" light, we need to use non-destructive testing.

  • One way of doing that is to make a super dark, super cold box and cover the inside with a

  • really really shiny mirror - a mirror so excellently reflective that photons of light bounce back

  • and forth more than a BILLION times before being absorbed. In that time, they'll travel

  • a distance equivalent to one trip around earth.

  • This box is also so cold and dark that only occasionally will there even be one photon

  • inside. And if there is one, how do we tell without destroying it?

  • Well, we send an atom through the box, an atom in a superposition of two different atomic

  • states, just like Schrödinger's cat! If there's no photon inside the mirror box, then when

  • the atom comes out the other side, we'll most likely measure it as being in a certain one

  • of the states - let's call it "dead". But if there is a photon in there, and we carefully

  • send the atom through so it doesn't actually destroy the photon, the atom-photon interactions

  • changes the odds - so now it's an overwhelming chance that we see that atom as "alive." After

  • sending through a few atoms, if they're mainly in the "alive" state, then we know there's

  • a photon in the box! And if they're "dead": no photon.

  • It's kind of like sending a pinwheel through a dark chamber, and if it comes out the other

  • side spinning, you know the wind is blowing. If not? No wind.

  • In fact, once we know there's a photon in there, we can use this cat measurement technique

  • to measure and manipulate other things about the photon: we can see how long it bounces

  • back and forth before it gets absorbed, check if it's in a superposition and even force

  • it into a superposition like Schrödinger's cat itself - so, not only can we see light,

  • we can now use Schrödinger's cat to measure Schrödinger's cat: Quantum Catception!

In order for us to see something, it either needs to emit light directly (like the sun

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B1 photon atom superposition bounce mirror measure

2012 Nobel Prize: How Do We See Light?

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    Why Why posted on 2013/03/28
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