Quantum Superposition: Where the Magic Begins

Alright, here’s my first attempt to talk about quantum physics while still staying within my 500-word limit.* Wish me luck and brevity!**

Recall: a quantum object is any extremely small object–like a single atom. They’re interesting because, unlike a “normal” sized object (physicists call them “classical”) a quantum object can have two or more different, seemingly contradictory properties at the same time– at least as long as you’re not looking at it.

Compare, for example, a classical penny and a “quantum penny.” Suppose you can’t decide between having sushi or  tempura for dinner. If you flip a regular penny and cover it with your hand, you’re done. Underneath your hand is either a head or a tail. Now, you won’t know which one it is until you look, but it is definitely one or the other, just waiting there to be seen.

A quantum penny would not have to behave this way. You could flip a quantum penny, cover it, and insist (correctly) that you hadn’t made a decision yet: underneath your hand, the quantum penny is no more committed than you are to raw versus battered fish. It is essentially both “heads” and “tails,” since both are possible. This is what we call a “superposition” of the two. But never fear; you won’t be left staring at a menu all night. As soon as you lift your hand and look, it will randomly settle (physicists say “collapse”) into one or the other.

You may feel like there’s no functional difference between saying the coin has been heads the whole time and saying that it only decided to be heads when you lifted your hand, since both appear identical whenever you’re looking. But there’s a real, testable way to see the difference.

Suppose I gave you ten coins, and said that they were either all fair coins, or a mixture of some coins with two heads and some with two tails. If I tossed all ten, could you tell me whether the coins were fair? I don’t think so. Even if you got 9 heads, you might suspect they were mostly two-headed, but you couldn’t be sure it wasn’t just the result of fair flips.

But here’s the thing about quantum coins: Given the same setup, you can build an apparatus that takes the coins and uses quantum mechanics to tell the difference.*** But even more amazingly, the machine only works if you don’t look at the coins before feeding them in. If you look, the coins “collapse” to being just heads and tails, like the classical case. But if you don’t do that — if you leave them in their superposition–you can apparently learn something about them that you couldn’t otherwise. Hence, there was something special about their state before you peeked.

All clear? If you’re quantum, and you’re considering complementary states of being (heads or tails, up or down, Beatles fan or terrible human being) then whenever no one’s looking, you can be all of them at once. That’s the essence of quantum superposition, and in many ways the essence of quantum mechanics itself.

Boom! 500 words exactly :-).

___

* This little introduction doesn’t count.

** Neither do footnotes.

*** The above is a very broad analogy for a famous kind of quantum experiment called a “double slit” experiment, in case you want to read more about the details. I’m doing my best to recast it in the more comfortable “coin” picture, but the actual set up is physically quite different. For experts (and if you are one, please feel free to critique), the parallel I’m trying to draw is this: each side of the coin represents one of two paths taken by a particle in a 2-slit experiment, so each fair coin collectively represents a particle randomly allowed to travel through either slit. An “unfair” coin corresponds to a case where one slit is blocked, guaranteeing one path over the other. Run these two with billiard balls (targeting random slits vs. randomly closing off one slit at a time) and you won’t see a difference in their detected locations. Run it with electrons and you will, since only the first case will allow an interference pattern to form. What’s more, if you “look at the coins before they’re detected” (i.e., if you measure which slit the electron enters along the way) you will destroy your ability to distinguish the cases because no pattern will form. Hence, there is something different between the unobserved quantum state and the classical analogue.

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About Colin West
Colin West is a graduate student in quantum information theory, working at the Yang Institute for Theoretical Physics at Stony Brook University. Originally from Colorado (where he attended college), his interests outside of physics include politics, paper-folding, puzzles, playing-cards, and apparently, plosives.

5 Responses to Quantum Superposition: Where the Magic Begins

  1. Brendan Long says:

    It’s also worth mentioning that “looking at” a quantum object tends to mean interacting with it in almost any way (like shooting it with a laser, or waiting for it to run into a wall), not “a human looking at it with their eyes” like pseudoscientists like to claim.

  2. Colin West says:

    A very, very good point from Brendan. There’s nothing magical about a human being observing the quantum object– the issue is whether the object has been probed in any way. Anton Zeilinger, an extremely brilliant quantum physicist, puts it this way: a quantum object has been “observed” if the information about its state of being “is accessible in principle from the experiment or even if it is dispersed in the environment and beyond any technical possibility to be recovered, but in principle still ‘’out there.'”

    In other words, all that it takes is for something to interact with the object in a way that depends upon it’s state. For example, if I throw a bouncy ball at a quantum penny, it might bounce off of the “heads” side in a slightly different way than it would bounce off the tails side, due to differences in the curvature and shape. Is there any realistic chance that I could tell the difference just by watching the ball bounce? No. But in PRINCIPLE the information is now “out there and available,” so the penny will have to collapse into one state or the other. This, by the way, is also why quantum superpositions are often described as being extremely fragile. Almost any little thing can cause them to revert to a “normal” state.

  3. livinginaquantumworld says:

    Reblogged this on Living in a Quantum World and commented:
    An excellent concise explanation of quantum superposition and all its glory.

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