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 :-).

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* 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.

“So, when will you be able to teleport me out to visit you?”

That’s a question my friends and family ask me (jokingly) on a regular basis, given that my graduate studies involve quantum mechanics and given that “quantum teleportation” is perhaps second in popularity only to “exoplanets” as a topic for  popular science writing.

Seriously, there are so many articles about it. And that’s not a bad thing: I love anything that gives more exposure to the genuine awesomeness of quantum physics. Plus the field is undeniably hot right now: just as it seems like a new exoplanet is discovered every month, so to are different teams of physicists seemingly breaking teleportation records for size and distance.

But despite the current media exposure, the idea is, I think, still poorly understood. True, most folks  are all well-informed enough by now to understand that when physicists say “quantum teleportation,” they don’t intend you to imagine them saying it in a Scottish accent, because just about every article on the subject dutifully clarifies that this ain’t quite the same thing as that shimmery trick Scotty does on “Star Trek.” But while everyone’s clear on what quantum teleportation is not, understanding just what it is can be a mite more difficult.

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The Very Model of a Modern Maser General

This past week I had the rare experience of realizing that the government was going to do something that I’ve thought for a long time would be a really good idea. On September 13th, the inaugural “Golden Goose Awards” were given out on Capitol Hill. The awards, brainchild of Tennessee Congressman Jim Cooper, are a response to the Golden Fleece “awards” used by Senator William Proxmire to draw attention to projects he saw as wastes of government spending– hence the vaguely unserious name. But aside from the unfortunate decision to trade gravitas for snarky wordplay, I could not be more excited about them.

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Are You Smarter Than an Italian Education Minister?

You are if you read my previous post about neutrinos, or really any account of the neutrino experiment that has been published in any of the newspapers I’ve seen

After all, I bet even those of you that didn’t have time for my 3,500 word treatise on the subject picked up on the fact that the scientists were just watching these particles do their thing, measuring their speed as they go about their business, only to discover that this speed was larger than the speed of light. I bet you also know by now that neutrinos can travel through the earth itself, because they’re so unwilling to interact with other matter.  If you’re really on the ball you might even know that this is considered a perk: Since neutrinos alone possess this extreme earth-tunneling power, a detector buried deep underground can be certain it’s seeing almost exclusively the neutrinos it wants to see, and not something else like a pion or a cosmic ray.

On the other hand, as some of my Italian friends have pointed out to me, anyone who got their news exclusively from Italian Education Minister Mariastella Gelmini might have a different impression. Here‘s her official statement on the subject, which I would translate as follows:

 

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Faster Than a Speeding Photon?

So all the buzz in the physics department yesterday was this announcement by organizers of an experiment called OPERA (Oscillation Project with Emulsion-tRacking Apparatus) that they’ve measured the speed of the neutrinos being produced at LHC particle accelerator in Europe– and that the speed is larger than the speed of light, Einstein’s famous speed limit for the universe. The one advocated by the bumper sticker above.

For some reason, the story really seems to have taken root outside the scientific community as well; it was featured prominently on a Washington Post, my family tells me it made the front page of the Denver Post, and to top it all off, an acquaintance I know only indirectly posted about it on my Facebook wall– a sign of total societal penetration if ever there was one. I don’t know quite why this bit of news has had such an impact, but I would guess it’s because the story lives in a happy place between being intriguingly futuristic and prohibitively complex. Lots of folks are aware of the “cosmic speed limit,” and the idea of someone breaking it (even someone subatomic!) is the kind of thing that would be right at home in an episode of Star Trek. Indeed, science fiction writers have made such prolific use of the concept of a “tachyon” (a blanket term for hypothetical particles that might travel faster than the speed of light) that there’s an entire wikipedia page devoted to their appearances in popular fiction.

But unfortunately, the apparent simplicity of the concept seems to have lulled the media into a false sense of security, with the result that many of the mainstream articles fail to give any of the interesting details, the context, or the implications, to the point that many of them could just be replaced with an extended headline reading “Scientists See Particles Moving Faster Than Light; Einstein Wrong? Carl Sagan Once Said Something About Extraordinary Claims.”

They aren’t all so bad. This piece from ScienceNow, reprinted in Wired, is the best I’ve seen in popular press, and not just because it quotes a professor I currently work for . And of course one can always turn to the actual publication of the result. But just in case your taste for detail falls somewhere between the two, I’d like to offer some clarifying information about the context of this little puzzle.

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Birds: Better Than Physicists

Alright, here’s one I’d really really like to write a more extensive explanation of, but don’t know if I’ll have time for. I’ve actually put off posting this twice now because I didn’t have time to write an accompanying explanation, but I recently found a nontechnical piece on the subject in Wired Science that does a decent job. Of course there are things I would like to have added, but it will have to do. Ask lots of questions if it seems incomplete :-).

The gist of it is: Scientists studying how Birds’ “inner compasses” work during migration have discovered reason to believe that they may be relying on a very complicated quantum-mechanical effect called “entanglement” to turn the atoms in their eyes into magnetometers. And interestingly, if that’s really how they do it, it means that birds are at least 25% better than the best of all physics collaborations at creating and maintaining this delicate state of entanglement.

I always knew there was a reason that birds were my favorite animal. I guess I had assumed it had to do with the wonderful birdwatching trips I used to take with my dad when I was younger.

A New Particle?

So I have two questions for the scientists at Fermilab: First, if they were going to discover a new subatomic particle, why didn’t they do that during the last two years, when I was working there? And second, if they had to make a discovery like this after I was no longer in any way affiliated with them, why did they wait to announce it until a weekend when I’m too busy to blog about it?

I’ll try to say something more substantial about the topic in a few days; hopefully scientists will know more at that point too. I’m feeling a bit cautious myself: this potential “particle” seems just a little TOO weird, and there’s not nearly enough evidence to say for sure that it exists at this point. Its sort of like finding a big smudge in the mud that looks like a two-food long lizard footprint. Could it be evidence of a six-foot-long lizard? Definitely. But then you have all kinds of other questions to ask, including the all-important “if there are six-foot-long lizards running around, how come all anyone has ever seen is this one footprint?”

Still, if it turns out they’ve got a new particle, it would be the moment physicists have been waiting for for probably 20 years, and it would mean I picked a really, really exciting time to be a graduate student :-).

So, expect some more explanations of the science from me in the next week or so. Until then, I’ve selected the two best articles on the subject for your edification.