I Don't Understand This

I don't understand this. I may never understand this.

Uncertain Principles :You Can't Get There From Here: Category: Quantum Optics: Buried beneath some unseemly but justified squee-ing, Scalzi links to an article about "counterfactal computation", an experiment in which the group of Paul Kwiat group at Illinois managed to find the results of a quantum computation without running the computer at all. Really, there's not much to say to that other than "Whoa." The article describing the experiment is slated to be published in Nature, so I don't have access to it yet, but I'll try to put together an explanation when I get a copy. The experiment involves a phenomenon know as the "Quantum Zeno Effect".... [M]ake the measurement a much shorter time after the excitation-- a tenth of a second, say. The probability that the atom has already decayed is really, really small-- 0.002%-- so you're really likely to find it in the excited state, after which the atom is entirely in the excited state again, and the decay clock starts over.... If you keep making measurements at short intervals, you can keep the atom in the excited state basically forever.

The cool thing is, you can do this sort of thing with passive measurements. You don't have to bounce a photon off the atom to prove that it's in the excited state-- instead, you can send in a photon that will only be absorbed by a ground-state atom, and see what happens. If it isn't absorbed (and it most likely won't be), that's just as effective at keeping the atom in the excited state as if you'd done something more active to detect the excited-state atom.... If you're really clever about it (and Paul Kwiat is a really clever guy)... computing without running a computer should come as no surprise...

Comments

[mere mortal]

Yup. Niels Bohr was not kidding around when he said "Anyone who is not shocked by quantum theory has not understood it."

Though perhaps he did not go far enough. I'm pretty sure I don't understand quantum theory, but I'm still shocked by it.

[mere mortal]

Come to think of it, perhaps Arthur C. Clarke has a better take on this one:

"Any sufficiently advanced technology is indistinguishable from magic."

[John H. Morrison]

I don't feel like working through the quantum mechanics of this process right now. However, there is a certain issue to consider: the probability cited (.002%) is not exactly zero.

The more mundane process of radioactive decay works exactly like the process described. At the first measurement one-tenth of a second after the excitation, 99.998% of the time the atom is still in the excited state. Repeated a gazillion times, 99.998% of the atoms are in their excited states. Then repeat the process on those that remain -- the two-tenth-second measurement will have 99.998% of the first 99.998% still in the excited state. The third time, it's 99.998% of 99.998% of 99.998% remaining.

We have the mathematics of exponential decay. The decay constant -- tau in exp(-t/tau) -- is going to be 0.1 sec/0.00002 or 500 sec. Come to think of it, that sounds fairly close to the decay time of a free neutron.

[John H. Morrison]

Okay, try tau = 5000 sec, rather than 500 sec. I dropped a decimal point in my mental arithmetic.

[Doctor Jay]

This talk of "keeping the atom excited" is what's misleading, and a category error. Think of it this way. The atom has no memory. It doesn't it remember how long it's been excited. So any time you find one that's excited is exactly the same as any other time.

When compared to the speed of computers, any individual atom will very likely stay excited for an entire computation, which is what's needed. In the low-but-not-zero probability event of the atom decaying before you wanted, the computation is lost and must be attempted again. Or something like that.

Of course, when you put billions of such atoms together in a lump of uranium, some of them will decay during each period, creating radiation.

What's really mysterious about this is the notion that the decay is completely random. There is no causation for it. It can't be predicted. This gives a Newtonian fits.

[Doctor Jay]

Ok, I did a bit more digging and got a much better idea of what the Quantum Zeno Effect is from the source of Uncertain Principles, namely Paul Kwiat's web page http://www.physics.uiuc.edu/People/Faculty/profiles/Kwiat/Interaction-Free-Measurements.htm

He has a much better example. Namely, imagine a series of 6 polarization rotators, each of which changes polarization angle by 15 degrees. They are set so that a photon with horizontal polarization entering the first has a 100% probability of exiting the last with vertical polarization, and thus zero probability of passing through a horizontal polarization filter at the end.

But if you insert a horizontal polarization filter between each of the polarization rotators, the probability that a photon that enters the apparatus horizontal will pass through the horizontal polarizer at the end rises to about 66 percent! And the way the math works, you can probably push this probability higher by adding more rotators and filters with smaller angles between them. This doesn't seem so wierd to me, a polarization filter isn't a benign thing.

[Maynard Handley]

Doctor Jay has exactly the right idea. There is a certain segment of the population that is upset that there are no longer witch doctors and alchemists getting all the respect and tries to use quantum physics to hustle up that same "reverence". Some of these are in fact physicists but many are journalists or teachers or authors.

The standard technique used by these people is the (sometimes subtle, sometimes completely unsubtle) redefinition of words (but not explicitly of course) so as to appear to be saying something that is not in fact the case. The last big such case of this was people jumping up and down with excitemen about "frozen light" and "making photons stand still", which all sounds rather less dramatic if you substitute the *correct* technical term, "dressed photon" for "photon" . What's a dressed photon you ask? Essentially it's a coherent collection of photons and charged particles that are constantly emitting and absorbing those photons; the details are not important. The point is that the big drama of this announcement, that light supposedly "stood still" instead of moving at "the speed of light" is in fact a complete misinterpretation of what happened.

Before that we had something else, I forget the details, but I seem remember that it basically hinged upon a rate of change of refractive index going negative --- an unusual event, but hardly the earth-shattering mystical event it was dressed up as once less technical language was used.

I've no idea what was done here, and I can't say I much care. I imagine that, once again, if the details were actually presented clinically and responsibly it would come across as simply one more experiment --- interesting, worthy of congratulation, but neither evidence of the fundamental mysticism of the world, nor evidence of the obsolesence of traditional digital computers over the next five years. The world is full of interesting physics and I'll be damned if I take some PR firm's rantings as my guide to the best order in which to investigate papers and phenomena.

Finally let's consider what will be next in the overhyped sweepstakes. I keep waiting for negative temperature to have it s day. Oh my --- negative temperature, can you imagine such a thing. How can it be colder than absolute zero? But wait, it gets even more mysterious for those wacky scientists because negative temperatures are actually super HOT not super cold. What a bizarre world we live in, and isn't it wonderful that there are people better and smarter than us to protect us, keep us safe, and tell us what to do? Remember reader, always listen to and trust your superiors --- they know and understand things you do not!
As for negative temperature, go look at
http://en.wikipedia.org/wiki/Negative_temperature
which gives a not great, but adequate account of the phenomenon.

[Rob Grigjanis]

Maynard, this has nothing to do with mysticism, nor does anyone I know/read present it as such.

Quantum phenomena are "weird" because they depart so drastically from our (classical) experience. I suppose the best known example of this weirdness is the "Feynman" double-slit thought experiment. Particles behave like waves.

http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DoubleSlit/DoubleSlit.html

That there might be very counterintuitive macro consequences to such micro phenomena should not be too surprising. Now, if you want to equate "very counterintuitive" to "mystical", that's fine with me.

[MTC]

The issue seems to be a very slippery use of the word "run" by the researchers. If by "run" one means "instigate the entanglement, apply the algorithm, then return the system to the ground state, marking one tick of the computational clock" then the system described manages to deliver answers without ever "running".

[Neil']

Well, note that negative or "Renninger" measurements can affect the wave function, since the wave readjusts to reflect not having been detected at a certain point. That means (as per quantum "seeing in the dark") that putting an obstacle in the path of a split-beam interferometer will change the interference pattern (blocking one path for example allows hits on a detector that otherwise would be in the interference "dark" zone.)

In 2000 I put out a quantum measurement paradox on refereed discussion group sci.physics.research that (now improved) postulated the following: what if we sent a polarized photon many times through half-wave plates. The degree of accumulated angular momentum would show how circularly polarized the photon wave was (as in fully circular, elliptical, linear etc. - based on relative proportions of RH and LH basis states) and not just answers to yes/no tests as usually believed possible. (This is what Y. Aharanov et al call a "weak measurement.")

It got enough comment and play that now (last I checked) typing "quantum measurement paradox" into Google brings it up first.

[Maynard Handley]

"
managed to find the results of a quantum computation without running the computer at all. Really, there's not much to say to that other than "Whoa." The article describing the experiment is slated to be published
"

This is not mysticism? You're splitting semantics when you want to claim that "counter-intuitive" and "weird" are not synonyms for "mystic". The world is full of non-intuitive things, but QM is unique in the amount of nonsense written about it. The various crap regarding string theory that we read about every day is not presented in the tones I described? The description of Schrodinger's cat is not presented in terms that are basically mystical: "there is another world out there beyond the world we can see, where everything is different from the every day world"?

Or lets go to the Cosmic Variance http://cosmicvariance.com/2006/02/27/quantum-interrogation/ discussion of the article which tells us things like:
"
Quantum mechanics, according to the conventional interpretations that are good enough for our purposes here, says three crucial and amazing things.

First, objects can exist in “superpositions” of the characteristics we can measure about them.
"

Amazing? That things can exist in a superposition? All of elctromagnetism is based on this (what do you think the resolution of a function in terms of any sort of basis is), and no-one spouted this sort of nonsense back in the 1880s.

There are unusual things about the small world, sure. QM superposition is one of them, though the prime issue is the way this plays out in QFT, not the trivialities of Schrodingers equation and double slits which have nothing to do with this and simply reflect the fact that the world is made up of fields, not particles --- and that includes electrons.
The transition from action-at-a-distance to field theory was a substantial revision of how one ought to view the world but occurred pretty smoothly; the transition from a world of particle to a world of fields would, by rights, have followed the same pah but it didn't because too many people preface every freaking world they say about quantum mechanics not with the statements "here is the way the world is, ain't it cool" but rather with the words "here is the way the world is, ain't it bizarre".
And yes, I claim that this is not completely an accident --- there are sociological and psychological reasons for why things are presented in this "oooh, it's all so spooky" language.

Cosmic Variance, like so many other writers, does not help its case by presenting it in terms of every-day objects and then saying "well isn't is bizarre that puppies behave this way"? Well no, it isn't because puppies don't behave that way --- electrons behave this way, not puppies.

The small world is different from what the untutored mind would expect - period. So what? The large world is also different from what the untutored mind would expect. The world of classical physics is different from what the untutored mind would expect. The untutored mind is a freaking idiot, and what else would you expect from something that's the result of 3 billion years of random evolutionary tinkering. But that's a statement about the untutored mind more than it's a statement about the small.

[Robert Grigjanis]

"The world of classical physics is different from what the untutored mind would expect"

"The world of classical physics" is the world we experience. Our intuitive understanding of it depends on the types of experience we have. What on earth does "untutored" mean? Not having taken 3rd year Classical Mechanics? Not knowing what a Hamiltonian is?

I dunno. Maybe I just don't see as many "ain't it bizarre" sites as you do. I also tend to avoid rightwing sites. Sanity maintenance, you know.

[orionATL]

not only do i not understand this,

i am absolutely confident i alway will not understand it.

nonetheless,

having a physics audience (potentialy, of course ) presents an opportunity to ask what, for non-physicist me, is the central question of modern physical interpretation of the world.

how can you, or anyone, be sure that the english (dutch, singhalese, chinese, finnish, french...) words attached to any explanation of quantum mechanics accurately describes the phenomenon?

the language i know (english, in case you doubted)

seems to me to be far too ambiguous, and far too much an instrument of individual interpretation, to serve as an accurate translator for mathematical depiction of microscopic (electron variety) reality.

now for a question about economics

this seems like a situation in which "less falling to less" is enough to make lots happen. the situation may not be akin to perpetual energy, but seems like a relative.

so this raises the question in my mind

could we possibly have a quantum economy?

what would quantum economics look like?

well, actually,

we're on our way,

we already have discussions of dark matter in national accounts.

perhaps,

less work, but sufficient output?

which suggests

quantum jobs

might be in demand.

quantum monetary policy??

hmmm

lots of work to be done.