Chad Orzel tells us that we are creeping closer to quantum computing. Whether we will ever get there, however, is unclear:
Uncertain Principles: I almost skipped "Neutral Atom Quantum Register" in PRL, too, as that number of Russian-sounding names in the author list is usually an indicator of Inscrutable Theory. I recognized the PI as an experimentalist, though, and it was indeed worth a look.
This is an experimental paper about a potential scheme for using neutral atoms in a quantum computer. As previously noted, a good quantum computing scheme needs to provide five things: long decoherence times (so the atoms stay in superposition states long enough for the computation to unfold); addressability (so you can target specific bits of your computer); a readout scheme (so you can extract an answer at the end of the computation); scalability (so you can add enough bits to do something worthwhile); and entangling operations (so you can create the cool quantum states that make the whole thing work). The scheme described in this paper gets four out of five, but the fifth is a doozy.
The method they propose is to trap single atoms in the anti-nodes of a standing wave laser beam, and use the two hyperfine levels of the ground state as the "0" and "1" states for the computation. This is great for decoherence, as these states have huge lifetimes, and are relatively easy to shield from outside influences. Readout is done by detecting the fluorescence when the atoms are hit by a laser, and is a well-established technology. In this paper, they demonstrate addressability by applying a linearly increasing magnetic field across the sample, so that each of the trapped atoms has a slightly different resonant frequency. They show pretty convincingly that they can pick out an arbitrary atom in the middle of the sample, and flip it from "0" to "1" in a fairly short time. Scaling is just a matter of adding more atoms to the sample, which is easy enough to do.
The one thing missing is an entangling operation. They've got a great scheme for storing neutral atoms in such a way that they can be manipulated and measured at will, but they don't have a good way to entangle two of the atoms-- that is, they can't do an operation where the state of one of the atoms ends up depending on the state of a different atom without measuring it. And without that, it's not really a quantum computer. They do mention a bit of an idea (moving atoms around to bring them close together where some reasonable interaction might entangle them), but it's not really there yet.
Of course, neither are any of the other experimental quantum computing schemes, so it's not like they're being blown away by the competition. And if they can come up with a workable way to entangle the atoms, this system will make somebody a really neat toy.
Someday I would like to understand "decoherence," and figure out whether the development of "decoherence" has moved practicing physicists out of the "shut up and calculate" school as far as the interpretation of quantum mechanics is concerned...
Posted by DeLong at October 12, 2004 08:30 AM | TrackBackBrad, did you accidentally delete the first part of this post?
Posted by: David Weman at October 12, 2004 09:01 AMThere's a formatting problem with this post in Mozilla (though not in IE.) For various stupid reasons, HTTP comments (of the "<!-- -->" type) can't contain a double dash "--" in them. Mozilla obeys this properly, but IE doesn't. On Mozilla, the page goes straight from the title of the next post up ("Not the Dismal Science--the Shrill Science") to "Of course, neither are any of the other experimental quantum computing schemes"; everything else is interpreted as a comment.
Posted by: Jon Lennox at October 12, 2004 09:10 AMThere is an old saw that QM is not only stranger than you think, it is stranger than you can think. Having met a few types who can go beyond the shut up and calculate method, let me say that any resemblence between them and normal humans is, well, Lewis Carrol said it best and first
One day Alice came to a fork in the road and saw a Cheshire cat in a tree. "Which road do I take?" she asked. "Where do you want to go?" was his response. "I don't know," Alice answered. "Then," said the cat, "it doesn't matter."
- Lewis Carroll, In Decisions
Posted by: Eli Rabett at October 12, 2004 09:27 AMTo do quantum computing you need your qubits in a "pure state" - in which there are special correlations between the states of different qubits. (i.e., coherent phase relations between their wavefunctions.)
Decoherence simply refers to the tendency of the qubits (in this case, probably Cesium atoms) to lose those correlations due to interaction with the environment.
Practical aspects of quantum computing can be understood completely within the "shut up and calculate" Copenhagen interpretation. However, some (like David Deutsch) would argue that a deeper, philosophical, understanding of their parallelism is highly suggestive of the many-worlds interpretation.
Posted by: steve at October 12, 2004 09:32 AMRe: missing paragraphs
For those still foolishly resisting IE assimilation, you can read the post by selecting "View", "Page Source", and then selecting "View" "Wrap Long Lines".
Posted by: Silent E at October 12, 2004 10:01 AM"Someday I would like to understand "decoherence," and figure out whether the development of "decoherence" has moved practicing physicists out of the "shut up and calculate" school as far as the interpretation of quantum mechanics is concerned..."
The short answer to your (implieD) question is: "No."
The medium answer is "Not really."
Orzel should expect a visit from the "Union of Russian-Sounding Physicists" flying squad any day now. Those guys play for keeps, especially the Ukrainian nationalists in the group.
Posted by: Zizka at October 12, 2004 11:32 AMBrad, first of all, I must thank you. It was your musings on quantum mechanics that led me to channel my many, many procrastinatory impulses that arise as I pursue a PhD in quantum computing at MIT toward something vaguely constructive: blogging. Basically, after writing several long comments on your site, I thought: "Gee, if I'm writing this much, I should get my own darn blog."
(Those curious about my previous long comments on this site regarding quantum mechanics can see them collated and clarified here on my own blog
http://williamkaminsky.typepad.com/too_many_worlds/2004/04/ideas_for_destr.html
Given that my blog is entitled Too Many Worlds and carries the droll subtitle "...and they're interfering with what I trying to do in this one," I think y'all can guess my metaphysical preferences.)
Second of all, the most important aspect of decoherence is the following idea which greatly reduces the mismatch between quantum mechanical formalism as it's treated in first-year courses and our ever-present experience that reality permits persistent, determinate records:
**************
(Informally) It's usually the case that before you get to observe a system, the rest of the universe ("the environment") has gotten a chance to interact with it significantly. Thus, the only observations about a system you can register are those that correspond to system states which are stable with respect to this omnipresent interaction with the environment.
(Formally) This interaction with the environment generically makes all information about the system unavailable *with only one exception*: observables of the system that commute with the Hamiltonian coupling the system and the environment cannot be disturbed by the system-environment coupling and thus can still in principle can be interrogated by your apparatus.
**************
Third and finally, the party line for physicists is still resolutely positivist. We're supposed to care first and foremost about making verfiable (and preferably precisely quantifiable) predictions about observable phenomena. Thus when we're harried and pressed for time, we'll say, "Shut up and calculate!" When we have a little more time for reflection, we'll say something like: "There usually exists more than 1 theory able to explain current observations to within the margins of present experimental uncertainty, and there's usually more than 1 interpretation that one can straightforwardly place atop a theory. Or as Jeffrey Barrett, a philosopher of physics at UC-Irvine, puts it: '...not only are physical theories are typically underdetermined by empirical evidence, but one's ontological commitments are typically underdetermined by the physical theory one adopts.' " All that said, though, physicists on the whole have a pet peeve for the ad hoc. Rules that produce the right results but have no other discernable justification usually annoy us to no end. And the biggest ad hoc rule of 20th century physics was the "collapse postulate" of quantum mechanics, this notion that systems deterministically and continuously obey Schroedinger's nice, familiar, and safe-looking differential equation when nobody's "observing" them, but that they randomly and suddenly collapse into an eigenstate of that Hamiltonian in Schroedinger's equation as soon as you "observe" them (whenever the hell that is exactly... when your machine goes ping? when you hear the ping and read the gauge on it? when your mind has consciously realized that it's heard the ping and read the gauge?! Aaaahhh!!) The basic notion of decoherence---namely, that the constrained set of possibilities you can observe in a system is the result of the system's interaction with the rest of the universe---and our ability to make quantiative predictions about what these constrained possibilities are and how fast the constraint arises has convinced many physicists (especially the ones working in quantum computation) that collapse is indeed basically some sort of ad hoc rule that somehow can eventually be purged from physics.
However, doing away with collapse isn't so easy. Philosophers of physics like Jeffrey Bub will bemoan the "new orthodoxy" (or perhaps better said "new complacency") of physicists that decoherence somehow means that the theory now fully supplies its own interpretation. You can read this online in Chapter 8 of his book Interpreting the Quantum World, which is full-text viewable at Amazon.com at:
http://www.amazon.com/gp/reader/052165386X/ref=sib_dp_pt/103-2162882-1244610#reader-link
Bill, with all due respect, shut up and calculate.
Posted by: Fabio Lanzoni at October 12, 2004 12:55 PMDavid D is nuts. He has made the mistake that we were warned about in high school. You cannot apply wave theory to photons, you cannot apply quantum theory to waves.
Posted by: old ari at October 12, 2004 01:02 PMWow! Fabio Lanzoni (better known as just Fabio), the Italian male model, I-Can't-Believe-It's-Not-Butter spokesman, favored muse of thousands of romance novel cover artists, and overall hero to millions, knows quantum mechanics and has just taken the time out of his busy, busy schedule to remind me I should really stick to my actual PhD work.
Point taken! I shall redouble my efforts to actually finish my PhD, Fabio. Thank you, sir!
Posted by: Bill at October 12, 2004 04:25 PMWell, at least we can all agree that Bush is more decoherent than Kerry, can't we?
Posted by: tedb at October 12, 2004 06:49 PMTry the Wikipedia articles on quantum logic and quantum operations.
Posted by: CSTAR at October 12, 2004 09:04 PMBill: I'm with you; I come from a "hard science" background myself, with an intent to become some variety of engineer. However, starting to follow links to DeLong's blog from other blogs reawakened in me a distant memory of having taken a first place prize in Economics in a high school competition way back in the day (Reagan was president, 'nuff said), which caught me totally off guard, since I already thought of myself as a hard sciences and math sort at the time. Lately, I've been considering combining engineering and economics in some sort of major-minor or double major combination. (I've still got way too much time to make a final decision.) I find it somewhat amusing (not at all in a condescending way) that DeLong produces the occasional physics related post as well.
CSTAR: I'll make things simple for our fellow readers, being a Wikipedia veteran myself.
http://en.wikipedia.org/wiki/Quantum_logic
http://en.wikipedia.org/wiki/Quantum_operations
Oops, try again on that second link:
http://en.wikipedia.org/wiki/Quantum_operation
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