What's So Interesting About Quantum Phenomena?

Third of the five research categories within DAMOP that I talked about is Quantum Phenomena. This is a little bit of a catch-all, as there are a few different things going on in this area. They are all unified, though, by the fact that they end up making quantum mechanical effects manifest in some way, either as a means to an end, or just for the sake of showing that quantum mechanics is really weird.

What do I mean "making quantum mechanical effects manifest?" Basically, demonstrating one of the essential elements that I talked about last year: showing the wave nature of matter, demonstrating the discreteness of quantum states, showing quantum superposition, demonstrating non-local correlations through quantum entanglement, etc. There are efforts underway to do all of these things in a variety of AMO systems.

Research within this category falls into three basic groups of topics:

--Quantum Information Processing: This is, basically, work on building a quantum computer. There are lots of candidate systems in which this might be done (I've written about a bunch of them: ion traps, optical lattices, NMR, Rydberg atoms, diamond), and lots of funding in this part of the field, because if you can make a practical quantum computer, it has huge implications for things like code-breaking.

Depending on the exact system under consideration, work in this area takes on a slight different appearance, but they're all stages of the same process: you need to first demonstrate the ability to control your qubit, putting it in any arbitrary superposition of 0 and 1; then you need to be able to faithfully detect the state; then you need to be able to do entangling "gates" involving two qubits; then you need to be able to scale things up to involve more qubits. Different candidate systems are at different points in this progression: most neutral-atom systems are still demonstrating entangling operations, while ion traps are in the scale-up stage. Those are the general areas you can expect to hear about when you read about or listen to a talk on quantum information.

--Quantum Communications: This is the process of moving quantum information from place to place, which is related to but not identical to the processing of quantum information. If you're going to make a useful quantum computer, you may very well need to be able to shift superposition states from place to place, but moving quantum information can also be useful in its own right.

One big area within this subfield is quantum cryptography, which uses the non-local correlations between entangled particles as a means of generating a random number that is shared by two distant people, but unknowable to anyone else. These numbers then serve as the basis for encryption keys, making effectively unbreakable codes. This has been demonstrated numerous times, and is pushing towards commercialization. A lot of current effort is devoted to trying to extend the distance over which quantum keys can be distributed-- there was a very nice talk at DAMOP by Richard Hughes from Los Alamos on efforts to make a practical system to send quantum keys to and from satellites in orbit. They've got just about everything worked out except for the part where they put the whole system in space (which is the expensive part...).

The other big area in quantum communications involves things like quantum teleportation, and the construction of "quantum repeaters" to enable the sending of information over long distances. This will involve storing then re-encoding quantum information, and there was a suprisingly low-key talk at DAMOP by Alex Kuzmich at Georgia Tech about a really impressive recent experiment where they stored a superposition state in a sample of cold atoms, then extracted it as two entangled photons, converted one photon to a telecom wavelength, sent it through a long optical fiber, converted it back to the original near-IR wavelength, and then tested it to confirm that it was still entangled with the other photon from the original pair. They confirmed a violation of Bell's inequality by about 5 standard deviations, which even a particle physicist will believe. Here's a pretty picture from their paper:


--"Macroscopic" Quantum Effects Not all that closely related to the other two subcategories, but a very active area of research is the topic of making quantum effects more apparent. This typically involves increasing the size or number of particles involved. One particualrly hot area of this sort of thing is cavity opto-mechanics, where you use light bouncing back and forth between two mirrors to control the quantum state of something in the system well enough to potentially put it in the quantum ground state, and make superposition states with it. These typically involve micron-scale membranes and that sort of thing, but the goal is to push this up as large as can be managed, and the LIGO folks have demonstrated this sort of thing with gram-scale mirrors. The idea is to push the boundaries of unambiguously quantum behavior up as far as possible, to begin to test speculative ideas about wavefunction collapse and the like.

Also, it would just be unimaginably cool to put a macroscopic object into a superposition of two different positions at the same time.

There are some other projects in this same vague area that put the scare quotes around "Macroscopic"-- things like demonstrating entanglement with up to double-digit numbers of particles, or interference experiments with organic molecules, and that kind of thing. the basic goal is the same, though: to show unambiguous quantum behavior in larger and more complicated systems.

Names to Conjure With: If you want a short list of people to follow, or names to drop to make you sound more informed about the field, here are some suggestions: Anton Zeilinger in Vienna is probably the world's best demonstrator of weird quantum phenomena.Dave Wineland's group at NIST is arguably the best in the world at quantum information experiments. Jeff Kimble at Caltech is the king of cavity QED, and Jack Harris at Yale is one of the pioneers of the cavity opto-mechanics thing. Misha Lukin at Harvard does a bunch of amazing quantum stuff, and the Joint Quantum Institute at Maryland has "Quantum" right there in the title, which tells you they're working in this area.

A lot of quantum information stuff is still highly theoretical, so I will even throw out the names of a couple of theorists: Peter Zoller has investigated just about every interesting possible quantum information system, as has Ignacio Cirac. If they're not studying it, there's probably a very good reason why it won't work. And, of course, there's that whole Perimeter Institute thing.

I am, of course, leaving out all sorts of brilliant people, but if you want to get a sense of the kinds of things that are going on in the world of quantum phenomena, these names will give you a good start.


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All very interesting, what I heard lately is that three-particle entanglement is supposed to reveal some new insights. At http://fqxi.org/community/forum/topic/949 I describe an experiment that would show whether loss of interference in a quantum superposition leads to mixture output. (My apologies for a needlessly confusing piece about it elsewhere ;-) Just aside from sympathies here and there or just arguing about it, the experiment is easily doable and this issue should be checked out empirically. I'm not aware of any other practical way to do so and it would "make manifest" what has been just argument over "interpretations" of the same facts.

I hate that I'm too old and too stupid to ever enter this field.

Roger, I'm not so sure. It hasn't gotten around a lot, but reading this article should show it's worth doing. However you have a point if suggesting proponents need to make a good case in practice and not just proclaim usefulness.

BTW good blog and quantum posts you've got. Check my stuff out too, see what you think. For more on Einstein's blunder (the one about dice, not the CC "error" he referred to himself) see "People Keep Making Einstein's Greatest Blunder" on Scienceblogs. I'm debating decoherence etc. there.