Monday, March 21, 2011

Bell-Type Experiments With No Loopholes?

In case you missed it, there's a very fascinating article in the March 18, 2011 issue of Science (p.1380). It describes the physics of quantum entanglement, which is essentially 2 separate phenomena - superposition and non-locality - and the Bell-type experiments that have demonstrated it (i.e. violation of Bell inequality), but up to a certain point. The article deals with the 2 types of loopholes - detection and locality loopholes - that still plague the experiments, and the efforts to design experiments that are devoid of these loopholes.

With their eyes on the prize, a group led by Paul Kwiat of the University of Illinois, Urbana-Champaign, has been collaborating with engineers at the U.S. National Institute of Standards and Technology (NIST) in Boulder, Colorado, to develop photon detectors with near 100% efficiency. “Those are good enough to perform a loophole-free test,” says team member Joseph Altepeter of Northwestern University in Evanston, Illinois. The struggle now is to chain these components together with optical fibers across a large enough distance to keep the communication loophole shut. “Essentially the pieces are all in place, but the devil is in the detail,” Altepeter says.

Meanwhile, Weinfurter and his colleagues are tackling the problem from an entirely different angle. They were inspired by an experiment, carried out in 2001 by David Wineland's team at NIST, that successfully closed the detection loophole using atoms rather than photons. Because atoms are far more hefty than flighty photons, Wineland realized, they are less likely to escape the apparatus, so they provide a potentially perfect detection rate. The team performed a Bell test that compared how often the energy levels—high or low—of electrons in entangled pairs of atoms matched up. Once again, quantum mechanics was hailed victorious, as the level of correlations exceeded Bell's inequalities. But it was not a resounding win because the atoms were close enough together to have influenced each other. In other words, the researchers had closed the detection loophole but in the process were forced to leave the communication loophole open.

Building on Wineland's experiment, Weinfurter's group is attempting to tie up both loopholes at once, by weaving photons together with atoms to reap the benefits of both. The idea is to start with two initially unentangled atoms in separate laboratories—ideally more than 100 meters apart, so that the atoms cannot influence each other over the course of the test. Each atom emits a photon; the two photons are captured and transmitted along optical fibers to a third location, where they are entangled. “The magic is that as soon as the photons are entangled, their parent atoms automatically become entangled, too,” explains Weinfurter's collaborator Marek Zukowski at the University of Gdansk in Poland.

These newly entangled atoms can then take the Bell test, with a perfect detection rate, while sitting far enough apart to keep the communication loophole closed. “The setup is being tried in two neighboring labs right now,” Zukowski says. “When we are happy that everything is working, we will try it in two distant labs.

Of course, the article then threw another wrench in the possible closure of these loopholes by pointing out the possibility of a "freedom-of-choice" loophole that can go back "... far back as the big bang..." Oy vey!

I think such superdeterminism needs to be shown to be influential for me to start putting any degree of validity on it.

It is a good article if you have access to it.


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