Tag Archives: Entangled states

Going Into Overtime

Posted on by

Entangled Particles Face Sudden Death

[I]n a paper published today in the journal Science, two physicists show that entangled particles can suddenly and irrevocably lose their connection, a phenomenon called Entanglement Sudden Death, or ESD.

“The degree of information entangled can disappear faster than the information itself,” said Joseph Eberly, a physicist at the University of Rochester, who, along with Ting Yu, co-authored the paper. “It’s completely non-classical physics.”

I don’t do experiments with entangled particles and I haven’t read the paper yet, but I was a little surprised to read that the model up to this point had been that entanglement was lost slowly. I had always gotten the impression that entanglement was much more a binary condition, so you wouldn’t describe particles as being a little bit entangled, any more than you would say someone was a little pregnant. I suspect this has to be tied to the question of how fast a wave function collapses.

Science 30 January 2009:
Vol. 323. no. 5914, pp. 598 – 601
DOI: 10.1126/science.1167343

New Quantum Teleportation Result

Posted on by

Via both Physics and Physicists and Uncertain Principles, I see that there is a new result in quantum teleportation between ions that were about a meter apart. Both posts have short summaries (along with Chad considering doing a more thorough write-up) and other links.

I think the Science Daily or Eureka Alert (which I think are identical) are the better ones, since they actually explain how the entanglement occurs:

You excite the two ions so they well drop back down into one of two complementary states, and in doing so they release photons that would be different in energy if they represent the two different transitions.

Before reaching the beamsplitter, each photon is in a superposition of states. After encountering the beamsplitter, four color combinations are possible: blue-blue, red-red, blue-red and red-blue. In nearly all of those variations, the photons cancel each other out on one side and both end up in the same detector on the other side. But there is one – and only one – combination in which both detectors will record a photon at exactly the same time.

In that case, however, it is physically impossible to tell which ion produced which photon because it cannot be known whether the photon arriving at a detector passed through the beamsplitter or was reflected by it.

Thanks to the peculiar laws of quantum mechanics, that inherent uncertainty projects the ions into an entangled state. That is, each ion is in a correlated superposition of the two possible qubit states

The B-Movie View of Quantum Analogies

Posted on by

Chad reminds us that pop-sci explanations of quantum mechanics are like the convoluted, contrived plots of B-movies. And he offers his own in The Teleporter’s Dilemma

Imagine that you and a friend are out hiking, and find yourselves kidnapped by a sinister conspiracy of some sort. You’re taken to a remote island, and shown an apparatus consisting of a dial on the floor and a remote control with a single button. You press the button, and the needle on the dial turns in a clockwise direction. There’s a mark on the rim of the dial at one position, but no other distinguishing features.

Your captors explain that there is another dial/ remote system elsewhere on the island, identical in every respect except it doesn’t have a mark on it. You are told that your friend will be taken to that apparatus, and given the remote. Your task is to get the needles on both dials pointing in exactly the direction indicated by the mark on the first dial. If you succeed, you’ll be set free and given a billion dollars. If you fail, a nuclear weapon will be detonated in Los Angeles. (If we’re going to do convoluted thriller plots, here, why not play for high stakes?).

The catch:

– – – fade to black – – –

Can’t give away the catch in a trailer!