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Πέμπτη, 18 Απριλίου 2013

Physics Psychics? How Alice And Bob Talk, Without Saying A Word

In quantum physics, objects can be in more than one place at a time and future events can change the past - don't get caught up too much in that or you won't sleep at night. 

A new paper says quantum physics can be even spookier. Using the “chained” quantum Zeno effect, the researchers write that they have discovered a form of "almost psychic communication" in which information can be exchanged between two parties without photons, or any physical particles, traveling between them.

Let's invoke "Alice" and "Bob", the most famous couple in physics, to illustrate what they mean. The basic set-up works like this: A photon leaving Alice's station can, on Bob's end, have its polarization state switched or kept the same. In other words, Bob has the choice of either absorbing the bit of information or not absorbing it. Depending on what Bob chooses, different detectors are set off on Alice's end. So Alice can figure out Bob's choice by checking her own detectors. But these researchers created a counterintuitive result: A communication occurred between Alice and Bob, but the photon never left Alice's station. It's long been assumed in physics that for information to travel in empty space between two parties, Alice and Bob, physical particles have to travel between them. But by using a series of beam splitters, the researchers created a setup in which information can be exchanged between Alice and Bob without any photons, tiny units of electromagnetic light, actually passing through the communication channel, introducing what Texas A&M physics professor M. Suhail Zubairy calls a "new paradigm" in quantum communication.

"In technical terms, we call this 'counterfactual,'" Zubairy said. "It's been believed that if two parties want to communicate, something needs to be sent, and something is allowing for its passage. That's been an underlying assumption in every communications system. This is the first possible setup where that doesn't exist. That's what I mean by 'almost psychic.'"

 Here BS stands for beam splitter and SW stands for ideal switches. In the transmission channel, the photon is accessible to Eve. (a) The BSs have large reflectivity, R = cos2 θ=cos2 (π/2N) with N being the total number of beam splitters. (b) By using a chained version of the setup shown in (a), we can achieve direct counterfactual quantum communication. Two kinds of BSs are used. One is BSM for M big cycles. The other is BSN for N small cycles within each M cycle. There are a total of M × N cycles for one signal. As discussed in the text, the probability of finding a signal photon in the transmission channel is nearly zero. Clicks at D1 or D2 reveal to Alice Bob’s bit choices. Note that in this case, the photon is accessible to "Eve," the group's token name for the possible third party in the "Alice" and "Bob" scenario -- an eavesdropper. (Credit: Salih et al., arXiv:1206.2042

Zubairy says they were inspired by the development in recent years of the branch of quantum cryptography called quantum key distribution, a system that allows secure communication between two parties by the exchange of "keys" that allow for decoding of messages sent through a public channel.

"What we are proposing goes a couple steps beyond that," Zubairy said. "We are talking about direct communication, not through a key. Now we have a protocol for eliminating the middle man. From a fundamental point of view, this is amazing. It sort of brings up these old questions, such as what is a photon?"

"The concept of two parties communicating without exchanging particles is mind-boggling and highly counterintuitive," said co-author Mohammad Al-Amri of the National Center for Mathematics and Physics at the King Abdulaziz City for Science and Technology in Saudi Arabia. "It raises interesting questions: How could Alice sense Bob's moves when her photon never left her station?"

Citation: Hatim Salih, Zheng-Hong Li, M. Al-Amri, and M. Suhail Zubairy, 'Protocol for direct counterfactual quantum communicatiarXiv:1206.2042
http://www.science20.com/

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