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‘Beautiful Idea’ Eliminates Middleman in Quantum Communication

Photonics.com
Apr 2013
COLLEGE STATION, Texas, April 23, 2013 — Information can now be exchanged between two parties without any physical particles traveling between them, essentially eliminating the middleman in quantum communication, a new study has found.

In quantum physics, objects can be in more than one place at a time, and future events can change the past. New research out of Texas A&M University has discovered an “almost psychic communication” that could play a major role in communication security.

“Right now, this is a new, beautiful idea,” said M. Suhail Zubairy, a professor in the department of physics and astronomy, and holder of the Munnerlyn-Heep Chair in Quantum Optics. “I'm looking at it like a painter or poet looks at art or poetry. Could there be use? Yes, but our main goal right now was simply to understand the basic science first. Who knows what kinds of applications could be envisioned in the future.”

Zubairy drew inspiration from recent developments in a 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. (See: Toward an ‘Unbreakable’ Message Exchange)

“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 middleman. From a fundamental point of view, this is amazing. It sort of brings up these old questions, such as what is a photon?”

It has 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. However, using a series of beamsplitters, Zubairy and colleagues from the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia created a setup in which information can be exchanged between the two parties without any photons actually passing through the communication channel, introducing what Zubairy calls a “new paradigm” in quantum communication.

“In technical terms, we call this ‘counterfactual,’ ” he 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.’ ”


In this figure from their Physical Review Letters paper, the Texas A&M researchers diagram the transmission channel, in which "BS" and "SW" stand for "beamsplitter" and "ideal switches," respectively. 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. Courtesy of Salih et al, Phys Rev Lett.

The simple setup works like this: A photon leaving Alice’s station can, on Bob’s end, have its polarization state switched or kept the same, meaning that Bob has the choice of either absorbing the bit of information or not absorbing it. Depending on Bob’s choice, different detectors are set off on Alice’s end. Alice can figure out Bob’s choice by checking her own detectors. The result: Communication was exchanged between Alice and Bob, but the photon never left Alice’s station.

“The concept of two parties communicating without exchanging particles is mind-boggling and highly counterintuitive,” said Mohammad Al-Amri of KACST’s National Center for Mathematics and Physics. “It raises interesting questions: How could Alice sense Bob's moves when her photon never left her station?”

The findings appeared in Physical Review Letters.  

For more information, visit: www.tamu.edu


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