lunes, 8 de junio de 2009
Codes that can't be hacked without revealing the hackers may be on the horizon, thanks to a team of Austrian physicists who sent pairs of entangled photons across a distance of 89 miles (144 kilometers).
Entangled photons are pairs of ordinary light particles that are mysteriously connected at the quantum level.
For each pair, one photon seems to "know" what has happened to the other no matter how far apart they are, an effect Albert Einstein once referred to as "spooky action at a distance."Cryptographers believe that this property makes entangled photons ideal for sending secret messages.
While the method won't prevent people from intercepting a communiqué, if someone does, the entangled pair will instantly reveal the spy.
"You immediately know that there was somebody on the line," said team member Anton Zeilinger of the University of Vienna.
Finding the Photons
Transmitting photons over long distances is difficult, because the beam quickly loses intensity, like the fading reach of a flashlight. This makes the entangled photons harder to detect the farther they travel.
"We lose many photons by scattering in the atmosphere and absorption," Zeilinger said. "Only about one in ten million arrive on the other side."
Making detectors that can "find" the key photons among the background light is therefore a crucial part of the experiment.
The team had previously managed to detect lone members of entangled pairs sent over a 90-mile (144-kilometer) distance. For their new research, published last week in Nature Physics, Zeilinger and colleagues made their detectors sensitive enough to send both members of a pair and find them together at a defined location.
The next step would be to send each entangled photon to a different receiver, opening the door for distant allies to send coded messages to each other via a satellite link.
E-Security and "Spookygrams"
Award-winning science fiction writer and futurist Robert J. Sawyer thinks the biggest short-term benefit of quantum messages will be in e-commerce, where coded data transmissions are vital for theft prevention.
But coded transmissions spanning Earth are just the beginning.
"Theoretically, entangled particles will retain their bonding regardless of how far apart they are," Sawyer said.
Thus, they might someday be used for interstellar communications.
Carl Frederick, a physicist and science fiction writer from Ithaca, New York, is dubious that entangled particles can ever be used to communicate "real" messages.
That's because the changes in the linked photons are more like the results of flipping coins: The "flips" might be the same at both ends, but the result is basically random.
"If you flipped a coin, you don't have any control over the result, and therefore you cannot use that as a volitional message such as, Meet me at noon," Frederick said.
Science fiction writer Jerry Oltion of Eugene, Oregon, disagrees.
"You simply have to establish a code ahead of time," he said by email. "If I alter the photon stream for a tenth of a second, it's a dot. Two tenths of a second is a dash. With that established up front, you can send messages in Morse code.
"A little more sophistication would allow text messages—more would allow voice or even video," Oltion added.
"With the right encoding, I see no reason why complex, real time messages can't be sent via 'spookygram.'"