Satellite links may don quantum cloaks

Today’s most powerful methods for protecting secret communications may not remain secure tomorrow. That’s because they rely on the difficulty of gnarly calculations that may someday succumb to faster computers, scientists say. However, secrecy based on the inviolable laws of nature—if such protection proves technically feasible—will keep spies completely in the dark.

Researchers now present the first experimental evidence that laws of quantum mechanics could shield signals all the way from the ground to satellites in low orbits. This potential channel for totally secure communications may appeal to military and government agencies, banks, and other security-conscious organizations, says William T. Buttler of Los Alamos (N.M.) National Laboratory.

In the June 12 Physical Review Letters, he and his colleagues describe their recent implementation of quantum-key distribution, a step in the transmission of secure communications.

“This is a convincing demonstration,” comments William P. Risk of the IBM Almaden Research Center in San Jose, Calif. The Los Alamos researchers “understand the difficult technical challenges associated with Earth-to-satellite quantum-key distribution and have devised practical ways of overcoming them.”

On a New Mexico mesa in daylight, the scientists tested whether they could transmit a code cloaked in quantum secrecy. They sent it from a red-light laser to a telescope 1.6 kilometers away.

To take advantage of quantum protection, they dimmed their laser pulses to less than one photon on average—so that many pulses are blanks—and polarized the pulses to represent binary 1s or 0s. Because photons are indivisible, an eavesdropper siphoning data would cause a noticeable intensity drop at the receiver. Other aspects of quantum mechanics prevent spies from surreptitiously measuring polarizations or copying them onto other photons (SN: 2/10/96, p. 90).

In open-air transmissions of laser beams, atmospheric turbulence typically causes trouble by wiggling and distorting the light. The pulses in the Los Alamos experiment passed through even more turbulence from laser to telescope than they would between a laser on a mountaintop and a satellite, Buttler says. That’s because small eddies, common near the ground but not higher up, disrupt laser beams most strongly.

Despite all that air, the telescope successfully received a randomly generated string of bits, called a key, that serves as a shared guide for encoding and decoding messages. Although the key arrived more slowly than data on a cheap Internet phone-line connection, “even this rate is useful. What makes it so is the security of the bits,” says coauthor Richard J. Hughes of Los Alamos.

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