Quantum Communication Between the Stars?

May. 22, 2003
by Seth Shostak - Senior Astronomer

Earthlings haven't made many deliberate broadcasts to extraterrestrials, but in 1974, as part of a ceremony at the economy-sized Arecibo radio telescope, the observatory staff arranged to beam a three-minute message to a few hundred thousand stars in the constellation of Hercules.

The message consisted of a simple picture showing the structure of our solar system and the structure of ourselves DNA and its chemical building blocks. Innocuous enough.

What was not so innocuous was the reaction. Englands Astronomer Royal was aghast at the thought of our freelance pinging of unknown galactic inhabitants. Despite the fact that the message was short and directed to a globular cluster 21,000 light-years distant, he felt that we might be endangering ourselves by "shouting in the jungle."

Given the brevity and remote target of this broadcast, such concerns were surely overwrought. But the point is worth considering: Would anyone deliberately beam high-powered signals into space? Can we assume that extraterrestrial societies would broadcast in ways that would mark their location as plainly as a flag on a golf green?

Maybe they dont have to. Walter Simmons, a physicist at the University of Hawaii, together with his colleague, Professor Sandip Pakvasa, have come up with a clever scheme that would allow interstellar broadcasters to keep the coordinates of their home planet secret. These two scientists have been researching quantum information theory for a while. Their trick is to forego conventional electromagnetic signals (light or radio) made up of large, organized "waves" of photons in favor of individual, quantum-entangled photons.

There is more to this than merely substituting a small task force for a large army. Individual photons can be quantum mechanically related they can have buddies, if you will, with which they share information. Each buddy is sent separately by the broadcaster, and reunited with his pal at the receiving end. They deliver their message only when theyre brought together.

In practical terms, the way this might be accomplished is that each member of a photon pair is sent in opposite directions from the broadcasters home planet. One might be beamed a light-year to the left, and the other a light-year to the right. They would be aimed at mirrors that would redirect them to the target star system. Additional non-paired photons could be sent along as well, to swamp the presence of these message bearers, somewhat like using disorganized street crowds to hide a task force.

At the receiving end, the photons that came from one mirror or the other would be indistinguishable from cosmic background noise. But the quantum-entangled buddy photons would unite to form a microscopic image a picture. The picture, of course, could contain all sorts of interesting information that sophisticated aliens might wish to share with us or others in the Galaxy.

As noted, the image would be quite small. Looking at it would disturb it in such a way that Heisenbergs famous uncertainty principle would come into play. In fact, by reading the message, all the information about its origin would be lost. Its somewhat akin to communicating with bottled messages thrown into the sea. The message arrives, but the sender keeps his location secret.

In fact, this new scheme is better than bobbing bottles. Theres no ocean to scramble the lines of communication. According to Simmons, "its fairly straightforward to target specific recipients, and it wouldnt be hard for the transmitting society to methodically send messages to large numbers of star systems, one after the other."

Could quantum messaging dominate interstellar communication? Could this be the preferred way to get in touch with unknown cosmic beings? If so, it offers an appealing resolution of the famous Fermi Paradox, which asks "if the Galaxy is teeming with intelligence, why dont we see evidence for it everywhere?" Perhaps the evidence is everywhere washing over us right now in a shower of quantum-encrypted messages.

We dont have the technology to look for such signals today, although Simmons expects that we could possibly construct it within a decade or so. "Meanwhile, we should continue our SETI searches," he adds, "we should absolutely do that." After all, there are many ways to get in touch. Its just that some of them don't carry a return address.