and in practice

The first photon teleportation was achieved by a team of physicists from the University of Vienna, led by Anton Zeilinger, in the second half of 1997. Many other teams subsequently repeated the experiment. However, the distances over which the photons were transmitted were not impressive—only millimeters or centimeters.

The fundamental difficulty here is transmitting entangled photons over long distances. In optical fiber, losses always occur. Worse still, the entanglement that connects the photons as they travel apart weakens or even completely disappears along the way. With weakly entangled particles, we are unable to teleport or transmit secret information. To date, the longest distances that entangled pairs can be transmitted are only a few kilometers.

That's why physicists are constantly inventing new ways to transmit entangled pairs over greater distances. In a recent issue of "Nature," Zeilinger's team demonstrated for the first time a simple and practical method—one that can be implemented immediately.

He used a mirror that transmits half the photons of light and reflects the other half. This has been a component of countless optical devices for more than a century. In optics jargon, it is called a beam splitter, or BS for short.

Zeilinger figured out how to use a beam splitter to separate the wheat from the chaff, that is, to assess which photons are most strongly entangled (physicists call this photon purification or distillation). Simply create enough entangled photon pairs, send them to the recipient of the message, and they will select from the cornucopia the best ones, those that, even after being separated by great distance, have retained their magical bond.