European Space Agency's Optical Ground Station on Tenerife in the
Canary Islands was used as a receiver in recent quantum teleportation
experiments. Credit: ESA
Two teams of researchers have extended the reach of quantum teleportation
to unprecedented lengths, roughly equivalent to the distance between
New York City and Philadelphia. But don’t expect teleportation stations
to replace airports or train terminals—the teleportation scheme shifts
only the quantum state of a single photon. And although part of the
transfer happens instantaneously, the steps required to read out the
teleported quantum state ensure that no information can be communicated
faster than the speed of light.
Quantum teleportation relies on the phenomenon of entanglement, through which quantum particles share a fragile, invisible link across space.
Two entangled photons, for instance, can have correlated, opposite
polarization states—if one photon is vertically polarized, for instance,
the other must be horizontally polarized. But, thanks to the
intricacies of quantum mechanics, each photon’s specific polarization
remains undecided until one of them is measured. At that instant the
other photon’s polarization snaps into its opposing orientation, even if
many kilometers have come between the entangled pair.
An entangled photon pair serves as the intermediary in the standard
teleportation scheme. Say Alice wants to teleport the quantum state of a
photon to Bob. First she takes one member of a pair of entangled
photons, and Bob takes the other. Then Alice lets her entangled photon
interfere with the photon to be teleported and performs a polarization
measurement whose outcome depends on the quantum state of both of her
Because of the link between Alice and Bob forged by entanglement,
Bob’s photon instantly feels the effect of the measurement made by
Alice. Bob’s photon assumes the quantum state of Alice’s original
photon, but in a sort of garbled form. Bob cannot recover the quantum
state Alice wanted to teleport until he reverses that garbling by
tweaking his photon in a way that depends on the outcome of Alice’s
measurement. So he must await word from Alice about how to complete the
teleportation—and that word cannot travel faster than the speed of
light. That restriction ensures that teleported information obeys the
cosmic speed limit.
Even though teleportation does not allow superluminal communication,
it does provide a detour around another physics blockade known as the
no-cloning theorem. That theorem states that one cannot perfectly copy a
quantum object to, for instance, send a facsimile to another person.
But teleportation does not create a copy per se—it simply shifts the
quantum information from one place to another, destroying the original
in the process.
Teleportation can also securely transmit quantum information even
when Alice does not know where Bob is. Bob can take his entangled
particle wherever he pleases, and Alice can broadcast her instructions
for how to ungarble the teleported state over whatever conventional
channels—radio waves, the Internet—she pleases. That information would
be useless to an eavesdropper without an entangled link to Alice.
Physicists note that quantum entanglement and teleportation could one
day form the backbone of quantum channels linking hypothetical quantum
processors or enabling secure communications between distant parties.
But for now the phenomenon of teleportation is in the gee-whiz
exploratory phase, with various groups of physicists devising new tests
to push the limits of what is experimentally possible.
In the August 9 issue of Nature, a Chinese group reports achieving quantum teleportation across Qinghai Lake in China, a distance of 97 kilometers. (Scientific American
is part of Nature Publishing Group.) That distance surpasses the
previous record, set by a group that included several of the same
researchers, of 16 kilometers.
But a more recent study seems to have pushed the bar even higher. In a paper posted May 17 to the physics preprint Web site arXiv.org, just eight days after the Chinese group announced their achievement on the same Web site, a European and Canadian group claims to have teleported information
from one of the Canary Islands to another, 143 kilometers away. That
paper has not been peer-reviewed but comes from a very reputable
Both teams of physicists faced serious experimental
challenges—sending a single photon 100 kilometers and then plucking it
out of the air is no easy task. In practical terms, both groups’ Alices
and Bobs needed laser-locked telescopes for sending and receiving their
photons, as well as complex optics for modifying and measuring the
photons’ quantum states.
But that’s nothing compared to what the physicists have in mind for
future experiments. Both research groups note that their work is a step
toward future space-based teleportation, in which quantum information
would be beamed from the ground to an orbiting satellite.
About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.