For more than a decade, physicists have been developing quantum-mechanical methods to pass secret messages without fear that they could be intercepted. But they still haven't created a true quantum network—the fully quantum-mechanical analog to an ordinary telecommunications network in which an uncrackable connection can be forged between any two stations or "nodes" in a network. Now, a team of researchers in Germany has built the first true quantum link using two widely separated atoms. A complete network could be constructed by combining many such links, the researchers say.
"These results are a remarkable achievement," says Andrew Shields, an applied physicist and assistant managing director at Toshiba Research Europe Ltd. in Cambridge, U.K., who was not involved in the work. "In the past, we have built networks that can communicate quantum information, but convert it into classical form at the network switching points. [The researchers] report preliminary experiments towards forming a network in which the information remains in quantum form."
Quantum communication schemes generally take advantage of the fact that, according to quantum theory, it's impossible to measure the condition or "state" of a quantum particle without disturbing the particle. For example, suppose Alice wants to send Bob a secret message. She can do the encrypting in a traditional way, by writing out the message in the form of a long binary number and zippering it together in a certain mathematical way with a "key," another long stream of random 0s and 1s. Bob can then use the same key to unscramble the message.
But first, Alice must send Bob the key without letting anybody else see it. She can do that if she encodes the key in single particles of light, or photons. Details vary, but schemes generally exploit the fact that an eavesdropper, Eve, cannot measure the individual photons without altering their state in some way that Alice and Bob can detect by comparing notes before Alice encodes and sends her message. Such "quantum key distribution" has already been demonstrated in networks, such as a large six-node network in Vienna in 2008, and various companies offer quantum key distribution devices.
Such schemes suffer a significant limitation, however. Although the key is passed from node to node in a quantum fashion, it must be read out and regenerated at each node in the network, leaving the nodes vulnerable to hacking. So physicists would like to make the nodes of the network themselves fully quantum mechanical—say, by forming them out of individual atoms.
According to quantum mechanics, an atom can have only certain discrete amounts of energy depending on how its innards are gyrating. Bizarrely, an atom can also be in two different energy states—call them 0 and 1—at once, although that uncertain two-states-at-once condition "collapses" into one state or the other as soon as the atom is measured. "Entanglement" takes weirdness to its absurd extreme. Two atoms can be entangled so that both are in an uncertain two-ways-at-once state, but their states are perfectly correlated. For example, if Alice and Bob share a pair of entangled atoms and she measures hers and finds it in the 1 state, then she'll know that Bob is sure to find his in the 1 state, too, even before he measures it.
Obviously, Alice and Bob can generate a shared random key by simply entangling and measuring their atoms again and again. Crucially, if entanglement can be extended to a third atom held by Charlotte, then Alice and Charlotte can share a key. In that case, if Eve then tries to detect the key by surreptitiously measuring Bob's atom, she'll mess up the correlations between Alice's and Charlotte's atoms in a way that will reveal her presence, making the truly quantum network unhackable, at least in principle.
But first, physicists must entangle widely separated atoms. Now, Stephan Ritter of the Max Planck Institute of Quantum Optics in Garching, Germany, and colleagues have done just that, entangling two atoms in separate labs on opposite sides of the street, as they report online today in Nature.
As simple as this may sound, the researchers still needed a complete lab room full of lasers, optical elements, and other equipment for each node. Each atom sat between two highly reflective mirrors 0.5 mm apart, which form an "optical cavity." By applying an external laser to atom A, Ritter's team caused a photon emitted by that atom to escape from its cavity and travel through a 60-meter-long optical fiber to the cavity across the street. When the photon was absorbed by atom B, the original quantum information from the first atom was transferred to the second. By starting with just the right state of the first atom, the researchers could entangle the two atoms. According to the researchers, the entanglement could in principle be extended to a third atom, which makes the system scalable to more than two nodes.
"Every experimental step had to be just right to make this work," says Ritter, who works in the group of Gerhard Rempe. "Take, for example, the optical cavity. All physicists agree that atoms and photons are great stuff for building a quantum network, but in free space they hardly interact. We needed to develop the cavity for that."
"This is a very important advance," says Toshiba's Shields, because it would enable technologists to share quantum keys on networks where the intermediate nodes can't be trusted, and it could also lead to more complex multiparty communication protocols based on distributed entanglement. "However," Shields cautions, "there is still a great deal of work to be done before the technology is practical." Miniaturization of the components that constitute one node will no doubt be on the researchers' wish list.