Taming photons, electrons paves way for quantum internet

WASHINGTON — A quantum internet could one day allow ultrasecure communications worldwide — but first, scientists must learn to tame unruly quantum particles such as electrons and photons. Several new developments in quantum technology, discussed at a recent meeting, have brought scientists closer to such mastery. Scientists are now teleporting particles’ properties across cities, satellite experiments are gearing up for quantum communications in space, and other scientists are developing ways to hold quantum information in memory.

In one feat, scientists achieved quantum teleportation across long distances through metropolitan areas. Quantum teleportation transfers quantum properties of one particle to another instantaneously. (It doesn’t allow for faster-than-light communication, though, because additional information has to be sent through standard channels.)
Using a quantum network in Calgary, scientists teleported quantum states of photons over 6.2 kilometers. “It’s one step towards … achieving a global quantum network,” says Raju Valivarthi of the University of Calgary in Canada, who presented the result at the International Conference on Quantum Cryptography, QCrypt, on September 12.

A second group of scientists recently teleported photons using a quantum network spread through the city of Hefei, China. The two teams published their results online September 19 in Nature Photonics.

The weird properties of quantum particles make quantum communication possible: They can be in two places at once, or can have their properties linked through quantum entanglement. Tweak one particle in an entangled pair, and you can immediately seem to affect the other — what Albert Einstein called “spooky action at a distance.” Using quantum entanglement, people can securely exchange quantum keys — codes which can be used to encrypt top-secret messages. (SN: 11/20/10, p. 22). Any eavesdropper spying on the quantum key exchange would be detected, and the keys could be thrown out.

In practice, quantum particles can travel only so far. As photons are sent back and forth through optical fibers, many are lost along the way. But certain techniques can be used to expand their range. Quantum teleportation systems could be used to create quantum repeaters, which could be chained together to extend networks farther. But in order to function, quantum repeaters would also require a quantum memory to store entanglement until all the links in the chain are ready, says Ronald Hanson of Delft University of Technology in the Netherlands. Using a system based on quantum entanglement of electrons in diamond chips, Hanson’s team has developed a quantum memory by transferring the entanglement of the electrons to atomic nuclei for safekeeping, he reported at QCrypt on September 15.

Satellites could likewise allow quantum communication from afar. In August, China launched a satellite to test quantum communication from space; other groups are also studying techniques for sending delicate quantum information to space and back again (SN Online: 6/5/16), beaming up photons through free space instead of through optical fibers. “A free-space link is essential if you want to go to real long distance,” Giuseppe Vallone of the University of Padua in Italy said in a session at QCrypt on September 14. Particles can travel farther when sent via quantum satellite — due to the emptiness of space, fewer photons are absorbed or scattered away.
Quantum networks could also benefit from processes that allow the use of scaled-down “quantum fingerprints” of data, to compare files without sending excess data, Feihu Xu of MIT reported at QCrypt on September 12. To check if two files are identical — for example, in order to find illegally pirated movies — one might compare all the bits in each file. But in fact, a subset of the bits — or a fingerprint — can do the job well. By harnessing the power of quantum mechanics, Xu and colleagues were able to compare messages using less information than classical methods require.

The quantum internet relies on the principles of quantum mechanics, which modern-day physicists generally accept — spooky action and all. In 2015, scientists finally confirmed that a key example of quantum weirdness is real, with a souped-up version of a test known as a Bell test, which closed loopholes that had weakened earlier Bell tests (SN: 9/19/15, p. 12). Loophole-free Bell tests were necessary to squelch any lingering doubts, but no one expected any surprises, says Charles Bennett of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. “In a certain sense it’s beating a dead horse.”

But Bell tests have applications for the quantum internet as well — they are a foundation of an even more secure type of quantum communication, called device-independent quantum key distribution. Typically, secure exchanges of quantum keys require that the devices used are trustworthy, but device-independent methods do away with this requirement. This is “the most safe way of quantum communication,” says Hanson. “It does not make any assumptions about the internal workings of the device.”