A team of physicists has succeeded in teleporting the quantum state of a photon over a distance of more than six kilometres – well beyond the previous record of 800 metres, and a feat that could help pave the way for more secure communication networks.
Wolfgang Tittel, a senior fellow in the Quantum Information Science program and a physicist at the University of Calgary, led the team that published the research in Nature Photonics. For their experiment, they used Calgary’s municipal fiber optic network, which normally carries telephone calls and Internet traffic.
The experiment involves three people in communication over the network – dubbed Alice, Bob, and Charlie – each located in a different facility in metropolitan Calgary. In the experiment, pairs of entangled photons were created by Bob, who sends one member of each pair to Charlie. Meanwhile, Alice sends photons to Charlie, which interfere with the photons received from Bob through something called a “Bell-state measurement.” The end result is that the quantum state of the photon created by Alice is “teleported” to Bob.
“The measurement at Charlie modifies the quantum state of Bob’s remaining photon. This happens without any object travelling between Charlie and Bob,” Tittels says.
Aside from being an impressive feat of pure physics, the work could prove to be an important step toward building a “quantum repeater” – a vital component of a quantum information network. In a classical communication network (for example, using laser pulses – which are comprised of photons – sent along a fibre optic cable), signals get weaker with distance. The classical signal can be boosted at regular intervals using amplifiers, known as “repeaters.”
In a quantum system, however, this sort of amplification would alter the quantum state of the photons, garbling the messages being sent. Quantum teleportation offers a way around this problem, by sending quantum information without loss across large distances.
Such information networks can also be made extremely secure – because if a quantum message is intercepted, the content is damaged and the intended recipient will know the information has been altered.
“We can verify if someone has eavesdropped or not,” Tittel says.
The technique could be used for fundamentally secure encrypted communication. It could even be used to send problems to a quantum computer, without even the computer knowing what it was working on, allowing for more secure problem-solving.
The study was carried out by researchers at the University of Calgary, the Jet Propulsion Laboratory, and the National Institute of Standards and Technology.