Teleportation goes the distance

By Eric Smalley, Technology Research News

You can't get from one place to another without passing through every point in between. This is true for all matter and energy, whether planets, people or quantum particles.

You can, however, do the quantum equivalent of faxing particles from one place to another, if the particles in question are photons. Teleportation makes it possible to transmit the quantum states, or structural information, of photons from one place to another.

And making photons from one location materialize at another without traveling the distance between opens the way for sending perfectly secure messages long distances.

Researchers at the University of Geneva in Switzerland and the University of Aarhus in Denmark have teleported photons from one laboratory to another lab 55 meters away, and their setup simulated a distance of two kilometers. Previous teleportation experiments have been limited to short distances within laboratories.

Quantum states, which dictate the ultimate structure of objects, can be teleported, said Nicholas Gisin, a professor of physics at the University of Geneva. The key to teleportation is that only this information is transported. "Objects can be transferred from one place to another without ever existing anywhere in between. But only the structure is teleported. The original object is destroyed and reconstructed," he said.

Teleportation relies on entanglement, a weird aspect of quantum physics. Entanglement links one or more physical properties of two or more particles, for example the polarizations, or orientations, of a pair of photons.

Particles become entangled when they are in superposition, which is a mixture of all possible quantum states. Superposition occurs when particles are isolated from their environments. A photon can be polarized in one of two opposite directions, for example, but in superposition it is polarized in some mix of both.

When a pair of particles in superposition come into contact with each other, they can become entangled. When one of the particles comes into contact with the environment and is knocked out of superposition, it is in one definite quantum state. At the same instant, regardless of the distance between them, the other particle is also knocked out of superposition and assumes the same quantum state.

Previous teleportation experiments have used photons whose polarizations are entangled. The Geneva researchers' method relied on time bins, or short time windows, said Gisin. The researchers generated photons using ultra-short laser pulses, counted time in these small increments, or bins, and timed the pulses to occur in specific bins.

Photons in superposition reside in two time bins at once, Gisin said. And photons in superposition can be entangled. The key to the researchers' teleportation experiment was entangling these photons based on time bins, because this allows them to survive transmission over fiber-optic lines better than polarization-entangled photons, he said. A pair of entangled particles can serve as transmitter and receiver to teleport a third particle.

The researchers entangled a pair of infrared photons and sent one to the second lab, then teleported a third photon by bringing it into contact with the entangled photon in the first lab. The third photon was destroyed and the entangled photon in the second lab became a replica of the third photon.

The researchers used photons of the same wavelengths used in ordinary optical communications, and they transmitted the entangled photon over a two-kilometer fiber-optic cable, proving that it is possible to teleport particles over distances.

Researchers are aiming to use teleportation to build quantum relays in order to extend the reach of quantum communications systems. Ordinary optical communications lines use repeaters to boost fading signals, but repeaters make copies of the fading photons and quantum states can't be copied without being destroyed.

Quantum relays would be a big boost for quantum cryptography, which is by far the most advanced quantum communications application, said Gisin.

Quantum cryptography allows a sender and receiver to tell for sure whether the encryption key they are using has been compromised by an eavesdropper. An encryption key is a string of numbers used to lock and unlock messages.

Last year, the Geneva researchers demonstrated a quantum cryptography system that transported a secure key over ordinary phone lines spanning 67 kilometers between Geneva and Lausanne. However, the quantum states of photons can't survive longer distances, making quantum relays necessary for long distance quantum cryptography.

The Geneva researchers are working on finding the limits for the distances between relays and determining the trade-offs between distance and performance for practical applications, said Gisin. They are also working on improving the stability of their experimental setup, he said.

Practical applications could be ready in five to ten years, said Gisin.

Gisin's research colleagues were Ivan Marcikic, Hugues de Reidmatten and Hugo Zbinden of the University of Geneva, and Wolfgang Tittel of the University of Geneva and the University of Aarhus in Denmark. They published the research in the January 30, 2003 issue of the journal Nature. The research was funded by the Swiss National Science Foundation and the European Community.

Timeline:   5-10 years
Funding:   Government
TRN Categories:  Quantum Computing and Communications
Story Type:   News
Related Elements:  Technical paper, "Long-Distance Teleportation of Qubits at Telecommunication Wavelengths," Nature, January 30, 2003.


February 12/19, 2003

Page One

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