Monday, June 10, 2013

Quantum Teleportation

Quantum teleportation involves the exchange of information in the form of quantum states, not matter, between two points with the use of quantum entanglement. In principle, the distances can be arbitrarily long and works even if the location of the recipient is not known.  Cryptography is a key element in quantum communications and quantum computers. It takes advantage of quantum entanglement, the phenomenon in which two objects, such as photons, are connected in such a way, even at great distances, that changing the state of one instantly changes the state of the other. If an eavesdropper intercepts the message, this entangled state is disrupted and the aggression is thus noticed.

The latest article in a recent series of breakthroughs in quantum teleportation involves researchers at the Niels Bohr Institute who have reported in Nature Physics (1) that they have succeeded in teleporting information between two clouds of gas atoms, and they have done so every single time they attempted. The experiment involves two glass containers, each containing caesium gas atoms. Information is teleported from one glass container to the other by means of a laser beam of light which becomes entangled with the gas atoms. Even though the glass containers were only a half meter apart, it “is entirely due to the size of the laboratory,” explains Eugene Polzik. “We could increase the range if we had the space and, in principle, we could teleport information, for example, to a satellite.” (2)

Anton Zeilinger’s group in Vienna have succeeded in teleporting information over long distances. In a paper in Nature in 2012 (3), an international team led by Zeilinger reported successfully transmitting quantum states over a distance of 143 kilometers, between two Canary Islands.

The limit to distance, in these experiments, has been due to the challenge of preventing the compromise of the fragile quantum state of the photons through the atmosphere. The next step is experiments involving teleportation of information to satellites, a task that involves longer distances (satellites fly in low earth orbit between 200 and 1200 km. above the Earth’s surface) but less interference by the atmosphere. This could lead to a secure global quantum communications system.

One step toward that goal was recently achieved by a group from Ludwig-Maximilians-Universitaet in Munich who have, for the first time, transmitted a secure quantum code from an aircraft to a ground station. (4) The effects from the rate of signal loss, angular velocity of the aircraft and air turbulence were comparable to those expected from satellite to ground transmissions. This success represents another step closer to satellite quantum communications. Apparently a race is already underway in developing the first global quantum communications network. (6)

And in yet another paper in Physical Review Letters (7,8), researchers have worked out how entanglement can be “recycled” to increase efficiency between entangled objects. Previous protocols either involved sending scrambled information that require correction by the receiver or “port-based” teleportation that does’t require correction but does require a large amount of entanglement to the point that each object sent would destroy the entangled state. The “recycling” of the entangled state allows for the teleportation of multiple objects. These researchers have even created a protocol in which multiple qubits can be teleported in bulk, although the entangled states degrade proportionally to the amount of qubits sent.

It is clear that our knowledge of quantum teleportation is not yet to the point that would allow the realization of sci-fi writers’ dreams of teleporting humans to another planet. But the creation of a world-wide quantum communications system using satellites, as well as quantum computers, is closer than some might think. The dream set by writers may still be a long way off but, in this case as well as in many others, reality may eventually follow fiction.

1.     H. Krauter, D. Salart, C. A. Muschik, J. M. Petersen, Heng Shen, T. Fernholz, E. S. Polzik. Deterministic quantum teleportation between distant atomic objects. Nature Physics, 2013; DOI: 10.1038/nphys2631
2.     University of Copenhagen - Niels Bohr Institute (2013, June 6). Quantum teleportation between atomic systems over long distances. ScienceDaily. Retrieved June 10, 2013, from­ /releases/2013/06/130606140844.htm
3.     Xiao-Song Ma, Thomas Herbst, Thomas Scheidl, Daqing Wang, Sebastian Kropatschek, William Naylor, Bernhard Wittmann, Alexandra Mech, Johannes Kofler, Elena Anisimova, Vadim Makarov, Thomas Jennewein, Rupert Ursin, Anton Zeilinger. Quantum teleportation over 143 kilometres using active feed-forward. Nature, 2012; DOI: 10.1038/nature11472
4.     Sebastian Nauerth, Florian Moll, Markus Rau, Christian Fuchs, Joachim Horwath, Stefan Frick, Harald Weinfurter. Air-to-ground quantum communication. Nature Photonics, 2013; DOI: 10.1038/nphoton.2013.46
5.     Ludwig-Maximilians-Universitaet Muenchen (LMU) (2013, April 3). Quantum cryptography: On wings of light. ScienceDaily. Retrieved June 10, 2013, from­ /releases/2013/04/130403071950.htm
6.     Institute of Physics (IOP) (2013, March 1). Space race underway to create quantum satellite. ScienceDaily. Retrieved June 10, 2013, from­ /releases/2013/02/130228194653.htm
7.     Sergii Strelchuk, MichaƂ Horodecki, Jonathan Oppenheim. Generalized Teleportation and Entanglement Recycling. Physical Review Letters, 2013; 110 (1) DOI: 10.1103/PhysRevLett.110.010505
8.     University of Cambridge (2013, January 16). Mathematical breakthrough sets out rules for more effective teleportation. ScienceDaily. Retrieved June 10, 2013, from­ /releases/2013/01/130116111744.htm 

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