Teleportation, but not as we know it
13 June 2007
NewScientist.com news service
Quantum teleportation, evoking as it does images of Star Trek-like transporters, hasn't quite lived up to its promise. It has never managed to transport more than a few fragile photons over short distances. Now there's a system that can potentially teleport thousands of substantial particles - without any quantum hang-ups.
Teleportation usually refers to a process that relies on quantum entanglement, where two particles are linked regardless of the distance between them, so that any change to the state of one instantly changes its twin. However, entangled particles are difficult to prepare and delicate to handle, imposing severe limits on quantum teleportation.
Now physicist Ashton Bradley's team at the Australian Research Council Center of Excellence for Quantum Atom Optics in Brisbane is proposing a technique that avoids quantum entanglement entirely. "We're talking about a beam of about 5000 particles disappearing from one place and appearing somewhere else," says Bradley. "We feel that our scheme is closer in spirit to the original fictional concept," he adds. While the technique can also transmit quantum information in the beam, the technique itself does not rely on the quantum properties of particles, so the team have dubbed the new method "classical teleportation". (Watch an animation here)
They hit upon the idea by accident. "We were messing around with a way to sensitively measure the quantum properties of an atom beam when we realised we could efficiently use the technique to transport matter," says Bradley.
In their method, a beam of rubidium atoms is fired into a "sender" device, also made from rubidium atoms (see Diagram). At the same time, a "control" laser pulse is fired at the sender. This laser traps the incoming atoms from the rubidium beam, exciting them to a high energy state.
Normally, if the sender were made of ordinary matter, the incoming atoms would simply lose this extra energy by releasing photons in every direction. "This wouldn't be much use if you hoped to ever reconstruct the matter beam," says team member Simon Haine.
The team's set-up prevents scattering by making the sender from rubidium atoms in a special low-temperature state called a Bose-Einstein condensate. In a BEC, all atoms are in their lowest possible quantum state. "When a bunch of new atoms from the matter beam hit the BEC they want to join them in this lowest state, but they can only do that by shedding all their extra energy as photons released in a very directed pulse," says Haine.
This outgoing "messenger" pulse of light can then be transmitted down an optical fibre, carrying with it all the information about the original matter beam, including the number of atoms it contained, their momentum and energy, and quantum properties such as their phase. "The only limitation on how far this messenger pulse can travel is the length of the optical fibre," says Bradley. "We're transmitting the information at the speed of light."
In theory, the original matter beam can then be reconstructed at a distant location when the messenger pulse strikes a BEC "receiver", which is controlled by a second laser pulse. The team's calculations show that once the messenger pulse hits the receiver its atoms are excited and they eject a second matter beam with identical properties to the original (www.arxiv.org/abs/0706.0062).
John Close, an expert on atomic laser physics at the Australian National University in Canberra, is impressed. "Using entangled atomic states looks pretty tough in comparison." Close wants to set up an experiment to test the system, but estimates it will take at least four years.
Warwick Bowen, an expert on quantum optics at the University of Otago in New Zealand, believes the method will be useful for setting up networks of quantum computers. "This is a system that can transfer the state of an atomic system to an optical system and then back to a second atomic system - a key requirement of quantum information networks."
From issue 2608 of New Scientist magazine, 13 June 2007, page 16
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