Quantum teleportation, evoking as it does images of Star Trek-like transporters, hasn鈥檛 quite lived up to its promise. It has never managed to transport more than a few fragile photons over short distances. Now there鈥檚 a system that can potentially teleport thousands of substantial particles 鈥� 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鈥檚 team at the Australian Research Council Center of Excellence for Quantum Atom Optics in Brisbane is proposing a technique that avoids quantum entanglement entirely. 鈥淲e鈥檙e talking about a beam of about 5000 particles disappearing from one place and appearing somewhere else,鈥� says Bradley. 鈥淲e feel that our scheme is closer in spirit to the original fictional concept,鈥� 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 鈥渃lassical teleportation鈥�. (Watch an )
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They hit upon the idea by accident. 鈥淲e 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,鈥� says Bradley.
In their method, a beam of rubidium atoms is fired into a 鈥渟ender鈥� device, also made from rubidium atoms (see Diagram). At the same time, a 鈥渃ontrol鈥� 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. 鈥淭his wouldn鈥檛 be much use if you hoped to ever reconstruct the matter beam,鈥� says team member Simon Haine.
The team鈥檚 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. 鈥淲hen 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,鈥� says Haine.
This outgoing 鈥渕essenger鈥� 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. 鈥淭he only limitation on how far this messenger pulse can travel is the length of the optical fibre,鈥� says Bradley. 鈥淲e鈥檙e transmitting the information at the speed of light.鈥�
In theory, the original matter beam can then be reconstructed at a distant location when the messenger pulse strikes a BEC 鈥渞eceiver鈥�, which is controlled by a second laser pulse. The team鈥檚 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 ().
John Close, an expert on atomic laser physics at the Australian National University in Canberra, is impressed. 鈥淯sing entangled atomic states looks pretty tough in comparison.鈥� 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. 鈥淭his is a system that can transfer the state of an atomic system to an optical system and then back to a second atomic system 鈥� a key requirement of quantum information networks.鈥�
