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Teleportation Becoming a Reality


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Theorists from the UQ (Ashton Bradley, Simon Haine, Murray Olsen) and ANU Faculties (Joseph Hope) nodes of the ARC Centre of Excellence for Quantum-Atom Optics (ACQAO) have come up with a scheme to teleport quantum states of collections of atoms from one position to another by converting the quantum state to light and back again. This work has been highlighted in a recent article in New Scientist.

 

When an object is transferred from one location to another, by a method other than physically moving the object itself, it is said to have undergone "teleportation". A fax machine might be said to teleport a piece of paper, but it doesn't have perfect resolution, so the paper you send to the receiver is always a little different to the piece you started with. You might think that measuring the paper with increasing accuracy might eventually lead to the perfect 'teleporter'. If you get all the atoms exactly positioned, it doesn't matter if they are the 'same' atoms as the original piece of paper, because quantum mechanics dictates that all particles of a given "type" are fundamentally indistinguishable (i.e., the universe is exactly the same if you swap two electrons).

 

Unfortunately, quantum mechanics, in particular the Heisenberg uncertainty principle, dictates that we can never measure the quantum state of a system perfectly. This "quantum noise" will make it impossible to accurately teleport something.

 

However, in 1993, a team of Physicists (C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, A. Peres and W. K. Wootters (read the original paper here), devised a scheme to avoid this problem by using quantum entanglement. Quantum entanglement is a bizarre property of quantum mechanics. When two particles are entangled, measurements on one of the particles instantaneously effect the quantum state of the other particle, no matter how far away it is. Entanglement is used to implement quantum teleportation as follows:

 

The sender (usually referred to as "Alice") combines the particle she wants to teleport with one half of an entangled pair of particles, and then measures the properties of the system, disrupting the quantum state of the system in the process. The added quantum noise from this process 'hides' the original quantum state from Alice. She then sends the results from her measurement to the receiver (bob), who possesses the other half of the entangled pair. Bob then uses this information to perform operations on his half of the entangled pair to retrieve the original quantum state. This works because the quantum noise of his half of the entangled pair is exactly right to cancel the noise added by Alice's half. Experiments using this scheme, or variations of it, have been performed with single photons, beams of light (also done in the first Australian teleportation experiment) , trapped ions, and the nuclear spin states of an ensemble of atoms.

 

In theory this scheme can lead to the quantum state being perfectly teleported. However, in practice quantum entanglement is never perfect, and this limits the fidelity of current teleportation experiments. So far, teleportation experiments have been limited to a fidelity of 85%.

 

Our scheme is different as it does not rely on Alice and Bob sharing quantum entanglement. We have shown that it may be possible to teleport a group of about 5 thousand cold atoms by transferring their quantum state onto a laser beam, which is then 'beamed' to a new location where the receiver can use this laser beam to recreate the original group of atoms almost exactly. The scheme relies on the sender and receiver each having a reservoir of extremely cold atoms, known as a Bose-Einstein condensate (BEC). BEC is a state of matter that occurs when atoms become very cold, (about 100 Billionths of a degree about absolute zero). Due to a phenomenon known as Bose-Enhancement, all the atoms like to act the same way. This causes the atoms to act as one macroscopic matterwave, rather than a collection of individual atoms.

 

Usually, decoherence is generated from the coupling with an outer environment. However, a macroscopic object generically possesses its own environment in itself, namely the complicated dynamics of internal degrees of freedom. We address a question: when and how the internal dynamics decohere interference of the center of mass motion of a macroscopic object. We will show that weak localization of a macroscopic object in disordered potentials can be destroyed by such decoherence.

 

In the early 1920s Satyendra Nath Bose was studying the new idea (at that time) that the light came in little discrete packets (we now call these "quanta" or "photons"). Bose assumed certain rules for deciding when two photons should be counted up as either identical or different. We now call these rules "Bose statistics" (or sometimes "Bose-Einstein statistics").

 

Einstein guessed that these same rules might apply to atoms. He worked out the theory for how atoms would behave in a gas if these new rules applied. What he found was that the equations said that generally there would not be much difference, except at very low temperatures. If the atoms were cold enough, something very unusual was supposed to happen. It was so strange he was not sure it was correct. Einstein did not realize the most important effects that his equations were predicting. What Einstein's equations predicted was that at normal temperatures the atoms would be in many different levels. However, at very low temperatures, a large fraction of the atoms would suddenly go crashing down into the very lowest energy level.

 

It is only at the special incredibly low temperatures needed for BEC that they lose their individual identities and coalesce into a single blob. Some people have called this a "super atom." Working steadily for the past six years, the Colorado scientists used that cooling principle to create the Superatom. They supercooled atoms of the element rubidium to the coldest temperature ever--nearly -273 [degrees] C (-460 [degrees] F), also known as absolute zero.

 

By capturing the action on video--and with the aid of a computer--the scientists could 'see' the atoms slow to a virtual standstill. When that happened, the atoms merged into a cloudy blob--a Superatom, says Cornell. Atoms in the Superatom state move in a single wavelike pattern, instead of bouncing around independently, as do the atoms in the other phases.

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