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RE: Bell inequality violation
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Quantum communication networks show great promise in becoming a highly secure communications system. By carrying information with photons or atoms, which are entangled so that the behaviour of one affects the other, the network can easily detect any eavesdropper who tries to tap the system.

Physicists at the Georgia Institute of Technology have just reached an important
milestone in the development of these systems by entangling a photon and a single atom located in an atomic cloud. Researchers believe this is the first time an entanglement between a photon and a collective excitation of atoms has passed the rigorous test of quantum behaviour known as a Bell inequality violation. The findings are a significant step in developing secure long-distance quantum communications. They appear in the July 22, 2005 edition of the Physical Review of Letters.


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Physicists have just reached an important milestone in the development of quantum communications networks by entangling a photon and a single atom located in an atomic cloud. This is the first time an entanglement between a photon and a collective excitation of atoms has passed the rigorous test of quantum behaviour known as a Bell inequality violation.

Relying on photons or atoms to carry information from one place to another, network security relies on a method known as quantum cryptographic key distribution. In this method, the two information-carrying particles, photonic qubits or atomic qubits, are entangled. Because of the entanglement and a rule in quantum physics that states that measuring a particle disturbs that particle, an eavesdropper would be easily detected because the very act of listening causes changes in the system.

But many challenges remain in developing these systems, one of which is how to get the particles to store information long enough and travel far enough to get to their intended destination. Photonic qubits are great carriers and can travel for long distances before being absorbed into the conduit, but theyíre not so great at storing the information for a long time. Atomic qubits, on the other hand, can store information for much longer. So an entangled system of atoms and photons offers the best of both worlds. The trick is how to get them entangled in a simple way that requires the least amount of hardware.

Physicists Alex Kuzmich and Brian Kennedy think that taking a collective approach is the way to go. Instead of trying to isolate an atom to get it into the excited state necessary for it to become entangled with a photon, they decided to try to excite an atom in a cloud of atoms.

"Using a collective atomic qubit is much simpler than the single atom approach. It requires less hardware because we donít have to isolate an atom. In fact, we donít even know, or need to know, which atom in the group is the qubit. We can show that the system is entangled because it violates Bell inequality" - Alex Kuzmich, , assistant professor of physics at Georgia Tech.

"With single atoms, its much more difficult to control the system because there is so much preparation that must be done. For the collective excitation, the initial preparation of the atoms is minimal. You donít have to play too much with their internal state Ė something thatís usually a huge concern" - Brian Kennedy, professor of physics at Georgia Tech.

In addition to having the system pass the rigorous test of Bell inequality, researchers said they were able to increase the amount of time the atomic cloud can store information to several microseconds. Thatís fifty times longer than it takes to prepare and measure the atom-photon entanglement.

Another challenge of quantum communication networks is that since photons can only travel so far before they get absorbed into the conduit, the network has to be built in nodes with a repeater at each connection.

"A very important step down the road would be to put systems like this together and confirm they are behaving in a quantum mechanical way" - Brian Kennedy.

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-- Edited by Blobrana at 15:21, 2005-08-14

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Quantum entanglement
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Quantum entanglement could be responsible for mass, and could finally explain why the fundamental particles of matter have the mass they do.
Sometimes, the interaction of two particles, say electrons, causes their individual properties, such as spin, to become "entangled".

If you then change the spin of one particle it will instantly affect the spin of the other, regardless of the distance between them.
There is mounting evidence that entanglement has consequences in the macroscopic world.
The physicist Vlatko Vedral of the University of Leeds, UK, showed that entanglement is involved in superconductivity.
Entanglement can explain one of the defining traits of superconductivity - the Meissner effect, in which a magnet will levitate above a piece of superconducting material. The magnetic field induces a current in the surface of the superconductor, and this current effectively excludes the magnetic field from the interior of the material, causing the magnet to hover.

Only a current composed of entangled electrons in the superconductor can achieve this effect. The current halts the photons of the magnetic field after they have travelled only a short distance through the superconductor.
For the normally massless photons it is as if they have suddenly entered treacle, effectively giving them a mass.
A similar mechanism may be behind the mass of all particles. The standard model of physics says that matter is made of particles such as electrons, neutrinos, and quarks, while the various forces in the universe, such as the strong and weak nuclear forces, act through `mediator` particles such as the gluon.
In theory, these mediators are all massless, and so all the fundamental forces should act over infinite distances. In reality, they do not: the forces have a limited range, and the mediator particles have mass.

Physicists believe that the source of this mass is something called the Higgs field that fills the universe and is mediated by a particle known as the Higgs boson. These bosons are thought to exist in a `condensed` state that excludes the mediator particles such as gluons in the same way that a superconductors entangled electrons exclude the photons of a magnetic field.
This exclusion by the Higgs field is what gives the mediator particles an effective mass, and also limits their range of influence.
But no one has yet seen a Higgs boson, let alone shown exactly how they exclude, say, gluons.
Entanglement could be the answer.
The condensation of the Higgs bosons and exclusion of the mediators requires entanglement between the Higgs bosons. Entanglement may be linked to the mass of not just the mediator particles, but all fundamental particles.
Different particles would interact differently with the entangled Higgs bosons, providing different `effective masses` for each particle.

"The connection (of entanglement) to the Higgs field, if it could be substantiated by a quantitative argument, is clearly intriguing."


-- Edited by Blobrana at 15:21, 2005-08-14

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