[Paleopsych] Science Blog: Physicists Entangle Photon and Atom in Atomic Cloud

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Physicists Entangle Photon and Atom in Atomic Cloud
http://www.scienceblog.com/cms/node/8532

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 behavior 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 behavior 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.

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 theyre 
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, said Kuzmich, assistant professor of physics at Georgia Tech.  It 
requires less hardware because we dont have to isolate an atom. In  fact, we 
dont 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.

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

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. Thats  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, said  Kennedy.

From Georgia Institute of Technology
Submitted by BJS on Tue, 2005-07-26 08:08.