[Paleopsych] Science Blog: Physicists Entangle Photon and Atom in Atomic Cloud
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Thu Jul 28 21:47:13 UTC 2005
Physicists Entangle Photon and Atom in Atomic Cloud
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.
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