Atlanta (July 26, 2005) — Quantum communication networks showgreat promise in becoming a highly secure communications system. Bycarrying information with photons or atoms, which are entangled so thatthe behavior of one affects the other, the network can easily detectany eavesdropper who tries to tap the system.
Physicists at the Georgia Institute of Technology have just reached an important
milestonein the development of these systems by entangling a photon and a singleatom located in an atomic cloud. Researchers believe this is the firsttime an entanglement between a photon and a collective excitation ofatoms has passed the rigorous test of quantum behavior known as a Bellinequality violation. The findings are a significant step in developingsecure long-distance quantum communications. They appear in the July22, 2005 edition of the Physical Review of Letters.
Relying onphotons or atoms to carry information from one place to another,network security relies on a method known as quantum cryptographic keydistribution. In this method, the two information-carrying particles,photonic qubits or atomic qubits, are entangled. Because of theentanglement and a rule in quantum physics that states that measuring aparticle disturbs that particle, an eavesdropper would be easilydetected because the very act of listening causes changes in the system.
Butmany challenges remain in developing these systems, one of which is howto get the particles to store information long enough and travel farenough to get to their intended destination. Photonic qubits are greatcarriers and can travel for long distances before being absorbed intothe conduit, but they’re not so great at storing the information for along time. Atomic qubits, on the other hand, can store information formuch longer. So an entangled system of atoms and photons offers thebest of both worlds. The trick is how to get them entangled in a simpleway that requires the least amount of hardware.
Physicists AlexKuzmich and Brian Kennedy think that taking a collective approach isthe way to go. Instead of trying to isolate an atom to get it into theexcited state necessary for it to become entangled with a photon, theydecided to try to excite an atom in a cloud of atoms.
“Using acollective atomic qubit is much simpler than the single atom approach,”said Kuzmich, assistant professor of physics at Georgia Tech. “Itrequires less hardware because we don’t have to isolate an atom. Infact, we don’t even know, or need to know, which atom in the group isthe qubit. We can show that the system is entangled because it violatesBell inequality.”
“With single atoms, its much more difficult tocontrol the system because there is so much preparation that must bedone,” said Kennedy, professor of physics at Georgia Tech. “For thecollective excitation, the initial preparation of the atoms is minimal.You don’t have to play too much with their internal state – somethingthat’s usually a huge concern.”
In addition to having the systempass the rigorous test of Bell inequality, researchers said they wereable to increase the amount of time the atomic cloud can storeinformation to several microseconds. That’s fifty times longer than ittakes to prepare and measure the atom-photon entanglement.
Anotherchallenge of quantum communication networks is that since photons canonly travel so far before they get absorbed into the conduit, thenetwork has to be built in nodes with a repeater at each connection.
“Avery important step down the road would be to put systems like thistogether and confirm they are behaving in a quantum mechanical way,”said Kennedy.
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