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Laser Enables Robust Quantum Communication

Photonics Spectra
Apr 2002
Richard Gaughan

Quantum communication is the key to such advances as quantum computing and encryption. In a nutshell, quantum communication means transferring entanglement -- identical quantum states -- over long distances. Unfortunately, the fidelity of those states will decrease exponentially with distance. A new proposal for entanglement transfer promises to improve fidelity.

At the heart of the system are pairs of atomic ensembles entangled by laser irradiation. An ensemble consists of a large number of identical ground-state atoms constrained to be optically thick in one axis.

The pump laser excitation, slightly detuned from the first excited-state transition, would propagate along the longer axis, and the resulting forward-scattered Stokes radiation would create a collective atomic state. The combination of geometry and collective-state interactions should lead to very low noise from spontaneous emission.

To generate entanglement, researchers from Universit├Ąt Innsbruck in Austria, USTC in Hefei, China, and Harvard University in Cambridge, Mass., suggest creating two ensembles, separating their Stokes light from the excitation pulse with polarization and wavelength filtering, and combining the forward-scattered emission at a 50/50 beamsplitter. The outputs from the beamsplitter would then be directed to single-photon detectors.

Ignacio Cirac, a member of the research team who is now at Max Planck Institute for Quantum Optics in Garching, Germany, explained, "If one detector clicks, the photon could have come from the first or the second ensemble. Since there is no way of finding out, we have a superposition of both possibilities, that is, an entangled state of both ensembles."

To implement a quantum communication scheme, however, the problems associated with the long-distance transfer of entanglement must be addressed. The researchers propose using 'entanglement swapping' with other pairs of ensembles, beamsplitters and detectors.

"Imagine A and B are entangled and C and D are entangled," Cirac said. "If we do entanglement swapping by measuring B and C, the result will be that A and D are entangled, but both B and C are no longer entangled [are out of the game]."

By continuing to swap entanglement in this manner along the transmission channel, they suggest, the distance can be extended arbitrarily.

Swapping does carry a cost, because the generation of entanglement at each stage is limited by a finite probability, and all stages must be entangled simultaneously for transfer. This implies that the communication time varies as the square of the distance.

But an advantage of the scheme is that the same detector clicks that establish entanglement also confirm it. That is, the system noise may decrease the probability of successful entanglement transfer, but it does not reduce the fidelity of that transfer once it has occurred.

Cirac believes that the components that are required to implement this scheme are within the scope of the current technology. Although the work is theoretical, research groups from around the world are investigating communication schemes that could follow this architecture.

For Cirac, an interesting outcome of the results is that using many atoms in the experiment seems to simplify the problem.

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