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Quantum Clean

Entanglements in the quantum realm are rather remarkable: the properties of two entangled photons, for instance, will stay thoroughly dependent on each other, regardless of how far apart those particles are in space. But like any entanglement, the quantum kind can get messy. "Entangled states tend not to remain pure, due to interactions with the environment," says Paul Kwiat, a former researcher from Los Alamos National Laboratory who is now a professor at the University of Illinois. "Just as the connection between two cell phones can become clouded with static and must be filtered, we have implemented a technique that cleans up the 'static' in entangled systems." He and his colleagues describe the technique--a potential boon to quantum information processing--in today's issue of Nature.

In developing the method, Kwiat and his team first passed a laser pulse through two adjacent nonlinear crystals to create pairs of polarization-entangled photons. They were able to tune the degree of entanglement by varying the linear polarization of the laser pulse. Then they applied a distillation procedure to only one photon in each pair, resulting in the noisier couples being filtered out. "We basically used the procedure to throw away the unwanted part of the contribution, and what remained was in a perfectly entangled state," Kwiat explains. "In this way, we demonstrated distillation of maximally entangled states from non-maximally entangled inputs.

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