Technique detects quantum state

The laws of physics make things that are very small -- like atomic particles -- act differently than objects in the larger world where we reside.

One weird quantum property is entanglement, which allows properties of particles like atoms, photons and electrons to remain linked, or synchronized, regardless of the physical distance between the particles. Entanglement is also very sensitive to disturbances and therefore difficult to measure.

Entanglement figures prominently in efforts to build quantum computers, which use properties of particles to compute. Quantum computers promise to be fantastically fast at certain types of large problems, including those that would render today's cryptography useless. Entanglement also figures in quantum cryptography schemes that offer theoretically perfect security.

Researchers from the University of Rome in Italy have pushed the schemes forward by demonstrating a method for detecting entanglement.

The researchers generated entangled photons using a crystal and a pair of laser beams. They showed that it was possible to detect entanglement using three independent local measurements. The method is particularly useful for determining if entanglement survived the transmission of photons over a fiber optic line.

The method could be used to measure entanglement in any kind of particles, including cold atoms, trapped ions, and electronic currents in superconducting devices, according to the researchers.

The method could be used practically in five to ten years, according to the researchers. The work appeared in the November 28, 2003 issue of Physical Review Letters.