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Two-photon source brings quantum cryptography closer

01 May 2007

A microstructured optical fiber is an efficient source of photon pairs for quantum information systems.

A team from the National Institute of Standards and Technology (NIST) in Maryland, US, has built a fiber-based two-photon light source capable of high brightness, broad spectral range and low background noise at room temperature. Key to the design is a microstructured optical fiber (MOF) that increases the efficiency of photon pair production. (Optics Express, 15, 6, 2915.)

"We used a picosecond pulsed laser at a wavelength of 740 nm and a power of less than 1 mW, along with a commercially available MOF," Jingyun Fan of NIST told optics.org. "We found that the MOF created photon pairs with a higher gain and a much lower noise level compared to conventional fiber."

"An efficient high-rate two-photon source is a key requirement for any workable quantum cryptography system," said Fan. "And these types of quantum links will also be necessary in quantum computing for moving information within and between processors."

Photon pairs can be produced in conventional fibers but the process is inefficient because most photons normally travel through them without interacting with each other. Raman scattering, in which individual photons bounce off the fiber's molecular structure and change their energies, makes things worse. Scattering produces photons that look as if they might be one half of a pair, but aren't.

In an MOF, the array of hollow channels increases the intensity of light in the central core and makes production of photon pairs more likely. This could provide a way to make the necessary pairs in realistic quantities.

"The mechanical properties of the fiber aren't an issue at this stage, and the MOF is only used as a relatively short length of 1.8 meters," Fan observes. "It's the optical properties that are important, the ability of the MOF to convert a particular pump wavelength into an output pair of photons with minimal background noise."

The team's two-photon source brightness is comparable to sources based on nonlinear crystals. They believe that engineering the fiber to increase the dispersion slope would produce even better results.

NIST will now apply the new source to a range of quantum encryption tests in a real-world setting, using a special testbed. "Beyond that, we'll aim to increase the accuracy of the two-photon states, and use them as building blocks for entangled states of more than two photons," said Fan. "We will see how far we can push this approach."

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