Many groups of researchers are working
on quantum communications systems, which use attributes of individual
photons to carry information.
Such systems are potentially very powerful because photons can
be entangled, or connected so that attributes like polarization remain
linked regardless of the distance between them.
Massachusetts Institute of Technology researchers have moved the
field forward with entangled photon beams that contain specific wavelengths
of light and are relatively bright.
Firing a laser into a certain type of crystal causes some single
photons to become a pair of lower-energy entangled photons. The researchers
generated 12,000 photon pairs per second per milliwatt of laser power
by using a continuous split laser beam that hit the crystal from two directions.
The method produces relatively many entangled pairs of photons
because it skips the filtering step usually required to remove unentangled
photons, according to the researchers. The researchers produced entangled-photon
beams at wavelengths of 795 nanometers, which is appropriate for quantum
memory, and 1,600 nanometers, which be transmitted down a standard telecom
The researchers' next step is to make a brighter beam by adding
an optical cavity, which amplifies light, to the device.
The project is part of a five-year program to transmit information
over long distances using entanglement. The researchers presented the
work at the Frontiers in Optics meeting of the Optical Society of America
(OSA) in Tucson, Arizona October 5 to 9.
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