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Ion Plasmas Simplify Atomic Clocks

Photonics Spectra
Jun 1998
Susan M. Reiss

Precision timekeeping and the understanding of white dwarf and neutron stars have been made easier through observations of a laser-cooled ion plasma created by researchers at the National Institute of Standards and Technology.

Led by physicist Wayne Itano, the team used Bragg scattering of laser light from a cold plasma of trapped beryllium ions to study what previously could be modeled only through calculation -- the crystalline structure of plasmas. "With a large enough cloud -- one made up of hundreds of thousands of ions -- the interior is like a state of bulk matter," Itano said.

Improved clocks could lead to better global positioning navigation systems, satellite relays and telecommunications.

A frequency-doubled continuous-wave ring dye laser cooled the ions, creating the plasma cloud, while an imaging photomultiplier tube captured the position and time of single photons striking a photocathode, enabling reconstruction of the Bragg diffraction pattern. A charge-coupled device along with an electronically gated image intensifier captured the diffraction pattern. Since the plasmas rotate at a high frequency (typically 100 kHz), observing a fixed pattern of diffraction spots requires accurate synchronization of the de- tection with the rotation. A rotating electric field was applied to control the frequency of the ion plasmas.

"This work is very significant," said Dan Dubin, a physicist at the University of California at San Diego, who has been applying mathematical models to estimate the ordering of the interior of plasma clouds. "The single-component plasma has been used for decades as a paradigm for condensed matter, but until now no one has ever actually observed the body-centered cubic order expected in large systems."

Astrophysicists predict that a similar body-centered cubic structure exists in dense plasmas in the outer crusts of neutron stars and in the interior of white dwarfs. With the interior crystalline structure of these plasma clouds verified, astrophysicists now have nearly direct verification of their models for white dwarf and neutron stars.

The ability to control exactly the frequency at which the beryllium ions spin also provides for a more precise atomic clock based on the hyperfine frequency of these ions, according to Itano. "One uncertainty of atomic clocks is the second-order Doppler shift, a small but still significant frequency shift due to the velocity of the ions," he said. "By controlling the spin, we can say that the plasma rotated 100,000 times per second and know it was exactly that -- not 99,999 -- and accurately calculate the shift."

Improved clocks could lead to better global positioning navigation systems, satellite relays and telecommunications. Knowing where the ions are located in the crystal also may be useful for applications in quantum computers, where the spin of each ion acts as a separately addressable "qbit." But as Dubin noted, "This type of application is still a few years down the road."

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