ALEXANDER TSIARAS / SCIENCE PHOTO LIBRARY ALEXANDER TSIARAS / SCIENCE PHOTO LIBRARY
David Wineland of the US National Institute of Standards and Technology (NIST) checking a helium/neon laser of the NIST-7 atomic clock. This clock uses a variety of different lasers. The He/Ne laser is used to count the oscillations of excited hydrogen ions held within an ion trap. Counting these oscillations is the key to the extreme accuracy of NIST-7, expected to be one billionth of a second per day. The ion trap allows ions to be observed for up to ten minutes at a time, a vast improvement on the 10 millisecond observation time in conventional caesium atomic clocks. NIST-7 is expected to become operational in 1992.
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