Atomic clocks
The nist-1 clock
The current most accurate clock (NIST-F1 fountain clock) neither gains nor loses a second in nearly 20 million years. Rather accurate. To understand how this super clock reaches such accuracy let’s first remind ourselves what a clock is. All clocks have two basic components: a constant process to mark the increments of time and a means of keeping track of the increments. For example, your grandfather’s pendulum clock increments time with the oscillation mechanism of the pendulum. Christian Huygens (a Dutch scientist) made the first one in 1656 and its error was less than one minute a day.
While Huygens’ clock (or your grandfather’s pendulum clock) is a mechanical clock, the atomic clock is a quantum-mechanical clock. In other words, its working is based on quantum physics (physics of the ultra small). Essentially, the incremental component is given by the radiation frequency that causes the maximum transitions of the clock’s atoms (NIST-F1 uses Caesium 133) to another state. In English? Well… According to quantum mechanics, sub-atomic particles can only exist in discrete states and not at in between states (electrons have two hyperfine states). Generally, an atom remains in its hyperfine state. But when prodded by electromagnetic radiation at a specific frequency, it will switch to the other state. For Caesium the specific radiation frequency is 9,192,631,770. Or in other words, when the radiation frequency is 9,192,631,770 a maximum number of Caesium atoms undergo a “hyperfine state” transition. As frequency is expressed in [s-1] (per second) it is obvious (…hopefully) that the 9,192,631,770 beats define exactly 1 second. The idea of using hyperfine states for a clock was first proposed by U.S. physicist Isador Rabi in 1945.
After this preamble, you can now understand that an atomic clock: 1) filters atoms by selecting those in one hyperfine state, 2) radiates the selected atoms by microwaves, 3) measures how many changed atoms there are (fluorescence detection), 4) tunes the microwave generator to the frequency that causes the maximum transitions. This optimal frequency (or number of oscillations) represents 1 second.
So now why do we need such accuracy? Innumerable communication and navigation systems as well as synchronising systems (between computers,…) rely on it. One example is the GPS (Global Positioning Satellites) system. GPS determine the position of a receiver anywhere on Earth by timing the arrival signals from four GPS satellites and doing a quick calculation to triangulate its position. Each nanosecond (billionth of a second) of error translates into a GPS error of one foot. Thus each of the 24 GPS satellites contains four atomic clocks.
More information on clocks: http://physics.nist.gov/GenInt/Time/time.html
Want to check your computer’s clock against the official U.S. atomic clock? http://www.time.gov/
