Atomic clock
Every clock before this took its standard from outside: the Sun, a pendulum, a crystal. An atomic clock takes it from nature. Every caesium atom in the universe rings at the same frequency, and since 1967 the second has been defined by it.
- 9,192,631,770periods in one second
- 1955the first caesium clock, National Physical Laboratory, England
- 10⁻¹⁸the accuracy of today's best optical clocks
A clock that checks itself against an atom
In 1955 Louis Essen and Jack Parry at the National Physical Laboratory in England started the first caesium clock. Twelve years later the world gave up the astronomical second and defined it by the atom.
On the right the clock is running: the feedback holds the frequency right on the peak, and the lamp on the cabinet glows green.
Atoms from an oven
On the left is a small oven holding a gram of caesium. It is heated to about a hundred degrees, and atoms fly out into a vacuum tube in a narrow beam. They travel at about two hundred metres a second and hardly ever hit each other.
Caesium's outer electron has two closely spaced states. The transition between them is the standard: 9,192,631,770 periods a second.
Sorting by state
A caesium atom is a tiny magnet, and in its two states it points different ways. The first magnet deflects the atoms and leaves only one state in the beam. The second, by the detector, separates them again: atoms that changed state fly up to the detector, the rest into the wall.
In the drawing the atoms that changed state are coloured.
Two strikes of microwaves
Between the magnets the beam passes twice through a microwave cavity. If the microwave frequency matches the atom's, the atom changes state. In 1949 Norman Ramsey had the idea of exposing the atoms in two places half a metre apart: the longer an atom flies between them, the sharper the resonance. It won him a Nobel Prize.
We have detuned the frequency by 120 Hz. Move the pointer or swipe sideways over the drawing, and the signal on the chart below it will drop.
A hundred hertz out of nine billion
The central peak on the chart is about two hundred hertz wide. That is two parts in a hundred million of the caesium frequency, and it is how the clock tells whether its frequency has wandered.
A detuning of 300 Hz has already sent the meter's needle down.
Quartz on a lead held by atoms
The atoms themselves don't tick. What ticks is the quartz oscillator in the cabinet, and a synthesiser multiplies its frequency up to 9.19 gigahertz. The electronics nudge the frequency to and fro, see on which side the signal is stronger, and steer the quartz to the top of the peak.
We have switched the feedback on: watch the dot on the chart climb to the top by itself. The button on the drawing switches it on and off.
More accurate than the Earth
The first caesium clocks drifted by about a second in three hundred years and were at once more regular than the Earth's rotation. Since then atomic time has occasionally been given a leap second, so that noon doesn't creep away from the Sun.
Today's caesium fountains toss a cloud of laser-cooled atoms upward and catch it on the way down: such clocks drift by a second in hundreds of millions of years. Optical clocks using strontium and ytterbium are a hundred times better still, and can tell that time runs faster when they are raised by a centimetre.
Every GPS satellite carries atomic clocks. The navigator in your phone finds you by comparing when the signals arrive, and an error of a millionth of a second would cost three hundred metres. Without a correction for relativity, the satellite clocks would run ahead by 38 microseconds a day.