How it works : Atomic Clock

ATOMIC CLOCK

The very accurate measurement of time is important in many areas of modern technology, such as space exploration, SATELLITE tracking, navigation, and scientific research; since the mid 1950s this has been provided by scientific instruments known as atomic clocks.

Modern precision time systems are based on the simpler QUARTZ CLOCK, which depends on the steady rate of vibration of a quartz crystal to keep it accurate. The frequency of the quartz crystal vibrations will, however, gradually alter over a period of time and need readjusting. In order to set a quartz clock to an exact frequency, an accurate frequency standard must he available for comparison, and this is obtainable from an atomic clock.

There is very little resemblance between an atomic clock and a normal one. It has no CLOCK face, digital or audible read-out normally associated with recording the time. It does not, therefore, tell the time, but provides a reference standard frequency which is used to calibrate other clocks. An atomic clock looks rather like a complex piece of laboratory equipment with vacuum pumps and electronic apparatus.

Principles of the atomic clock
All atoms in their natural state emit and absorb pulses, or quanta, of energy (see QUANTUM THEORY), owing to the switching back and forth of electrons from one orbit, or energy level, around the central NUCLEUS to another. When an electron changes from a high energy orbit to a lower energy orbit a pulse of electromagnetic radiation is emitted, and the frequency of this radiation increases proportionally to the change in energy. Conversely, to boost an electron from a low to a higher energy state a pulse of electromagnetic radiation must be 'fed into' the electron from an outside source. The energy change is proportional to the frequency of the radiation.

These energy levels of an atom are more constant than any other known natural phenomenon, not varying with temperature, pressure or gravity, and are therefore ideally suited to determining standard frequencies. Certain elements such as the ALKALI METALS caesium and rubidium, and HYDROGEN are especially useful for this work. They have only one electron in their outer orbit to emit and absorb energy, and their respective frequencies are relatively uncomplicated and easier to use than those from atoms with more electrons in this orbit.

The caesium clock
A caesium atom can be at one of two energy states depending on whether the outer electron is spinning in the same or opposite direction to that of the nucleus. These different states influence the path of a free caesium atom as it travels through a magnetic field.

Furthermore, if a caesium atom passes through an electromagnetic field which has a frequency equivalent to the difference between the caesium atom's two energy states, the atom will be induced to change from one state to the other. If a beam of caesium atoms is passed through such a field then the maximum number of state transitions will occur when the field frequency is tuned precisely to the 'natural frequency' of the caesium. The caesium clock measures indirectly the number of transitions that occur and constantly tries to achieve the greatest possible number by means of a feedback circuit to the field frequency controller. The field frequency is therefore 'locked' on to the caesium frequency, and by suitable electronic division of this very high frequency to a manageable rate, it can be used to control the accuracy of a quartz clock.

Top: a sketch of the atomic beam chamber showing the caesium oven, the deflecting magnets, cavity resonators and detector. The chamber is made from copper because this is a suitable material for vacuum work. The chamber is evacuated using three oil-diffusion pumps. Above: the operating principle of the atomic clock. If the resonators are tuned to the caesium frequency the maximum number of energy state transitions will occur and the current output from the detector unit ,will also be a maximum. If the resonant frequency changes the detector output falls - automatically readjusting the frequency.

In practice, the method by which this is done is to heat caesium, which has a very low melting point, in a small electric oven set inside a straight tube from which all the air has been removed by VACUUM pumps. Atoms stream out of a slit in the front of the oven and down the tube- there is no air to slow their progress. On the way they pass a magnet which deflects atoms in one state to one side, and atoms in the other state to the other side. At the same time it 'focuses' their path into a converging beam like that of light that has been passed through a convex lens.

The beam converges on a cavity resonator, a hollow aerial-[antenna-] like device which passes an electromagnetic field across it. This field is kept at a frequency of 9192 MHz (millions of cycles a second) by electronically multiplying the 5 MHz signal from a quartz clock. Provided that the quartz clock is running at exactly the right frequency, the field will change the spin direction of nearly all the atoms passing through it, but without affecting the tidy focusing of the beam in which they travel.

The beam comes to a focus at a narrow slit in the middle of the resonator, which stops any off-course atoms. The beam spreads out on the other side of the slit, but the focusing has made it cross over, and an atom that was on the left side of the beam before the slit is on the right side after it.

At the same time the resonator has changed the spin direction of nearly all the atoms. Since atoms in one state are on one side of the beam (and vice versa) the net result is that the two changes cancel each other out, and the beam emerges from the resonator with atoms in a particular state on the same side as when they entered.

The beam then passes through a second magnet, which focuses it to strike a detecting device at the end of the tube. The few atoms that have not changed their state are on the 'wrong' side of the beam to be focused, and are thrown out to the side. The atoms striking the detector cause it to emit a signal which is fed back to the quartz dock.

If the frequency of the quartz clock slips a little, the frequency of the resonator in the tube is no longer 9192 MHz and the spin direction of the atoms is not changed. As a result, all the atoms emerging from the resonator are on the 'wrong' side of the beam to be focused on to the detector.

Below: the first NPL atomic clock.This was accurate to one second in a 100 years.The chamber was mounted in a magnetic east-west direction and surrounded by two coils (the cage) to eliminate any component of the Earth's magnetic field which might upset the caesium beam.

None of them strikes it, and the absence of any signal causes the quartz clock to change its frequency until it receives the signal again, in this way the quartz clock is accurate to within one second in 1000 years.

Other atomic clocks
Other forms of atomic clock are in use throughout the world, but the caesium clock, developed at the National Physical Laboratory in England, provides the basis of some of the present international measurements of time. Other types employ masers (microwave amplification by stimulated emission of radiation) to produce their atomic frequency signals. The original types used ammonia masers, but these have now been superseded by hydrogen ones, which are more reliable and accurate. More use is being made of rubidium gas cell clocks, which use an optical system instead of microwaves. Commercial atomic clocks are available, and are in use in research institutes, broadcasting stations, scientific organizations, and industry. A typical portable caesium clock weighs about 65 lb (30 kg).

ATOMIC NUMBER (see periodic table)
ATOMIC WEIGHT (see chemistry)

Reproduced from HOW IT WORKS p182